Portrait of Professor Stephen Lowry

Professor Stephen Lowry

Professor of Astronomy and Planetary Science

About

Professor Stephen Lowry is based at the University of Kent’s Centre for Astrophysics and Planetary Science. He specialises in optical and infrared observations and modelling of Solar System Small Bodies, making use of both space-based and large ground-based telescope facilities. He is also involved in spacecraft exploration of these bodies. Stephen serves on the UK Science and Technology Facilities Council Solar System Research Advisory Panel, and has served on numerous international grant-review panels.

He completed his PhD in the area of cometary science at Queen's University Belfast in 2001, and from there he was a research fellow for several years in the UK and US. Before moving to the University of Kent, he was a research scientist at NASA’s Jet Propulsion Laboratory working on physical and compositional characterisations of cometary and asteroidal bodies; he supported a number of spacecraft missions to these bodies and the development of a new comet surface sample return mission. His publications include numerous research papers in Science and Nature and other leading astronomy journals, and was lead author in a chapter in the book The Solar System Beyond Neptune. Details of his research programmes can be found here and here.

Research interests

Professor Lowry's research programme focuses on observations of Small Solar System Bodies (SSSBs), with particular emphasis on Near-Earth Asteroids (NEAs), and comets, including Main Belt Comets (MBCs). This field is of particular importance as these bodies are the only surviving remnants of the formation era of our solar system, and several space missions have been launched to these bodies to help answer fundamental questions regarding their nature.

The research involves the use of large, ground-based optical and infrared telescope facilities, as well as space-based telescopes such as the Hubble Space Telescope and the Spitzer Space Infrared Telescope. Professor Lowry and his colleagues now use planetary radar facilities, including the Arecibo Observatory (Puerto Rico, USA). Analysis techniques used include: image analysis and photometry, spectroscopy, 3-D shape and spin-state modelling from optical and radar observations, and thermophysical modelling.

The group also has interest in spacecraft exploration of SSSBs.  Professor Lowry is currently part of the science analysis team for the OSIRIS instrument on board the recent Rosetta comet explorer spacecraft, and has supported many other spacecraft missions by helping to characterise the target bodies, either leading up to or during the encounter phase. The group is also involved in the development of several mission concepts to visit cometary bodies.

Teaching

Stephen is involved in the teaching of a number of astronomy modules; topics covered include data analysis techniques, solar system science and rocketry and human spaceflight.

Publications

Showing 50 of 92 total publications in the Kent Academic Repository. View all publications.

Article

  • Street, R., Bachelet, E., Tsapras, Y., Hundertmark, M., Bozza, V., Dominik, M., Bramich, D., Cassan, A., Horne, K., Mao, S., Saha, A., Wambsganss, J., Zang, W., JørgensenU., Longa-PeñaP., Peixinho, N., Sajadian, S., Burgdorf, M., Campbell-White, J., Dib, S., Evans, D., Fujii, Y., Hinse, T., Khalouei, E., Lowry, S., Rahvar, S., Rabus, M., Skottfelt, J., Snodgrass, C., Southworth, J. and Tregloan-Reed, J. (2019). OGLE-2018-BLG-0022: A Nearby M-dwarf Binary. The Astronomical Journal [Online] 157:215. Available at: https://doi.org/10.3847/1538-3881/ab1538.
    We report observations of the binary microlensing event OGLE-2018-BLG-0022, provided by the Robotic Observations of Microlensing Events (ROME)/Reactive Event Assessment (REA) Survey, which indicate that the lens is a low-mass binary star consisting of M3 (0.375 ± 0.020 M ⊙) and M7 (0.098 ± 0.005 M ⊙) components. The lens is unusually close, at 0.998 ± 0.047 kpc, compared with the majority of microlensing events, and despite its intrinsically low luminosity, it is likely that adaptive optics observations in the near future will be able to provide an independent confirmation of the lens masses.
  • Shvartzvald, Y., Yee, J., Skowron, J., Lee, C., Udalski, A., Novati, S., Bozza, V., Beichman, C., Bryden, G., Carey, S., Gaudi, B., Henderson, C., Zhu, W., Bachelet, E., Bolt, G., Christie, G., Maoz, D., Natusch, T., Pogge, R., Street, R., Tan, T., Tsapras, Y., Pietrukowicz, P., SoszyńskiI., SzymańskiM., MrózP., Poleski, R., KozłowskiS., Ulaczyk, K., Pawlak, M., Rybicki, K., Iwanek, P., Albrow, M., Cha, S., Chung, S., Gould, A., Han, C., Hwang, K., Jung, Y., Kim, D., Kim, H., Kim, S., Lee, D., Lee, Y., Park, B., Ryu, Y., Shin, I., Zang, W., Dominik, M., Helling, C., Hundertmark, M., JørgensenU., Longa-PeñaP., Lowry, S., Sajadian, S., Burgdorf, M., Campbell-White, J., Ciceri, S., Evans, D., Fujii, Y., Hinse, T., Rahvar, S., Rabus, M., Skottfelt, J., Snodgrass, C. and Southworth, J. (2019). Spitzer Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-rotating Low-mass Brown Dwarf. The Astronomical Journal [Online] 157:106. Available at: https://doi.org/10.3847/1538-3881/aafe12.
    The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens-parallax and finite-source effects are needed in order to recover the full information. Here, we combine the Spitzer satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either 18 ± 1 M J or 20 ± 1 M J . This is the lowest isolated-object mass measurement to date, only ~45% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either 3.9 ± 0.1 kpc or 4.1 ± 0.1 kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.
  • Jones, G., Agarwal, J., Bowles, N., Burchell, M., Coates, A., Fitzsimmons, A., Graps, A., Hsieh, H., Lisse, C., Lowry, S., Masters, A., Snodgrass, C. and Tubiana, C. (2018). The proposed Caroline ESA M3 mission to a Main Belt Comet. Advances in Space Research [Online] 62:1921-1946. Available at: https://doi.org/10.1016/j.asr.2018.02.032.
    We describe Caroline, a mission proposal submitted to the European Space Agency in 2010 in response to the Cosmic Visions M3 call for medium-sized missions. Caroline would have travelled to a Main Belt Comet (MBC), characterizing the object during a flyby, and capturing dust from its tenuous coma for return to Earth. MBCs are suspected to be transition objects straddling the traditional boundary between volatile–poor rocky asteroids and volatile–rich comets. The weak cometary activity exhibited by these objects indicates the presence of water ice, and may represent the primary type of object that delivered water to the early Earth. The Caroline mission would have employed aerogel as a medium for the capture of dust grains, as successfully used by the NASA Stardust mission to Comet 81P/Wild 2. We describe the proposed mission design, primary elements of the spacecraft, and provide an overview of the science instruments and their measurement goals. Caroline was ultimately not selected by the European Space Agency during the M3 call; we briefly reflect on the pros and cons of the mission as proposed, and how current and future mission MBC mission proposals such as Castalia could best be approached.
  • Han, C., Novati, S., Udalski, A., Lee, C., Gould, A., Bozza, V., MrózP., Pietrukowicz, P., Skowron, J., SzymańskiM., Poleski, R., SoszyńskiI., KozłowskiS., Ulaczyk, K., Pawlak, M., Rybicki, K., Iwanek, P., Albrow, M., Chung, S., Hwang, K., Jung, Y., Ryu, Y., Shin, I., Shvartzvald, Y., Yee, J., Zang, W., Zhu, W., Cha, S., Kim, D., Kim, H., Kim, S., Lee, D., Lee, Y., Park, B., Pogge, R., Kim, W., Beichman, C., Bryden, G., Carey, S., Gaudi, B., Henderson, C., Dominik, M., Helling, C., Hundertmark, M., JørgensenU., Longa-PeñaP., Lowry, S., Sajadian, S., Burgdorf, M., Campbell-White, J., Ciceri, S., Evans, D., Haikala, L., Hinse, T., Rahvar, S., Rabus, M. and Snodgrass, C. (2018). OGLE-2017-BLG-0329L: A Microlensing Binary Characterized with Dramatically Enhanced Precision Using Data from Space-based Observations. The Astrophysical Journal [Online] 859:82. Available at: https://doi.org/10.3847/1538-4357/aabd87.
    Mass measurements of gravitational microlenses require one to determine the microlens parallax πE, but precise πE measurement, in many cases, is hampered due to the subtlety of the microlens-parallax signal combined with the difficulty of distinguishing the signal from those induced by other higher-order effects. In this work, we present the analysis of the binary-lens event OGLE-2017-BLG-0329, for which πE is measured with a dramatically improved precision using additional data from space-based Spitzer observations. We find that while the parallax model based on the ground-based data cannot be distinguished from a zero-πE model at the 2σ level, the addition of the Spitzer data enables us to identify two classes of solutions, each composed of a pair of solutions according to the wellknown ecliptic degeneracy. It is found that the space-based data reduce the measurement uncertainties of the north and east components of the microlens-parallax vector pE by factors ∼18 and ∼4, respectively. With the measured microlens parallax combined with the angular Einstein radius measured from the resolved caustic crossings, we find that the lens is composed of a binary with component masses of either (M1, M2) ∼ (1.1, 0.8) Me or ∼(0.4, 0.3) Me according to the two solution classes. The first solution is significantly favored but the second cannot be securely ruled out based on the microlensing data alone. However, the degeneracy can be resolved from adaptive optics observations taken ∼10 years after the event.
  • Snodgrass, C., Jones, G., Boehnhardt, H., Gibbings, A., Homeister, M., Andre, N., Beck, P., Bentley, M., Bertini, I., Bowles, N., Capria, M., Carr, C., Ceriotti, M., Coates, A., Della Corte, V., Donaldson Hanna, K., Fitzsimmons, A., GutiérrezP., Hainaut, O., Herique, A., Hilchenbach, M., Hsieh, H., Jehin, E., Karatekin, O., Kofman, W., Lara, L., Laudan, K., Licandro, J., Lowry, S., Marzari, F., Masters, A., Meech, K., Moreno, F., Morse, A., Orosei, R., Pack, A., Plettemeier, D., Prialnik, D., Rotundi, A., Rubin, M., Sánchez, J., Sheridan, S., Trieloff, M. and Winterboer, A. (2018). The Castalia mission to Main Belt Comet 133P/Elst-Pizarro. Advances in Space Research [Online] 62:1947-1976. Available at: https://doi.org/10.1016/j.asr.2017.09.011.
    We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.
  • Tatsumi, E., Domingue, D., Hirata, N., Kitazato, K., Vilas, F., Lederer, S., Weissman, P., Lowry, S. and Sugita, S. (2018). Vis-NIR disk-integrated photometry of asteroid 25143 Itokawa around opposition by AMICA/Hayabusa. Icarus [Online] 311:175-196. Available at: https://doi.org/10.1016/j.icarus.2018.04.001.
    We present photometry of the S-type near-Earth asteroid 25143 Itokawa based on both ground-based observations in the UBVRI bands and measurements from the AMICA/Hayabusa spacecraft observations with ul-, b-, v-, w-, x-, and p-filters. Hayabusa observed Itokawa around opposition during the rendezvous, thus providing a unique set of observations of this asteroid. We fit the phase curve measurements with both the Classic Hapke Model (Hapke, 1981, 1984, 1986) and Modern Hapke Model (Hapke, 2002, 2008, 2012a) and thereby extract the physical properties of Itokawa's surface regolith. The single-scattering albedo (0.57?±?0.05) is larger than that derived for Eros (0.43?±?0.02), another S-type near-Earth asteroid visited by a spacecraft. Both models indicate a regolith that is forward-scattering in nature. From the hockey stick relationship derived for the single-particle phase function (Hapke, 2012b), both modeling results suggest a regolith comprised of rough surfaced particles with a low density of internal scatterers. Application of the Modern Hapke model derives porosity parameter values from 1 to 1.1, for BVR bands, which corresponds to porosity values between 77–79%. This suggests the surface of Itokawa is very fluffy and the large boulders may be bonded with smaller size particles, typical of the particle sizes observed in Muses Sea. Both models also provide similar geometric albedo values (0.27?±?0.02) at the V-band wavelength, which are equivalent to Eros’ geometric albedo.
  • Attree, N., Groussin, O., Jorda, L., NébouyD., Thomas, N., Brouet, Y., KührtE., Preusker, F., Scholten, F., Knollenberg, J., Hartogh, P., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., A’HearnM., Auger, A., Barucci, M., Bertaux, J., Bertini, I., Bodewits, D., Boudreault, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Deller, J., El-Maarry, M., Fornasier, S., Fulle, M., GutiérrezP., GüttlerC., Hviid, S., Ip, W., Kovacs, G., Kramm, J., KüppersM., Lara, L., Lazzarin, M., Lopez Moreno, J., Lowry, S., Marchi, S., Marzari, F., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Toth, I., Tubiana, C., Vincent, J. and Shi, X. (2018). Tensile strength of 67P/Churyumov–Gerasimenko nucleus material from overhangs. Astronomy & Astrophysics [Online] 611. Available at: https://doi.org/10.1051/0004-6361/201732155.
    We directly measured twenty overhanging cliffs on the surface of comet 67P/Churyumov–Gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support them against collapse under the comet’s gravity. We find extremely low strengths of around 1 Pa or less (1 to 5 Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features andthe implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67P nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties either with size over the ~10–100 m range studied here or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km).
  • Gicquel, A., Rose, M., Vincent, J., Davidsson, B., Bodewits, D., A’HearnM., Agarwal, J., Fougere, N., Sierks, H., Bertini, I., Lin, Z., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Barucci, M., Bertaux, J., Besse, S., Boudreault, S., Cremonese, G., Da Deppo, V., Debei, S., Deller, J., De Cecco, M., Frattin, E., El-Maarry, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., Gutiérrez-MarquezP., GüttlerC., Höfner, S., Hofmann, M., Hu, X., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lazzarin, M., Moreno, J., Lowry, S., Marzari, F., Masoumzadeh, N., Massironi, M., Moreno, F., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Preusker, F., Scholten, F., Shi, X., Thomas, N., Toth, I. and Tubiana, C. (2017). Modelling of the outburst on 2015 July 29 observed with OSIRIS cameras in the Southern hemisphere of comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society [Online] 469:S178-S185. Available at: http://dx.doi.org/10.1093/mnras/stx1441.
    Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov– Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) cameras since 2014 March using both the wide-angle camera and the narrow-angle camera (NAC). We use images from the NAC camera to study a bright outburst observed in the Southern hemisphere on 2015 July 29. The high spatial resolution of the NAC is needed to localize the source point of the outburst on the surface of the nucleus. The heliocentric distance is 1.25 au and the spacecraft–comet distance is 186?km. Aiming to better understand the physics that led to the outgassing, we used the Direct Simulation Monte Carlo method to study the gas flow close to the nucleus and the dust trajectories. The goal is to understand the mechanisms producing the outburst. We reproduce the opening angle of the outburst in the model and constrain the outgassing ratio between the outburst source and the local region. The outburst is in fact a combination of both gas and dust, in which the active surface is approximately 10 times more active than the average rate found in the surrounding areas. We need a number of dust particles 7.83 × 10^11 to 6.90 × 10^15 (radius 1.97–185 ?m), which correspond to a mass of dust (220–21) × 10^3 kg.
  • Pajola, M., Höfner, S., Vincent, J., Oklay, N., Scholten, F., Preusker, F., Mottola, S., Naletto, G., Fornasier, S., Lowry, S., Feller, C., Hasselmann, P., GüttlerC., Tubiana, C., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’HearnM., Barucci, M., Bertaux, J., Bertini, I., Besse, S., Boudreault, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Deller, J., Deshapriya, J., El-Maarry, M., Ferrari, S., Ferri, F., Fulle, M., Groussin, O., Gutierrez, P., Hofmann, M., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lin, Z., Lazzarin, M., Lucchetti, A., Lopez Moreno, J., Marzari, F., Massironi, M., Michalik, H., Penasa, L., Pommerol, A., Simioni, E., Thomas, N., Toth, I. and Baratti, E. (2017). The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse. Nature Astronomy [Online] 1:92. Available at: https://doi.org/10.1038/s41550-017-0092.
    Outbursts occur commonly on comets1 with different frequencies and scales2,3. Despite multiple observations suggesting various triggering processes4,5, the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked6 to explain some outbursts on comet 103P/Hartley 2, although the process required a pre-existing dust layer on the verge of failure. The Rosetta mission observed several outbursts from its target comet 67P/Churyumov–Gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs7,8. However, none of the aforementioned works included definitive evidence that landslides occur on comets. Amongst the many features observed by Rosetta on the nucleus of the comet, one peculiar fracture, 70?m long and 1?m wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan9. On 10 July 2015, the Rosetta Navigation Camera captured a large plume of dust that could be traced back to an area encompassing the Aswan escarpment7. Five days later, the OSIRIS camera observed a fresh, sharp and bright edge on the Aswan cliff. Here we report the first unambiguous link between an outburst and a cliff collapse on a comet. We establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies7. Moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus.

