Physical properties of ESA Rosetta target asteroid (21) Lutetia: Shape and flyby geometry Benoit Carry, Mikko Kaasalainen, Cedric Leyrat, William J. Merline, Jack D. Drummond, Al Conrad, Hal A. Weaver, Peter M. Tamblyn, Clark R. Chapman, Christophe Dumas, et al. To cite this version: Benoit Carry, Mikko Kaasalainen, Cedric Leyrat, William J. Merline, Jack D. Drummond, et al.. Physical properties of ESA Rosetta target asteroid (21) Lutetia: Shape and flyby geometry. Astronomy and Astrophysics - A&A, EDP Sciences, 2010, 523 (A94), 19 p. 10.1051/0004-6361/201015074. hal- 00618654 HAL Id: hal-00618654 https://hal.archives-ouvertes.fr/hal-00618654 Submitted on 14 Apr 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A&A 523, A94 (2010) Astronomy DOI: 10.1051/0004-6361/201015074 & c ESO 2010 Astrophysics Physical properties of the ESA Rosetta target asteroid (21) Lutetia II. Shape and flyby geometry, B. Carry1,2, M. Kaasalainen3,C.Leyrat1, W. J. Merline4,J.D.Drummond5,A.Conrad6,H.A.Weaver7, P. M. Tamblyn 4,C.R.Chapman4,C.Dumas8,F.Colas9, J. C. Christou10, E. Dotto11,D.Perna1,11,12, S. Fornasier1,2, L. Bernasconi13,R.Behrend14,F.Vachier9, A. Kryszczynska15 , M. Polinska15, M. Fulchignoni1,2,R.Roy16, R. Naves17, R. Poncy18, and P. Wiggins19 1 LESIA, Observatoire de Paris, 5 place Jules Janssen, 92190 Meudon, France e-mail: [email protected] 2 Université Paris 7 Denis-Diderot, 5 rue Thomas Mann, 75205 Paris Cedex, France 3 Tampere University of Technology, PO Box 553, 33101 Tampere, Finland 4 Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302, USA 5 Starfire Optical Range, Directed Energy Directorate, Air Force Research Laboratory, Kirtland AFB, NM 87117-577, USA 6 W.M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA 7 Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA 8 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile 9 IMCCE, Observatoire de Paris, 14 bvd de l’Observatoire, 75014 Paris, France 10 Gemini Observatory, Northern Operations Center, 670 N. A’ohoku Place, Hilo, HI, 96720, USA 11 INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy 12 Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy 13 Les Engarouines Observatory, 84570 Mallemort-du-Comtat, France 14 Geneva Observatory, 1290 Sauverny, Switzerland 15 Astronomical Observatory, Adam Mickiewicz University, Sloneczna 36, 60-286 Poznan, Poland 16 Blauvac Observatory, 84570 St-Estève, France 17 Observatorio Montcabre, C/Jaume Balmes 24, 08348 Cabrils, Barcelona, Spain 18 Le Cres Observatory, 2 rue des Écoles, 34920 le Cres, France 19 Wiggins Observatory, 472 Country Club, Tooele Utah 84074, USA Received 28 May 2010 / Accepted 26 August 2010 ABSTRACT Aims. We determine the physical properties (spin state and shape) of asteroid (21) Lutetia, target of the International Rosetta Mission of the European Space Agency, to help in preparing for observations during the flyby on 2010 July 10 by predicting the orientation of Lutetia as seen from Rosetta. Methods. We use our novel KOALA inversion algorithm to determine the physical properties of asteroids from a combination of optical lightcurves, disk-resolved images, and stellar occultations, although the last are not