    The evolution of the collapse of the Aswan cliff9, observed by the OSIRIS Narrow Angle Camera (NAC)10 and the Rosetta Navigation camera (NavCam), is shown in Fig. 1. We estimated a total outburst ejected mass of cometary material between 0.5?×?106 and 1.0?×?106?kg for the 10 July event. By applying stereo-photogrammetric methods11 using multiple OSIRIS images (Supplementary Table 1), we determined the total volume of material that collapsed from the Aswan cliff. In Fig. 2, the dataset that depicts the aspect of the cliff before and after the collapse is presented. By using pre- and post-collapse three-dimensional (3D) models (see Methods), we have been able to measure the dimensions of the collapsed overhang (Supplementary Figs 1–2), deriving a total volume of 2.20?×?104?m3, with a 1? uncertainty of 0.34?×?104?m3
  • Snodgrass, C., A’Hearn, M., Aceituno, F., Afanasiev, V., Bagnulo, S., Bauer, J., Bergond, G., Besse, S., Biver, N., Bodewits, D., Boehnhardt, H., Bonev, B., Borisov, G., Carry, B., Casanova, V., Cochran, A., Conn, B., Davidsson, B., Davies, J., de LeónJ., de Mooij, E., de Val-Borro, M., Delacruz, M., DiSanti, M., Drew, J., Duffard, R., Edberg, N., Faggi, S., Feaga, L., Fitzsimmons, A., Fujiwara, H., Gibb, E., Gillon, M., Green, S., Guijarro, A., Guilbert-Lepoutre, A., GutiérrezP., Hadamcik, E., Hainaut, O., Haque, S., Hedrosa, R., Hines, D., Hopp, U., Hoyo, F., HutsemékersD., Hyland, M., Ivanova, O., Jehin, E., Jones, G., Keane, J., Kelley, M., Kiselev, N., Kleyna, J., Kluge, M., Knight, M., Kokotanekova, R., Koschny, D., Kramer, E., López-MorenoJ., Lacerda, P., Lara, L., Lasue, J., Lehto, H., Levasseur-Regourd, A., Licandro, J., Lin, Z., Lister, T., Lowry, S., Mainzer, A., Manfroid, J., Marchant, J., McKay, A., McNeill, A., Meech, K., Micheli, M., Mohammed, I., MonguióM., Moreno, F., MuñozO., Mumma, M., Nikolov, P., Opitom, C., Ortiz, J., Paganini, L., Pajuelo, M., Pozuelos, F., Protopapa, S., Pursimo, T., Rajkumar, B., Ramanjooloo, Y., Ramos, E., Ries, C., Riffeser, A., Rosenbush, V., Rousselot, P., Ryan, E., Santos-Sanz, P., Schleicher, D., Schmidt, M., Schulz, R., Sen, A., Somero, A., Sota, A., Stinson, A., Sunshine, J., Thompson, A., Tozzi, G., Tubiana, C., Villanueva, G., Wang, X., Wooden, D., Yagi, M., Yang, B., Zaprudin, B. and Zegmott, T. (2017). The 67P/Churyumov–Gerasimenko observation campaign in support of the Rosetta mission. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences [Online] 375:20160249. Available at: https://doi.org/10.1098/rsta.2016.0249.
    We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov–Gerasimenko from before Rosetta's arrival until nearly the end of the mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively ‘well-behaved’ comet, typical of Jupiter family comets and with activity patterns that repeat from orbit to orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends—in this paper, we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies.
  • Kokotanekova, R., Snodgrass, C., Lacerda, P., Green, S., Lowry, S., Fernández, Y., Tubiana, C., Fitzsimmons, A. and Hsieh, H. (2017). Rotation of cometary nuclei: new light curves and an update of the ensemble properties of Jupiter-family comets. Monthly Notices of the Royal Astronomical Society [Online] 471:2974-3007. Available at: http://dx.doi.org/10.1093/mnras/stx1716.
    We report new light curves and phase functions for nine Jupiter-family comets (JFCs). They were observed in the period 2004–2015 with various ground telescopes as part of the Survey of Ensemble Physical Properties of Cometary Nuclei as well as during devoted observing campaigns. We add to this a review of the properties of 35 JFCs with previously published rotation properties. The photometric time series were obtained in Bessel R, Harris R and SDSS r? filters and were absolutely calibrated using stars from the Pan-STARRS survey. This specially developed method allowed us to combine data sets taken at different epochs and instruments with absolute-calibration uncertainty down to 0.02 mag. We used the resulting time series to improve the rotation periods for comets 14P/Wolf, 47P/Ashbrook–Jackson, 94P/Russell and 110P/Hartley 3 and to determine the rotation rates of comets 93P/Lovas and 162P/Siding Spring for the first time. In addition to this, we determined the phase functions for seven of the examined comets and derived geometric albedos for eight of them. We confirm the known cut-off in bulk densities at ?0.6 g cm?3 if JFCs are strengthless. Using a model for prolate ellipsoids with typical density and elongations, we conclude that none of the known JFCs requires tensile strength larger than 10–25 Pa to remain stable against rotational instabilities. We find evidence for an increasing linear phase function coefficient with increasing geometric albedo. The median linear phase function coefficient for JFCs is 0.046 mag deg?1 and the median geometric albedo is 4.2 per cent.
  • Agarwal, J., Della Corte, V., Feldman, P., Geiger, B., Merouane, S., Bertini, I., Bodewits, D., Fornasier, S., GrünE., Hasselmann, P., Hilchenbach, M., Höfner, S., Ivanovski, S., Kolokolova, L., Pajola, M., Rotundi, A., Sierks, H., Steffl, A., Thomas, N., A’Hearn, M., Barbieri, C., Barucci, M., Bertaux, J., Boudreault, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Deller, J., Feaga, L., Fischer, H., Fulle, M., Gicquel, A., Groussin, O., GüttlerC., GutiérrezP., Hofmann, M., Hornung, K., Hviid, S., Ip, W., Jorda, L., Keller, H., Kissel, J., Knollenberg, J., Koch, A., Koschny, D., Kramm, J., KührtE., KüppersM., Lamy, P., Langevin, Y., Lara, L., Lazzarin, M., Lin, Z., Lopez Moreno, J., Lowry, S., Marzari, F., Mottola, S., Naletto, G., Oklay, N., Parker, J., Rodrigo, R., Rynö, J., Shi, X., Stenzel, O., Tubiana, C., Vincent, J., Weaver, H. and Zaprudin, B. (2017). Evidence of sub-surface energy storage in comet 67P from the outburst of 2016 July 03. Monthly Notices of the Royal Astronomical Society [Online] 469:s606-s625. Available at: https://doi.org/10.1093/mnras/stx2386.
    On 3 July 2016, several instruments on board ESA’s Rosetta spacecraft detected signs of an
    outburst event on comet 67P, at a heliocentric distance of 3.32 AU from the sun, outbound
    from perihelion. We here report on the inferred properties of the ejected dust and the surface
    change at the site of the outburst. The activity coincided with the local sunrise and continued
    over a time interval of 14 – 68 minutes. It left a 10m-sized icy patch on the surface. The ejected
    material comprised refractory grains of several hundred microns in size, and sub-micron-sized
    water ice grains. The high dust mass production rate is incompatible with the free sublimation
    of crystalline water ice under solar illumination as the only acceleration process. Additional
    energy stored near the surface must have increased the gas density. We suggest a pressurized
    sub-surface gas reservoir, or the crystallization of amorphous water ice as possible causes.
  • Giacomini, L., Massironi, M., El-Maarry, M., Penasa, L., Pajola, M., Thomas, N., Lowry, S., Barbieri, C., Cremonese, G., Ferri, F., Naletto, G., Bertini, I., La Forgia, F., Lazzarin, M., Marzari, F., Sierks, H., Lamy, P., Rodrigo, R., Rickman, H., Koschny, D., Keller, H., Agarwal, J., A’Hearn, M., Auger, A., Barucci, M., Bertaux, J., Besse, S., Bodewits, D., Da Deppo, V., Davidsson, B., De Cecco, M., Debei, S., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., GüttlerC., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lopez Moreno, J., Magrin, S., Michalik, H., Oklay, N., Pommerol, A., Preusker, F., Scholten, F., Tubiana, C. and Vincent, J. (2016). Geologic mapping of the Comet 67P/Churyumov-Gerasimenko’s Northern hemisphere. Monthly Notices of the Royal Astronomical Society [Online] 462:S352-S367. Available at: http://dx.doi.org/10.1093/mnras/stw2848.
    The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS), the scientific imaging system onboard the Rosetta mission, has been acquiring images of the nucleus of the comet 67P/Churyumov-Gerasimenko since 2014 August with a resolution which allows a detailed analysis of its surface. Indeed, data reveal a complex surface morphology which is likely the expression of different processes which occurred at different times on the cometary nucleus. In order to characterize these different morphologies and better understand their distribution, we performed a geologic mapping of comet's 67P Northern hemisphere in which features have been distinguished based on their morphological, textural and stratigraphic characteristics. For this purpose, we used narrow-angle camera images acquired in 2014 August and September with a spatial scale ranging from 1.2 to 2.4 m pixel-1. Several different geologic units have been identified on the basis of their different surface textures, granulometry and morphology. Some of these units are distinctive and localized, whereas others are more common and distributed all over the Northern hemisphere. Moreover, different types of linear features have been distinguished on the basis of their morphology. Some of these lineaments have never been observed before on a comet and can offer important clues on the internal structures of the nucleus itself. The geologic mapping results presented here will allow us to better understand the processes which affected the nucleus' surface and thus the origin and evolutionary history of comet 67P/Churyumov-Gerasimenko. © 2016 The Authors.
  • Gicquel, A., Vincent, J., Agarwal, J., A’Hearn, M., Bertini, I., Bodewits, D., Sierks, H., Lin, Z., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Barucci, M., Bertaux, J., Besse, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., Deller, J., De Cecco, M., Frattin, E., El-Maarry, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., Gutiérrez-MarquezP., Guttler, C., Hofner, S., Hofmann, M., Hu, X., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., Kuhrt, E., Kuppers, M., Lara, L., Lazzarin, M., Lopez Moreno, J., Lowry, S., Marzari, F., Masoumzadeh, N., Massironi, M., Moreno, F., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Pommero, A., Preusker, F., Scholten, F., Shi, X., Thomas, N., Toth, I. and Tubiana, C. (2016). Sublimation of icy aggregates in the coma of comet 67p/churyumov-gerasimenko detected with the osiris cameras on board rosetta. Monthly Notices of the Royal Astronomical Society [Online] 462:S57-S66. Available at: http://dx.doi.org/10.1093/mnras/stw2117.
    Beginning in 2014 March, the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) cameras began capturing images of the nucleus and coma (gas and dust) of comet 67P/Churyumov-Gerasimenko using both the wide angle camera (WAC) and the narrow angle camera (NAC). The many observations taken since July of 2014 have been used to study the morphology, location, and temporal variation of the comet's dust jets. We analysed the dust monitoring observations shortly after the southern vernal equinox on 2015 May 30 and 31 with theWAC at the heliocentric distance Rh = 1.53 AU, where it is possible to observe that the jet rotates with the nucleus. We found that the decline of brightness as a function of the distance of the jet is much steeper than the background coma, which is a first indication of sublimation. We adapted a model of sublimation of icy aggregates and studied the effect as a function of the physical properties of the aggregates (composition and size). The major finding of this paper was that through the sublimation of the aggregates of dirty grains (radius a between 5 and 50 Œºm) we were able to completely reproduce the radial brightness profile of a jet beyond 4 km from the nucleus. To reproduce the data, we needed to inject a number of aggregates between 8.5 ?ó 1013 and 8.5 ?ó 1010 for a = 5 and 50 Œºm, respectively, or an initial mass of H2O ice around 22 kg. ¬© 2016 The Authors.
  • Snodgrass, C., Jehin, E., Manfroid, J., Opitom, C., Fitzsimmons, A., Tozzi, G., Faggi, S., Yang, B., Knight, M., Conn, B., Lister, T., Hainaut, O., Bramich, D., Lowry, S., RożekA., Tubiana, C. and Guilbert-Lepoutre, A. (2016). Distant activity of 67P/Churyumov-Gerasimenko in 2014: Ground-based results during the Rosetta pre-landing phase. Astronomy and Astrophysics [Online] 588:A80. Available at: http://dx.doi.org/10.1051/0004-6361/201527834.
    Context. As the ESA Rosetta mission approached, orbited, and sent a lander to comet 67P/Churyumov-Gerasimenko in 2014, a large campaign of ground-based observations also followed the comet. Aims. We constrain the total activity level of the comet by photometry and spectroscopy to place Rosetta results in context and to understand the large-scale structure of the comet's coma pre-perihelion. Methods. We performed observations using a number of telescopes, but concentrate on results from the 8 m VLT and Gemini South telescopes in Chile. We use R-band imaging to measure the dust coma contribution to the comet's brightness and UV-visible spectroscopy to search for gas emissions, primarily using VLT/FORS. In addition we imaged the comet in near-infrared wavelengths (JHK) in late 2014 with Gemini-S/Flamingos-2. Results. We find that the comet was already active in early 2014 at heliocentric distances beyond 4 au. The evolution of the total activity (measured by dust) followed previous predictions. No gas emissions were detected despite sensitive searches. Conclusions. The comet maintains a similar level of activity from orbit to orbit, and is in that sense predictable, meaning that Rosetta results correspond to typical behaviour for this comet. The gas production (for CN at least) is highly asymmetric with respect to perihelion, as our upper limits are below the measured production rates for similar distances post-perihelion in previous orbits. © ESO, 2016.
  • Lin, Z., Lai, I., Su, C., Ip, W., Lee, J., Wu, J., Vincent, J., La Forgia, F., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’HearnM., Barucci, M., Bertaux, J., Bertini, I., Bodewits, D., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., GüttlerC., Hviid, S., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lazzarin, M., López-MorenoJ., Lowry, S., Marzari, F., Michalik, H., Mottola, S., Naletto, G., Oklay, N., Pajola, M., RożekA., Thomas, N. and Tubiana, C. (2016). Observations and analysis of a curved jet in the coma of comet 67P/Churyumov-Gerasimenko. Astronomy & Astrophysics [Online] 588:L3. Available at: http://doi.org/10.1051/0004-6361/201527784.
    Aims. We analyze the physical properties and dynamical origin of a curved jet of comet 67P/Churyumov-Gerasimenko that was observed repeatedly in several nucleus rotations starting on May 30 and persisting until early August, 2015.
    Methods. We simulated the motion of dust grains ejected from the nucleus surface under the influence of the gravity and viscous drag effect of the expanding gas flow from the rotating nucleus.
    Results. The formation of the curved jet is a combination of the size of the dust particles (~0.1?1 mm) and the location of the source region near the nucleus equator. This enhances the spiral feature of the collimated dust stream after the dust is accelerated to a terminal speed on the order of m s-1.
  • Vincent, J., Oklay, N., Pajola, M., Höfner, S., Sierks, H., Hu, X., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., A’HearnM., Barucci, M., Bertaux, J., Bertini, I., Besse, S., Bodewits, D., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., El-Maarry, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., Gutiérrez-MarquezP., GüttlerC., Hofmann, M., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lazzarin, M., Lin, Z., Lopez Moreno, J., Lowry, S., Marzari, F., Massironi, M., Moreno, F., Mottola, S., Naletto, G., Preusker, F., Scholten, F., Shi, X., Thomas, N., Toth, I. and Tubiana, C. (2016). Are fractured cliffs the source of cometary dust jets? Insights from OSIRIS/Rosetta at 67P/Churyumov-Gerasimenko. Astronomy & Astrophysics [Online] 587:A14. Available at: http://doi.org/10.1051/0004-6361/201527159.
    Context. Dust jets (i.e., fuzzy collimated streams of cometary material arising from the nucleus) have been observed in situ on all comets since the Giotto mission flew by comet 1P/Halley in 1986, and yet their formation mechanism remains unknown. Several solutions have been proposed involving either specific properties of the active areas or the local topography to create and focus the gas and dust flows. While the nucleus morphology seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller jets (a few meters wide) that connect directly to the nucleus surface.
    Aims. We monitored these jets at high resolution and over several months to understand what the physical processes are that drive their formation and how this affects the surface.
    Methods. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets and linked them precisely to their sources on the nucleus.
    Results. We show here observational evidence that the northern hemisphere jets of comet 67P/Churyumov-Gerasimenko arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features and therefore of the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.
  • Ip, W., Lai, I., Lee, J., Cheng, Y., Li, Y., Lin, Z., Vincent, J., Besse, S., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’HearnM., Barucci, M., Bertaux, J., Bertini, I., Bodewits, D., Boudreault, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., El-Maarry, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., GüttlerC., Hviid, S., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., La Forgia, F., Lara, L., Lazzarin, M., López-MorenoJ., Lowry, S., Marchi, S., Marzari, F., Michalik, H., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Thomas, N., Toth, E. and Tubiana, C. (2016). Physical properties and dynamical relation of the circular depressions on comet 67P/Churyumov-Gerasimenko. Astronomy & Astrophysics [Online] 591:A132. Available at: http://doi.org/10.1051/0004-6361/201628156.
    Aims. We aim to characterize the circular depressions of comet 67P/Churyumov-Gerasimenko and investigate whether such surface morphology of a comet nucleus is related to the cumulative sublimation effect since becoming a Jupiter family comet (JFC).
    Methods. The images from the Rosetta/OSIRIS science camera experiment are used to construct size frequency distributions of the circular depression structures on comet 67P and they are compared with those of the JFCs 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2. The orbital evolutionary histories of these comets over the past 100?000 yr are analyzed statistically and compared with each other.
    Results. The global distribution of the circular depressions over the surface of 67P is charted and classified. Descriptions are given to the characteristics and cumulative size frequency distribution of the identified features. Orbital statistics of the JFCs visited by spacecraft are derived.
    Conclusions. The size frequency distribution of the circular depressions is found to have a similar power law distribution to those of 9P/Tempel 1 and 81P/Wild 2. This might imply that they could have been generated by the same process. Orbital integration calculation shows that the surface erosion histories of 81P/Wild 2, and 9P/Tempel 1 could be shorter than those of 67P, 103 P/Hartley 2 and 19P/Borrelly. From this point of view, the circular depressions could be dated back to the pre-JFC phase or the transneptunian phase of these comets. The north-south asymmetry in the distribution of the circular depressions could be associated with the heterogeneous structure of the nucleus of comet 67P and/or the solar insolation history.
  • Deller, J., Lowry, S., Snodgrass, C., Price, M. and Sierks, H. (2016). A new approach to modelling impacts on rubble pile asteroid simulants. Monthly Notices of the Royal Astronomical Society [Online] 455:3752-3762. Available at: http://dx.doi.org/10.1093/mnras/stv2584.
    Many asteroids with low bulk densities must have a rubble pile structure and internal voids. Although little is known about their internal structure, numerical simulations of impact events on these asteroids rely on assumptions on how the voids are distributed. We present a new approach to model impacts on rubble pile asteroids that explicitly takes into account their internal structure. The formation of the asteroid is modelled as a rubble pile aggregate of spherical pebbles of different sizes. This aggregate is then converted into a high-resolution smoothed particle hydrodynamics (SPH) model, accounting for macroporosity inside the pebbles. We compare impact-event outcomes for a large set of internal configurations to explore the parameter space of our model-building process. The analysis of the fragment size distribution and the disruption threshold quantifies the specific influence of each input parameter. The size distribution of the pebbles used in our model is a simple power law, containing three free parameters: the slope ?, the lower cut-off radius rmin and the upper cut- off radius rmax. The influence of all three parameters on the outcome is assessed in this paper. The existence of void space in our model increases the resistance against collisional disruption, a behaviour previously reported based on numerical simulations using a continuum description of porous material (Holsapple 2009). We show, for a set of asteroid collisions typical for small asteroids in the main belt, that no a priori knowledge of the exact size distribution of the pebbles inside the asteroid is needed, as the choice of the corresponding parameters does not directly correlate with the impact outcome.
  • Groussin, O., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., A’Hearn, M., Auger, A., Barucci, M., Bertaux, J., Bertini, I., Besse, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., El-Maarry, M., Fornasier, S., Fulle, M., GutiérrezP., GüttlerC., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lara, L., Lazzarin, M., Lopez Moreno, J., Lowry, S., Marchi, S., Marzari, F., Massironi, M., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Pommerol, A., Thomas, N., Toth, I., Tubiana, C. and Vincent, J. (2015). Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics [Online] 583:(A36) 1-4. Available at: http://dx.doi.org/10.1051/0004-6361/201527020.
    Aims. We report on the first major temporal morphological changes observed on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko in the smooth terrains of the Imhotep region. Methods. We used images of the OSIRIS cameras onboard Rosetta to follow the temporal changes from 24 May 2015 to 11 July 2015. Results. The morphological changes observed on the surface are visible in the form of roundish features that are growing in size from a given location in a preferential direction at a rate of 5.6-8.1 x 10-5 m s-1 during the observational period. The location where the changes started and the contours of the expanding features are bluer than the surroundings, which suggests that ices (H2O and/or CO2) are exposed on the surface. However, sublimation of ices alone is not sufficient to explain the observed expanding features. No significant variations in the dust activity pattern are observed during the period of changes. © ESO, 2015.
  • Vincent, J., Bodewits, D., Besse, S., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’Hearn, M., Auger, A., Barucci, A., Bertaux, J., Bertini, I., Capanna, C., Cremonese, G., Da Deppo, V., Davidson, B., Debei, S., De Cecco, M., El-Maarry, M., Ferri, F., Fornasier, S., Fulle, M., Gaskell, R., Giacomini, L., Giacomini, O., Guilbert-Lepoutre, A., Gutierrez-Marques, P., GutiérrezP., GüttlerC., Hoekzem, N., Höfner, S., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, R., KührtE., KüppersM., La Forgia, F., Lara, L., Lazzarin, M., Lee, V., Leyrat, C., Lin, Z., Moreno, J., Lowry, S., Magrin, S., Maquet, L., Marchi, S., Marzari, F., Massironi, M., Michalik, H., Moissl, R., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Preusjer, F., Scholten, F., Thomas, N., Toth, I. and Tubiana, C. (2015). Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse. Nature [Online] 523:63-66. Available at: http://dx.doi.org/10.1038/nature14564.
    Pits have been observed on many cometary nuclei mapped by spacecraft. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments and models cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts. Alternative mechanisms like explosive activity have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov–Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh come- tary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface.
  • Massironi, M., Simioni, E., Marzari, F., Cremonese, G., Giacomini, L., Pajola, M., Jorda, L., Naletto, G., Lowry, S., El-Maarry, M., Preusker, F., Scholten, F., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., A’Hearn, M., Agarwal, J., Auger, A., Barucci, M., Bertaux, J., Bertini, I., Besse, S., Bodewits, D., Capanna, C., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Ferri, F., Fornasier, S., Fulle, M., Gaskell, R., Groussin, O., GutiérrezP., GüttlerC., Hviid, S., Ip, W., Knollenberg, J., Kovacs, G., Kramm, R., KührtE., KüppersM., La Forgia, F., Lara, L., Lazzarin, M., Lin, Z., Moreno, J., Magrin, S., Michalik, H., Mottola, S., Oklay, N., Pommerol, A., Thomas, N., Tubiana, C. and Vincent, J. (2015). Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature [Online] 526:402-405. Available at: http://dx.doi.org/10.1038/nature15511.
    The factors shaping cometary nuclei are still largely unknown, but could be the result of concurrent effects of evolutionary and primordial processes. The peculiar bilobed shape of comet 67P/Churyumov–Gerasimenko may be the result of the fusion of two objects that were once separate or the result of a localized excavation by outgassing at the interface between the two lobes. Here we report that the comet’s major lobe is enveloped by a nearly continuous set of strata, up to 650 metres thick, which are independent of an analogous stratified envelope on the minor lobe. Gravity vectors computed for the two lobes separately are closer to perpendicular to the strata than those calculated for the entire nucleus and adjacent to the neck separating the two lobes. Therefore comet 67P/Churyumov–Gerasimenko is an accreted body of two distinct objects with ‘onion-like’ stratification, which formed before they merged. We conclude that gentle, low-velocity collisions occurred between two fully formed kilometre-sized cometesimals in the early stages of the Solar System. The notable structural similarities between the two lobes of comet 67P/Churyumov–Gerasimenko indicate that the early-forming cometesimals experienced similar primordial stratified accretion, even though they formed independently.
  • Davidsson, B., Gutierrez, P., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’Hearn, M., Barucci, M., Bertaux, J., Bertini, I., Bodewits, D., Cremonese, G., Da Deppo, V., Debei, S., De Cecco, M., Fornasier, S., Fulle, M., Groussin, O., Guttler, C., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., Kuhrt, E., Kuppers, M., La Forgia, F., Lara, L., Lazzarin, M., Lopez Moreno, J., Lowry, S., Magrin, S., Marzari, F., Michalik, H., Moissl-Fraund, R., Naletto, G., Oklay, N., Pajola, M., Snodgrass, C., Thomas, N., Tubiana, C. and Vincent, J. (2015). Orbital elements of the material surrounding comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics [Online] 583:(A16) 1-9. Available at: http://www.dx.doi.org/10.1051/0004-6361/201525841.
    We investigate the dust coma within the Hill sphere of comet 67P/Churyumov-Gerasimenko. We aim to determine osculating orbital elements for individual distinguishable but unresolved slow-moving grains in the vicinity of the nucleus. In addition, we perform photometry and constrain grain sizes. Methods. We performed astrometry and photometry using images acquired by the OSIRIS Wide Angle Camera on the European Space Agency spacecraft Rosetta. Based on these measurements, we employed standard orbit determination and orbit improvement techniques. Results. Orbital elements and effective diameters of four grains were constrained, but we were unable to uniquely determine them. Two of the grains have light curves that indicate grain rotation. Conclusions. The four grains have diameters nominally in the range 0.14-0.50 m. For three of the grains, we found elliptic orbits, which is consistent with a cloud of bound particles around the nucleus. However, hyperbolic escape trajectories cannot be excluded for any of the grains, and for one grain this is the only known option. One grain may have originated from the surface shortly before observation. These results have possible implications for the understanding of the dispersal of the cloud of bound debris around comet nuclei, as well as for understanding the ejection of large grains far from the Sun.
  • Lara, L., Lowry, S., Vincent, J., Gutierrez, P., RożekA., La Forgia, F., Oklay, N., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., Auger, A., A’Hearn, M., Barucci, M., Bertaux, J., Bertini, I., Besse, S., Bodewits, D., Cremonese, G., Davidsson, B., Da Deppo, V., Debei, S., De Cecco, M., El-Maarry, M., Ferri, F., Fornasier, S., Fulle, M., Groussin, O., Gutiérrez-MarquesP., GüttlerC., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., Lazzarin, M., Lin, Z., López-MorenoJ., Magrin, S., Marzari, F., Michalik, H., Moissl-Fraund, R., Moreno, F., Mottola, S., Naletto, G., Pajola, M., Pommerol, A., Thomas, N., Sabau, M. and Tubiana, C. (2015). Large-scale dust jets in the coma of 67P/Churyumov-Gerasimenko as seen by the OSIRIS instrument onboard Rosetta. Astronomy and Astrophysics [Online] 583:(A9) 1-11. Available at: http://dx.doi.org/10.1051/0004-6361/201526103.
    During the most recent perihelion passage in 2009 of comet 67P/Churyumov-Gerasimenko (67P), ground-based observations showed an anisotropic dust coma where jet-like features were detected at ~ 1.3 AU from the Sun. The current perihelion passage is exceptional as the Rosetta spacecraft is monitoring the nucleus activity since March 2014, when a clear dust coma was already surrounding the nucleus at 4.3 AU from the Sun. Subsequently, the OSIRIS camera also witnessed an outburst in activity between April 27 and 30, and since mid-July, the dust coma at rh ~ 3.7-3.6 AU preperihelion is clearly non-isotropic, pointing to the existence of dust jet-like features. We aim to ascertain on the nucleus surface the origin of the dust jet-like features detected as early as in mid-July 2014. This will help to establish how the localized comet nucleus activity compares with that seen in previous apparitions and will also help following its evolution as the comet approaches its perihelion, at which phase most of the jets were detected from ground-based observations. Determining these areas also allows locating them in regions on the nucleus with spectroscopic or geomorphological distinct characteristics. Methods. Three series of dust images of comet 67P obtained with the Wide Angle Camera (WAC) of the OSIRIS instrument onboard the Rosetta spacecraft were processed with different enhancement techniques. This was made to clearly show the existence of jet-like features in the dust coma, whose appearance toward the observer changed as a result of the rotation of the comet nucleus and of the changing observing geometry from the spacecraft. The position angles of these features in the coma together with information on the observing geometry, nucleus shape, and rotation, allowed us to determine the most likely locations on the nucleus surface where the jets originate from. Results. Geometrical tracing of jet sources indicates that the activity of the nucleus of 67P gave rise during July and August 2014 to large-scale jet-like features from the Hapi, Hathor, Anuket, and Aten regions, confirming that active regions may be present on the nucleus localized at 60° northern latitude as deduced from previous comet apparitions. There are also hints that large-scale jets observed from the ground are possibly composed, at their place of origin on the nucleus surface, of numerous small-scale features. © 2015 ESO.
  • Tubiana, C., Snodgrass, C., Bertini, I., Mottola, S., Vincent, J., Lara, L., Fornasier, S., Knollenberg, J., Thomas, N., Fulle, M., Agarwal, J., Bodewits, D., Ferri, F., GüttlerC., GutiérrezP., La Forgia, F., Lowry, S., Magrin, S., Oklay, N., Pajola, M., Rodrigo, R., Sierks, H., A’hearn, M., Angrilli, F., Barbieri, C., Barucci, M., Bertaux, J., Cremonese, G., Da Deppo, V., Davidsson, B., De Cecco, M., Debei, S., Groussin, O., Hviid, S., Ip, W., Jorda, L., Keller, H., Koschny, D., Kramm, R., KührtE., KüppersM., Lazzarin, M., Lamy, P., Lopez Moreno, J., Marzari, F., Michalik, H., Naletto, G., Rickman, H., Sabau, L. and Wenzel, K. (2015). 67P/Churyumov-Gerasimenko: Activity between March and June 2014 as observed from Rosetta/OSIRIS. Astronomy and Astrophysics [Online] 573:(A62) 1-11. Available at: http://dx.doi.org/10.1051/0004-6361/201424735.
    Aims. 67P/Churyumov-Gerasimenko is the target comet of the ESA's Rosetta mission. After commissioning at the end of March 2014, the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard Rosetta, started imaging the comet and its dust environment to investigate how they change and evolve while approaching the Sun.
  • Rotundi, A., Sierks, H., Della Corte, V., Fulle, M., Gutierrez, P., Lara, L., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., López-MorenoJ., Accolla, M., Agarwal, J., A’Hearn, M., Altobelli, N., Angrilli, F., Barucci, M., Bertaux, J., Bertini, I., Bodewits, D., Bussoletti, E., Colangeli, L., Cosi, M., Cremonese, G., Crifo, J., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Esposito, F., Ferrari, M., Fornasier, S., Giovane, F., Gustafson, B., Green, S., Groussin, O., GrünE., GüttlerC., Herranz, M., Hviid, S., Ip, W., Ivanovski, S., JerónimoJ., Jorda, L., Knollenberg, J., Kramm, R., KührtE., KüppersM., Lazzarin, M., Leese, M., López-JiménezA., Lucarelli, F., Lowry, S., Marzari, F., Epifani, E., McDonnell, J., Mennella, V., Michalik, H., Molina, A., Morales, R., Moreno, F., Mottola, S., Naletto, G., Oklay, N., Ortiz, J., Palomba, E., Palumbo, P., Perrin, J., RodríguezJ., Sabau, L., Snodgrass, C., Sordini, R., Thomas, N., Tubiana, C., Vincent, J., Weissman, P., Wenzel, K., Zakharov, V. and Zarnecki, J. (2015). Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the sun. Science [Online] 347:3905-4000. Available at: https://doi.org/10.1126/science.aaa3905.
    Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency's Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10-10 to 10-7 kilograms, and 48 grains of mass 10-5 to 10-2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails. © 2015, American Association for the Advancement of Science. All rights reserved.
  • Pommerol, A., Thomas, N., El-Maarry, M., Pajola, M., Groussin, O., Auger, A., Oklay, N., Fornasier, S., Feller, C., Davidsson, B., Gracia-Berna, A., Jost, B., Marschall, R., Poch, O., Barucci, M., Bertaux, J., La Forgia, F., Keller, H., KührtE., Lowry, S., Mottola, S., Naletto, G., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Agarwal, J., A’Hearn, M., Bertini, I., Boudreault, S., Cremonese, G., Da Deppo, V., De Cecco, M., Debei, S., GüttlerC., Fulle, M., GutiérrezP., Hviid, S., Ip, W., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., Kuppers, E., Lara, L., Lazzarin, M., Lopez Moreno, J., Marzari, F., Michalik, H., Preusker, F., Scholten, F., Tubiana, C. and Vincent, J. (2015). OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments. Astronomy and Astrophysics [Online] 583:(A25) 1-16. Available at: http://dx.doi.org/10.1051/0004-6361/201525977.
    Since OSIRIS started acquiring high-resolution observations of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko, over one hundred meter-sized bright spots have been identified in numerous types of geomorphologic regions, but mostly located in areas receiving low insolation. The bright spots are either clustered, in debris fields close to decameter-high cliffs, or isolated without structural relation to the surrounding terrain. They can be up to ten times brighter than the average surface of the comet at visible wavelengths and display a significantly bluer spectrum. They do not exhibit significant changes over a period of a few weeks. All these observations are consistent with exposure of water ice at the surface of boulders produced by dislocation of the weakly consolidated layers that cover large areas of the nucleus. Laboratory experiments show that under simulated comet surface conditions, analog samples acquire a vertical stratification with an uppermost porous mantle of refractory dust overlaying a layer of hard ice formed by recondensation or sintering under the insulating dust mantle. The evolution of the visible spectrophotometric properties of samples during sublimation is consistent with the contrasts of brightness and color seen at the surface of the nucleus. Clustered bright spots are formed by the collapse of overhangs that is triggered by mass wasting of deeper layers. Isolated spots might be the result of the emission of boulders at low velocity that are redepositioned in other regions. © 2015 ESO.
  • Lin, Z., Ip, W., Lai, I., Lee, J., Vincent, J., Lara, L., Bodewits, D., Sierks, H., Barbieri, C., Lamy, P., Rodrigo, R., Koschny, D., Rickman, H., Keller, H., Agarwal, J., A’Hearn, M., Barucci, M., Bertaux, J., Bertini, I., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., GüttlerC., Hviid, S., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J., KührtE., KüppersM., La Forgia, F., Lazzarin, M., López-MorenoJ., Lowry, S., Marzari, F., Michalik, H., Mottola, S., Naletto, G., Oklay, N., Pajola, M., RożekA., Thomas, N., Liao, Y. and Tubiana, C. (2015). Morphology and dynamics of the jets of comet 67P/Churyumov-Gerasimenko: Early-phase development. Astronomy and Astrophysics [Online] 583:(A11) 1-10. Available at: http://dx.doi.org/10.1051/0004-6361/201525961.
    Aims. The OSIRIS camera onboard the Rosetta spacecraft obtained close-up views of the dust coma of comet 67P. The jet structures can be used to trace their source regions and to examine the possible effect of gas-surface interaction. Methods. We analyzed the wide-angle images obtained in the special dust observation sequences between August and September 2014. The jet features detected in different images were compared to study their time variability. The locations of the potential source regions of some of the jets are identified by ray tracing. We used a ring-masking technique to calculate the brightness distribution of dust jets along the projected distance. Results. The jets detected between August and September 2014 mostly originated in the Hapi region. Morphological changes appeared over a timescale of several days in September. The brightness slope of the dust jets is much steeper than the background coma. This might be related to the sublimation or fragmentation of the emitted dust grains. Interaction of the expanding gas flow with the cliff walls on both sides of Hapi could lead to erosion and material down-fall to the nucleus surface. © ESO, 2015.
  • Hainaut, O., Boehnhardt, H., Snodgrass, C., Meech, K., Deller, J., Gillon, M., Jehin, E., Kuehrt, E., Lowry, S., Manfroid, J., Micheli, M., Mottola, S., Opitom, C., Vincent, J. and Wainscoat, R. (2014). Continued activity in P/2013 P5 PANSTARRS: Unexpected comet, rotational break-up, or rubbing binary asteroid?. Astronomy and Astrophysics [Online] 563:(A75) 1-11. Available at: http://dx.doi.org/10.1051/0004-6361/201322864.
    The object P/2013 P5 PANSTARRS was discovered in August 2013, displaying a cometary tail, but its orbital elements indicated that it was a typical member of the inner asteroid main belt. We monitored the object from 2013 August 30 until 2013 October 05 using the CFHT 3.6 m telescope (Mauna Kea, HI), the NTT (ESO, La Silla), the CA 1.23 m telescope (Calar Alto), the Perkins 1.8m (Lowell) and the 0.6 m TRAPPIST telescope (La Silla). We measured its nuclear radius to be r ≠0.25-0.29 km, and its colours g? - r? = 0.58 ± 0.05 and r? - i? = 0.23 ± 0.06, typical for an S-class asteroid, as expected for an object in the inner asteroid belt and in the vicinity of the Flora collisional family. We failed to detect any rotational light curve with an amplitude <0.05 mag and a double-peaked rotation period <20 h. The evolution of the tail during the observations was as expected from a dust tail. A detailed Finson-Probstein analysis of deep images acquired with the NTT in early September and with the CFHT in late September indicated that the object was active since at least late January 2013 until the time of the latest observations in 2013 September, with at least two peaks of activity around 2013 June 14 ± 10 d and 2013 July 22 ± 3 d. The changes of activity level and the activity peaks were extremely sharp and short, shorter than the temporal resolution of our observations (?1 d). The dust distribution was similar during these two events, with dust grains covering at least the 1-1000 ?m range. The total mass ejected in grains <1 mm was estimated to be 3.0 × 106 kg and 2.6 × 107 kg around the two activity peaks. Rotational disruption cannot be ruled out as the cause of the dust ejection. We also propose that the components of a contact binary might gently rub and produce the observed emission. Volatile sublimation might also explain what appears as cometary activity over a period of 8 months. However, while main belt comets best explained by ice sublimation are found in the outskirts of the main belt, where water ice is believed to be able to survive buried in moderately large objects for the age of the solar system deeply, the presence of volatiles in an object smaller than 300 m in radius would be very surprising in the inner asteroid belt. © ESO, 2014.
  • Mottola, S., Lowry, S., Snodgrass, C., Lamy, P., Toth, I., RożekA., Sierks, H., A’Hearn, M., Angrilli, F., Barbieri, C., Barucci, M., Bertaux, J., Cremonese, G., Da Deppo, V., Davidsson, B., De Cecco, M., Debei, S., Fornasier, S., Fulle, M., Groussin, O., GutiérrezP., Hviid, S., Ip, W., Jorda, L., Keller, H., Knollenberg, J., Koschny, D., Kramm, R., KührtE., KüppersM., Lara, L., Lazzarin, M., Lopez Moreno, J., Marzari, F., Michalik, H., Naletto, G., Rickman, H., Rodrigo, R., Sabau, L., Thomas, N., Wenzel, K., Agarwal, J., Bertini, I., Ferri, F., GüttlerC., Magrin, S., Oklay, N., Tubiana, C. and Vincent, J. (2014). The rotation state of 67P/Churyumov-Gerasimenko from approach observations with the OSIRIS cameras on Rosetta. Astronomy and Astrophysics [Online] 569:(L2) 1-5. Available at: http://dx.doi.org/10.1051/0004-6361/201424590.
    Aims. Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67P's nucleus. Methods. Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations. Results. Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque. We find that the sidereal rotation periods P1 = 12.76129 ± 0.00005 h and P2 = 12.4043 ± 0.0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates ? = 65° ± 15°, ? = + 59° ± 15°, corresponding to equatorial coordinates RA = 22°, Dec = + 76°. However, we find that the mirror solution, also prograde, at ? = 275° ± 15°, ? = + 50° ± 15° (or RA = 274°, Dec = + 27°), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane. © 2014 ESO.
  • Wolters, S., Weissman, P., Christou, A., Duddy, S. and Lowry, S. (2014). Spectral similarity of unbound asteroid pairs. Monthly Notices of the Royal Astronomical Society [Online] 439:3085-3093. Available at: http://dx.doi.org/10.1093/mnras/stu171.
    Infrared (IR) spectroscopy between 0.8 and 2.5 μ has been obtained for both components of three unbound asteroid pairs, using theNASA Infrared Telescope Facility with the SpeX instrument. Pair primary (2110) Moore-Sitterly is classified as an S-type following the Bus-DeMeo taxonomy; the classification for secondary (44612) 1999 RP27 is ambiguous: S/Sq/Q/K/Ltype. Primary (10484) Hecht and secondary (44645) 1999 RC118 are classified as V-types. IR spectra for Moore-Sitterly and Hecht are each linked with available visual photometry. The classifications for primary (88604) 2001 QH293 and (60546) 2000 EE85 are ambiguous: S/Sq/Q/K/L-type. Subtle spectral differences between them suggest that the primary may have more weathered material on its surface. Dynamical integrations have constrained the ages of formation: 2110-44612 > 782 kyr; 10484-44645 = 348 (+823,-225) kyr; 88604-60546 = 925 (+842,-754) kyr. The spectral similarity of seven complete pairs is ranked in comparison with nearby background asteroids. Two pairs, 17198-229056 and 19289-278067, have significantly different spectra between the components, compared to the similarity of spectra in the background population. The other pairs are closer than typical, supporting an interpretation of each pair's formation from a common parent body. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
  • Lowry, S., Weissman, P., Duddy, S., Rozitis, B., Fitzsimmons, A., Green, S., Hicks, M., Snodgrass, C., Wolters, S., Chesley, S., Pittichová, J. and Van Oers, P. (2014). The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up. Astronomy and Astrophysics [Online] 562:(A48) 1-9. Available at: http://dx.doi.org/10.1051/0004-6361/201322602.
    Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa's spin rate of dÏ?/dt = (3.54 ± 0.38) Ã? 10-8 rad day-2, equivalent to a decrease of its rotation period of ~45 ms year-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m-3 and 2850 ± 500 kg m-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this. © ESO, 2014.
  • Guilbert-Lepoutre, A., Schulz, R., RożekA., Lowry, S., Tozzi, G. and StüweJ. (2014). Pre-perihelion activity of comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics [Online] 567:(L2) 1-4. Available at: http://dx.doi.org/10.1051/0004-6361/201424186.
    Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system. Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008. Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU. Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion. © 2014 ESO.
  • Meech, K., Kleyna, J., Hainaut, O., Lowry, S., Fuse, T., A’Hearn, M., Chesley, S., Yeomans, D., Fernández, Y., Lisse, C., Reach, W., Bauer, J., Mainzer, A., Pittichová, J., Christensen, E., Osip, D., Brink, T., Mateo, M., Motta, V., Challis, P., Holman, M. and Ferrin, I. (2013). The demise of Comet 85P/Boethin, the first EPOXI mission target. Icarus [Online] 222:662-678. Available at: http://dx.doi.org/10.1016/j.icarus.2012.09.002.
    Comet 85P/Boethin was selected as the original comet target for the Deep Impact extended mission, EPOXI. Because this comet had been only observed at two apparitions in 1975 and 1986 and consequently had a large ephemeris error, an early intense recovery effort similar to that of 1P/Halley was undertaken beginning in 2005 using the ESO Very Large Telescopes (VLTs) in a distant comet program. These were challenging observations because of the low galactic latitude, and an error ellipse (the line of variations) that was larger than the CCD FOV, and the comet was not seen. Dedicated recovery observing time was awarded on the Subaru telescope in April and May 2006, and June 2007, in addition to time on the VLT and Canada–France–Hawaii telescopes during July–August 2007 with wide field mosaics and mosaicing techniques. The limiting V magnitudes from these observing runs ranged between 25.7 and 27.3 and again the comet was not seen in the individual nights. A new image processing technique was developed to stack images over extended runs and runs after distorting them to account for dilations and rotations in the line of variations using modifications of the world coordinate system. A candidate at V ? 27.3 was found in the CFHT data along the LOV, 2.5? west of the nominal ephemeris position. The EPOXI mission was unwilling to re-target the spacecraft without a confirmation. Additional time was secured using the Spitzer Space Telescope, the Gemini South 8-m telescope, the Clay and Baade (Magellan 6.5 m), CTIO 4 m, and SOAR 4 m telescopes during 2007 September and October A composite image made by stacking the new data showed no plausible candidate nucleus to a limiting magnitude of V = 28.5, corresponding to a nucleus radius between 0.1 and 0.2 km (assuming an albedo of 0.04). The comet was declared lost, presumably having disintegrated. Searches in the WISE data set revealed no debris trail, but no constraints on the possible time of disruption can be made. NASA approved the trajectory correction maneuver to go to Comet 103P/Hartley 2 on 2007 November 1. Many observers searched for the comet as it came to its December 2008 perihelion, but no trace of the nucleus was found.

    Based on observations collected at the Very Large Telescope, Chile, in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, in part using data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile, in part on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii, in part using data collected at the Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which are operated by the Association for Research in Astronomy, under contract with the National Science Foundation, and in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). This work is also based in part on observations taken with the Spitzer Space Telescope, which is operated by JPL/Caltech under a contract with NASA. © 2012 Elsevier Inc.
  • Fernández, Y., Kelley, M., Lamy, P., Toth, I., Groussin, O., Lisse, C., A’Hearn, M., Bauer, J., Campins, H., Fitzsimmons, A., Licandro, J., Lowry, S., Meech, K., Pittichová, J., Reach, W., Snodgrass, C. and Weaver, H. (2013). Thermal properties, sizes, and size distribution of jupiter-family cometary nuclei. Icarus [Online] 226:1138-1170. Available at: http://dx.doi.org/10.1016/j.icarus.2013.07.021.
    We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4-5. AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03. ±. 0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3. km radius, and even for comets that approach to ~2. AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ~2. km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around -1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3-6. km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects. © 2013 Elsevier Inc.
  • Rozitis, B., Duddy, S., Green, S. and Lowry, S. (2013). A thermophysical analysis of the (1862) Apollo Yarkovsky and YORP effects. Astronomy and Astrophysics [Online] 555:(A20) 1-12. Available at: http://dx.doi.org/10.1051/0004-6361/201321659.
    Context. The Yarkovsky effect, which causes orbital drift, and the YORP effect, which causes changes in rotation rate and pole orientation, play important roles in the dynamical and physical evolution of asteroids. Near-Earth asteroid (1862) Apollo has strong detections of both orbital semimajor axis drift and rotational acceleration. Aims. We produce a unified model that can accurately match both observed effects using a single set of thermophysical properties derived from ground-based observations, and we determine Apollo's long term evolution. Methods. We use light-curve shape inversion techniques and the advanced thermophysical model (ATPM) on published light-curve, thermal-infrared, and radar observations to constrain Apollo's thermophysical properties. The derived properties are used to make detailed predictions of Apollo's Yarkovsky and YORP effects, which are then compared with published measurements of orbital drift and rotational acceleration. The ATPM explicitly incorporates 1D heat conduction, shadowing, multiple scattering of sunlight, global self-heating, and rough surface thermal-infrared beaming in the model predictions. Results. We find that ATPM can accurately reproduce the light-curve, thermal-infrared, and radar observations of Apollo, and simultaneously match the observed orbital drift and rotational acceleration using: a shape model with axis ratios of 1.94:1.65:1.00, an effective diameter of 1.55 ± 0.07 km, a geometric albedo of 0.20 ± 0.02, a thermal inertia of 140-100 +140140-100+140 J m-2 K -1 s-1/2, a highly rough surface, and a bulk density of 2850-680 +480+480-680 kg m-3. Using these properties we predict that Apollo's obliquity is increasing towards the 180 YORP asymptotic state at a rate of 1.5 -0.5 +0.3 +0.3-0.5 degrees per 105 yr. Conclusions. The derived thermal inertia suggests that Apollo has loose regolith material resting on its surface, which is consistent with Apollo undergoing a recent resurfacing event based on its observed Q-type spectrum. The inferred bulk density is consistent with those determined for other S-type asteroids, and suggests that Apollo has a fractured interior. The YORP effect is acting on a much faster timescale than the Yarkovsky effect and will dominate Apollo's long term evolution. The ATPM can readily be applied to other asteroids with similar observational data sets. © 2013 ESO.
  • Kelley, M., Fernández, Y., Licandro, J., Lisse, C., Reach, W., A’Hearn, M., Bauer, J., Campins, H., Fitzsimmons, A., Groussin, O., Lamy, P., Lowry, S., Meech, K., Pittichová, J., Snodgrass, C., Toth, I. and Weaver, H. (2013). The persistent activity of Jupiter-family comets at 3-7AU. Icarus [Online] 225:475-494. Available at: http://dx.doi.org/10.1016/j.icarus.2013.04.012.
    We present an analysis of comet activity based on the Spitzer Space Telescope component of the Survey of the Ensemble Physical Properties of Cometary Nuclei. We show that the survey is well suited to measuring the activity of Jupiter-family comets at 3-7. AU from the Sun. Dust was detected in 33 of 89 targets (37. ±. 6%), and we conclude that 21 comets (24. ±. 5%) have morphologies that suggest ongoing or recent cometary activity. Our dust detections are sensitivity limited, therefore our measured activity rate is necessarily a lower limit. All comets with small perihelion distances (q<. 1.8. AU) are inactive in our survey, and the active comets in our sample are strongly biased to post-perihelion epochs. We introduce the quantity {small element of}f?, intended to be a thermal emission counterpart to the often reported Af?, and find that the comets with large perihelion distances likely have greater dust production rates than other comets in our survey at 3-7. AU from the Sun, indicating a bias in the discovered Jupiter-family comet population. By examining the orbital history of our survey sample, we suggest that comets perturbed to smaller perihelion distances in the past 150. yr are more likely to be active, but more study on this effect is needed. © 2013 Elsevier Inc.
  • Duddy, S., Lowry, S., Christou, A., Wolters, S., Rozitis, R. and Weissman, P. (2013). Spectroscopic observations of unbound asteroid pairs using the WHT. Monthly Notices of the Royal Astronomical Society [Online] 429:63-74. Available at: http://dx.doi.org/10.1093/mnras/sts309.
    Recently over 62 pairs of asteroids have been shown to have very similar orbital elements. Backward integration of their orbits indicates that the asteroids in each pair likely had very close encounters at low relative velocities, consistent with models of the spin-up and rotational fission of asteroids. Although linked dynamically, the observation of highly similar spectra would suggest that the asteroids share a common composition, which we would expect if they formed from a common parent body. We have begun an observational campaign whose aim is to obtain optical and/or NIR spectra of a large sample of these unbound asteroid pairs to determine whether the asteroids in each pair exhibit similar spectra. We present optical spectroscopic observations of four complete pairs obtained using the William Herschel Telescope. We find that the components of pairs 1979-13732 and 19289-278067 share very similar spectra and likely have a common origin. Our current spectra of 17198-229056 are sufficiently different to suggest that they do not have a common origin, although this is contrary to the strong dynamical linkup of these asteroids demonstrated in the current paper and previous studies. Further observations of this pair are encouraged to examine why the spectra are so different. It is unclear whether the spectra of the final pair, 11842-228747, are a match due to the low S/N of the secondary's spectrum. We discuss the process of space weathering and present newdynamical analyses which confirm the previously estimated ages of the observed pairs. The time-scale for space weathering appears to be longer than 1 Myr for at least some pairs.We also present an efficient method which can be used to determine the positional convergence of unbound asteroid pairs ©2012 The Authors.
  • Lowry, S., Duddy, S., Rozitis, B., Green, S., Fitzsimmons, A., Snodgrass, C., Hsieh, H. and Hainaut, O. (2012). The nucleus of Comet 67P/Churyumov-Gerasimenko: A new shape model and thermophysical analysis. Astronomy and Astrophysics [Online] 548:(A12) 1-15. Available at: http://dx.doi.org/10.1051/0004-6361/201220116.
    Context. Comet 67P/Churyumov-Gerasimenko is the target of the European Space Agency Rosetta spacecraft rendez-vous mission. Detailed physical characteristation of the comet before arrival is important for mission planning as well as providing a test bed for ground-based observing and data-analysis methods. Aims. To conduct a long-term observational programme to characterize the physical properties of the nucleus of the comet, via ground-based optical photometry, and to combine our new data with all available nucleus data from the literature. Methods. We applied aperture photometry techniques on our imaging data and combined the extracted rotational lightcurves with data from the literature. Optical lightcurve inversion techniques were applied to constrain the spin state of the nucleus and its broad shape. We performed a detailed surface thermal analysis with the shape model and optical photometry by incorporating both into the new Advanced Thermophysical Model (ATPM), along with all available Spitzer 8-24 ?m thermal-IR flux measurements from the literature. Results. A convex triangular-facet shape model was determined with axial ratios b/a = 1.239 and c/a = 0.819. These values can vary by as much as 7% in each axis and still result in a statistically significant fit to the observational data. Our best spin state solution has Psid = 12.76137 ± 0.00006 h, and a rotational pole orientated at Ecliptic coordinates ? = 78°(±10°), ? = + 58°(±10°). The nucleus phase darkening behaviour was measured and best characterized using the IAU HG system. Best fit parameters are: G = 0.11 ± 0.12 and H R(1,1,0) = 15.31 ± 0.07. Our shape model combined with the ATPM can satisfactorily reconcile all optical and thermal-IR data, with the fit to the Spitzer 24 ?m data taken in February 2004 being exceptionally good. We derive a range of mutually-consistent physical parameters for each thermal-IR data set, including effective radius, geometric albedo, surface thermal inertia and roughness fraction. Conclusions. The overall nucleus dimensions are well constrained and strongly imply a broad nucleus shape more akin to comet 9P/Tempel 1, rather than the highly elongated or "bi-lobed" nuclei seen for comets 103P/Hartley 2 or 8P/Tuttle. The derived low thermal inertia of <15 J m-2 K-1 s-1/2 is comparable with that measured for other comets scaled to similar heliocentric distances, and implies a surface regolith finer than lunar surface material. © 2012 ESO.
  • Bertini, I., Sabolo, W., Gutierrez, P., Marzari, F., Snodgrass, C., Tubiana, C., Moissl, R., Pajola, M., Lowry, S., Barbieri, C., Ferri, F., Davidsson, B. and Sierks, H. (2012). Search for satellites near (21) Lutetia using OSIRIS/Rosetta images. Planetary and Space Science [Online] 66:64-70. Available at: http://dx.doi.org/10.1016/j.pss.2011.12.022.
    On 2010 July 10 the ESA Rosetta mission flew by the large asteroid (21) Lutetia. One of the scientific goals of the onboard OSIRIS instrument was the search for satellites of the asteroid, with more than 20 images specifically dedicated to this topic. An observational campaign was devised with a selection of filters and exposure times tailored to maximize the possibility of detecting small companions and determining their bound orbits. Data were analyzed with suitable methods to remove cosmic ray hits and known background objects, in order to search for persistent detections of potential interesting flux sources. We found no unambiguous detections of a satellite larger than ?160m inside the entire sphere of gravitational influence. Our search confirmed the absence of bound companions larger than â?¼30m inside 20 primary radii. These limits are a factor of ?30 smaller than the values reported so far from large ground-based telescopes using adaptive optics and from the Hubble Space Telescope. © 2011 Elsevier Ltd.
  • Hsieh, H., Yang, B., Haghighipour, N., Kaluna, H., Fitzsimmons, A., Denneau, L., Novakovia, B., Jedicke, R., Wainscoat, R., Armstrong, J., Duddy, S., Lowry, S., Trujillo, C., Micheli, M., Keane, J., Urban, L., Riesen, T., Meech, K., Abe, S., Cheng, Y., Chen, W., Granvik, M., Grav, T., Ip, W., Kinoshita, D., Kleyna, J., Lacerda, P., Lister, T., Milani, A., Tholen, D., VereÅ¡, P., Lisse, C., Kelley, M., Fernández, Y., Bhatt, B., Sahu, D., Kaiser, N., Chambers, K., Hodapp, K., Magnier, E., Price, P. and Tonry, J. (2012). Discovery of main-belt comet P/2006 VW139 by Pan-STARRS1. Astrophysical Journal Letters [Online] 748:(L15) 1-7. Available at: http://dx.doi.org/10.1088/2041-8205/748/1/L15.
    The main-belt asteroid (300163) 2006 VW139 (later designated P/2006 VW139) was discovered to exhibit comet-like activity by the Pan-STARRS1 (PS1) survey telescope using automated point-spread-function analyses performed by PS1's Moving Object Processing System. Deep follow-up observations show both a short (~10'') antisolar dust tail and a longer (~60'') dust trail aligned with the object's orbit plane, similar to the morphology observed for another main-belt comet (MBC), P/2010 R2 (La Sagra), and other well-established comets, implying the action of a long-lived, sublimation-driven emission event. Photometry showing the brightness of the near-nucleus coma remaining constant over ~30 days provides further evidence for this object's cometary nature, suggesting it is in fact an MBC, and not a disrupted asteroid. A spectroscopic search for CN emission was unsuccessful, though we find an upper limit CN production rate of Q CN < 1.3 × 1024 mol s–1, from which we infer a water production rate of $Q_{\rm H_2O}<10^{26}$ mol s–1. We also find an approximately linear optical spectral slope of 7.2%/1000 Å, similar to other cometary dust comae. Numerical simulations indicate that P/2006 VW139 is dynamically stable for >100 Myr, while a search for a potential asteroid family around the object reveals a cluster of 24 asteroids within a cutoff distance of 68 m s–1. At 70 m s–1, this cluster merges with the Themis family, suggesting that it could be similar to the Beagle family to which another MBC, 133P/Elst-Pizarro, belongs. © 2012. The American Astronomical Society. All rights reserved.
  • Duddy, S., Lowry, S., Wolters, S., Christou, A., Weissman, P., Green, S. and Rozitis, B. (2012). Physical and dynamical characterisation of the unbound asteroid pair 7343-154634. Astronomy and Astrophysics [Online] 539:(A36) 1-11. Available at: http://dx.doi.org/10.1051/0004-6361/201118302.
    Context. Models have shown that asteroids can undergo fission if their rate of rotation is steadily increased. The forces acting to pull the asteroid apart exceed the material strength and gravitational force holding the asteroid together and material can escape from the surface of the asteroid. Initially forming a binary asteroid system, the components are capable of decoupling at low relative velocity from their mutual orbit if their mass ratio is less than 0.2. A number of asteroids with very similar orbital elements have been shown to have had very recent (<1 Myr) encounters at distances smaller than the Hill sphere radius of the larger of the asteroids. The mass ratio of the asteroids in each pair is estimated to be less than 0.2, suggesting that these unbound pairs are the result of rotational fission.
    Aims. We determine whether the asteroids in one such unbound pair, (7343) Ockeghem and (154 634) 2003 XX28, share a common composition, indicative of asteroids formed from a common parent and further constrain a likely formation age for this pair.
    Methods. We have obtained spectroscopic observations of each asteroid covering the wavelength range 0.45 to 1.0 microns. Using thermal observations we have measured the size and albedo of (7343) Ockeghem. Combined with optical lightcurve data of both asteroids, we have constrained the size and density of the asteroids and estimated the strength of the Yarkovsky force experienced by both. This improved physical information has been used in new dynamical simulations of the asteroids' orbits to better constrain a formation time of this pair.
    Results. We find that the asteroids have very similar spectra consistent with an S-type taxonomy. The geometric albedo of (7343) Ockeghem, 0.20 ± 0.06 is consistent with this classification. The mass ratio range of the asteroids assuming an equal density, 0.007 to 0.065, is consistent with models of unbound asteroid pair formation. A new dynamical analysis has indicated that an absolute lower limit for the age of this pair is 400 kyr with a more likely age around 560 kyr, lower than a previous estimate of 800 kyr. © 2012 ESO.
  • Wolters, S., Rozitis, B., Duddy, S., Lowry, S., Green, S., Snodgrass, C., Hainaut, O. and Weissman, P. (2011). Physical characterization of low delta-V asteroid (175706) 1996 FG3. Monthly Notices of the Royal Astronomical Society [Online] 418:1246-1257. Available at: http://dx.doi.org/10.1111/j.1365-2966.2011.19575.x.
    Asteroid (175706) 1996 FG3 is a binary asteroid and the baseline target for the proposed MarcoPolo-R sample return mission. We present thermal-infrared photometry obtained with the European Southern Observatory (ESO) Very Large Telescope using the VISIR instrument, together with optical photometry obtained with the ESO New Technology Telescope using the EFOSC2 instrument. An absolute visual magnitude HV= 17.833 ± 0.024 and phase parameter G=-0.041 ± 0.005 are derived. The near-Earth asteroid thermal model has been fitted to the measured fluxes to derive a geometric visual albedo ?v= 0.046 p± 0.014, effective diameter at the observed aspect Deff= 1.68 ± 0.25 km and beaming parameter ? = 1.15 for phase angle ?±= 117. The advanced thermophysical model (ATPM) has been fitted to the measured fluxes to derive a more accurate effective diameter Deff= 1.71 ± 0.07km and albedo ?v= 0.044 ± 0.004. Based on the ATPM results, assuming the same albedo for primary and secondary, we derive a primary mean spherical diameter D?= 1.69+0.18 - 0.12km, secondary diameter Ds= 0.51 ± 0.03 km and a secondary orbital semimajor axis a= 2.8+1.7 -0.7km. A low surface thermal inertia ?= 120 ± 50Jm-2s-1/2K-1 was also derived, suggesting a dusty surface and raising questions as to the binary formation mechanism of this asteroid. These physical properties are used to predict a Yarkovsky drift in semimajor axis of -60+31 -45myr-1. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Research report (external)

  • Arridge, C., Downes, H., Forsyth, B., Hood, A., Joy, K., Lowry, S., Matthews, C., Calcutt, S. and Wilson, C. (2015). Roadmap for Solar System Research July 2015. [Online]. Science and Technology Facilities Council. Available at: http://www.stfc.ac.uk/files/stfc-ssap-roadmap-20150706/.
  • O’Brien, P., Arridge, C., Lowry, S., Nelson, R., Pollaco, D., Sing, D., Tinetti, G. and Watson, C. (2015). Science and Technology Facilities Council Exoplanets Science Review Panel Report. [Online]. UK Science and Technology Facilities Council. Available at: http://www.stfc.ac.uk/about-us/how-we-are-governed/advisory-boards-panels-committees/exoplanets-uk-research-review/.
  • Jones, G., Lowry, S. and The Castalia Science Team, . (2013). Main Belt Comets - A New Class of Solar System Object. [Online]. European Space Agency - Cosmic Visions: White Papers submitted in response to ESA’s Call for Science Themes for the L2 and L3 Missions. Available at: http://sci.esa.int/cosmic-vision/52030-white-papers-submitted-in-response-to-esas-call-for-science-themes-for-the-l2-and-l3-missions/.
  • Grady, M., Arridge, C., Calcutt, S., Davis, C., Downes, H., Forsyth, B., Hood, A., Jain, R. and Lowry, S. (2012). Roadmap for Solar System Research. [Online]. UK Science and Technology Facilities Council. Available at: http://www.stfc.ac.uk/about-us/how-we-are-governed/advisory-boards-panels-committees/solar-system-advisory-panel/.

Thesis

  • RożekA. (2017). Understanding the Influence of Non-Gravitational Forces on the Physical Evolution of Near-Earth Asteroids and Comets.
    Near-Earth asteroids are the small rocky bodies orbiting the Sun in the vicinity of Earth's orbit. They are remnants of the planetesimals formed in the young Solar System, which repeatedly collided and underwent disruption. They form loosely-bound aggregates dubbed 'rubble piles'. Their dynamical and physical evolution is expected to be affected by a nongravitational torque called the YORP effect.

    The YORP effect is a torque due to the anisotropic emission of thermal photons on minor bodies in the Solar System. For small asteroids the radiation recoil torques can systematically modify rotational rates or shift spin axis orientations (Rubincam, 2000). The effect is crucial to understanding the dynamical and physical evolution of near-Earth asteroids, like the alignment of spin-axes (Slivan, 2002), the peculiar spin-top shapes observed for a few targets (Ostro et al., 2006; Scheeres et al., 2006), or rotational fission and evolution of asteroid binaries (Walsh et al., 2008; Pravec et al., 2010; Jacobson and Scheeres, 2011; Jacobson et al., 2016). The first direct detection of the asteroidal YORP effect on asteroid (54509) 2000 PH5 was possible thanks to the combination of radar and photometric lightcurve observations (Lowry et al., 2007; Taylor et al., 2007). Since then, YORP spin-up has also been detected on several other asteroids. However, the sample is still very small, and further observational data is needed to refine the YORP theories.

    The asteroids (1917) Cuyo and (85990) 1999 JV6, discussed here, were selected from a sample of nearly 40 YORP-detection candidates that were monitored photometrically, and in infra-red, through an ESO Large Programme (ESO LP) led by S. C. Lowry at the ESO New Technology Telescope and Very Large Telescope telescopes, and at other facilities with associated programmes.

    The ESO LP has been used to acquire photometric lightcurves of the asteroid (1917) Cuyo spanning the period between 2010 and 2013, which, combined with the 1989-2008 archive lightcurves, should provide a large enough time-base to constrain a possible YORP strength. However, the distribution of observations in time results in effectively having observations from just two epochs. This produces potential YORP values in the range of ?0.7 × 10?8 rad/d² (radians per day squared) and 1.5 × 10?8 rad/d². The rotation pole of the object is most likely located at l = 46°, b = -62°. The sidereal period was refined relative to earlier lightcurve estimates, to be (2.6897642 ± 0.0000035) h (hours). The shape of the object suggests the presence of an 'equatorial bulge', typical for an evolved system close to shedding mass due to fast rotation.

    For asteroid (85990) 1999 JV6, the data in the ESO LP span the period between 2007 and 2016. Additionally, the author has secured radar spectra and imaging observations with Arecibo and Goldstone planetary radars. Having radar observations permitted additional constraints on the shape and spin-state, but YORP spin-up was not detected. The asteroid is shown to have a bi-lobed shape, likely a result of two ellipsoidal components collapsing onto each other. The smaller lobe is close to spherical and has diameters (345 ± 9) m, (281 ± 8) m and (291 ± 9)m, and the larger is more elongated, with (580 ± 10) m, (322 ± 5) m and (332 ± 7) m. The rotation pole resides at negative latitudes in a circle of a 10° radius, close to the southern pole of the celestial sphere. The refined sidereal rotation period is (6.536787 ± 0.000006) h. No YORP-induced change in period was detected using the phase offset measurement using the radar model, however the global lightcurve-only analysis shows the object could be experiencing a spin-up of up to 7 × 10?8 rad/d².

    The shapes and spin-states developed here were used in further studies, beyond the scope of this thesis. Combined with the infra red observations the outcome of this work was used for thermophysical analysis by ESO LP collaborator B. Rozitis to constrain physical properties of both targets. The shape and rotation state of (1917) Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit. The detection of non-gravitational acceleration in the orbital motion of the asteroid (85990) 1999 JV6 combined with thermophysical modelling suggest a low, cometary-like density.

    The shape modelling and spin-state analysis tools were also applied to a Jupiter family comet, and the Rosetta mission target, 67P/Churyumov Gerasimenko. The author contributed to the confirmation of the seminal measurement of spin-rate change between previous perihelion approach and the arrival of Rosetta (Mottola et al., 2014, incl. A. Rozek). The detected 20 min decrease in the sidereal period, from ?12.7 h to ?12.4 h, was later linked to cometary activity (Keller et al., 2015b; Bertaux, 2015). Tools were also developed to assess the mean insolation of the comet’s surface, useful in calculations of nucleus dust production rates (Guilbert-Lepoutre et al., 2014, incl. A. Rozek), establish jet-activity source regions on the surface of the nucleus (Lara et al., 2015; Lin et al., 2015, 2016, incl. A. Rozek), and calibrate ground-based photometry using the spacecraft shape model (Snodgrass et al., 2016, incl. A. Rozek).
  • Deller, J. (2015). Hyper-Velocity Impacts on Rubble Pile Asteroids.
    Most asteroids in the size range of approximately 100m to 100km are rubble piles, aggregates of rocky material held together mainly by gravitational forces, and only weak cohesion. They contain high macroporosities, indicating a large amount of void space in their interiors. How these voids are distributed is not yet known, as in-situ measurements are still outstanding.

    In this work, a model to create rubble pile asteroid simulants for use in SPH impact simulations is presented. Rubble pile asteroids are modelled as gravitational re-aggregating remnant fragments of a catastrophically disrupted parent body, which are represented by spherical pebbles. It is shown that this approach allows to explicitly follow the internal restructuring of rubble pile asteroids during impact events, while preserving the expected properties of the bulk asteroid as known from observations and experiments. The bulk behaviour of asteroid simulants, as characterized by the stability against disruption and fragment size distribution, follows the expected behaviour and is not sensitive to the exact distribution of voids in the interior structure, but rather to the void fraction as the amount of consolidated void space in between the constituent fragment pebbles. No exact a priory knowledge of the fragment size distribution inside the body is therefore needed to use this model in impact simulations.

    Modelling the behaviour of the large-scale rubble pile constituents during impact events is used as a tool to infer the internal structure of asteroids by linking surface features like hills or pits to the creation of sub-catastrophic craters. In this work, the small rubble pile asteroid (2867) S?teins is analysed. The flyby of the Rosetta spacecraft at S?teins has revealed several interesting features: the large crater Diamond close to the southern pole, a hill like feature almost opposite to the crater, and a catena of crater pits extending radially from the rim of the crater.

    A possible link between these two structures and the cratering event is investigated in a series of impact simulations varying the interior of a plausible shape of S?teins prior to the event that formed crater Diamond. A connection between the cratering event and the hill is shown to be highly unlikely. Therefore, the hill is most likely a remnant of the formation of S?teins. Its size therefore helps to infer the initial size distribution of fragments forming the asteroid.

    The formation of a fracture radially from the crater can be observed for rubble pile simulants with highly collimated voids. This fracture could plausibly form the catena of pits observed on S?teins. This can therefore serve as a link between observable surface features and S?teins internal structure. The interior of S?teins is most likely an aggregate of fragments that themselves are only lightly fractured, and large void spaces might be found inside the asteroid. As S?teins seems to be a good example of a YORPoid, an asteroid that has been evolved to a top-like shape by radiative forces due to the YORP effect, this gives first insights in the distribution of voids in the interior of this class of rubble pile asteroids.

Forthcoming

  • RożekA., Lowry, S., Rozitis, B., Green, S., Snodgrass, C., Weissman, P., Fitzsimmons, A., Hicks, M., Lawrence, K., Duddy, S., Wolters, S., Roberts-Borsani, G., Behrend, R. and Manzini, F. (2019). Physical model of near-Earth asteroid (1917) Cuyo from ground-based optical and thermal-IR observations. Astronomy & Astrophysics [Online]. Available at: http://dx.doi.org/10.22024/UniKent/01.02.74005.
    Context. The near-Earth asteroid (1917) Cuyo was subject to radar and lightcurve observations during a close approach in 1989, and observed up until 2008. It was selected as one of our ESO
    Large Programme targets, aimed at observational detections of the YORP effect through longterm lightcurve monitoring and physical modelling of near-Earth asteroids.

    Aims. We aimed to constrain physical properties of Cuyo: shape, spin-state, and spectroscopic & thermophysical properties of the surface.

    Methods. We acquired photometric lightcurves of Cuyo spanning the period between 2010 and 2013, which we combined with published lightcurves from 1989-2008. Our thermal-infrared observations were obtained in 2011. Rotationally-resolved optical spectroscopy data were acquired in 2011 and combined with all available published spectra to investigate any surface material variegation.

    Results. We developed a convex lightcurve-inversion shape of Cuyo that suggests the presence of an equatorial ridge, typical for an evolved system close to shedding mass due to fast rotation. We determine limits of YORP strength through lightcurve-based spin-state modelling, including both negative and positive acceleration values, between −0.7 × 10−8 rad day−2 and 1.7 × 10−8 rad day−2. Thermophysical modelling with the ATPM provides constraints on the geometric albedo, pV = 0.24±0.07, the effective diameter D eff = 3.15±0.08 km, the thermal inertia,
    Γ = 44 ± 9 J m−2 s −1/2 K −1, and a roughness fraction of 0.52 ± 0.26. This enabled a YORP strength prediction of ν = (−6.39±0.96)×10−10 rad day−2. We also see evidence of surface compositional variation.

    Conclusions. The low value of YORP predicted by means of thermophysical analysis, consistent with the results of the lightcurve study, might be due to the self-limiting properties of rotational YORP, possibly involving movement of sub-surface and surface material. This may also be consistent with the surface compositional variation that we see. The physical model of Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit.
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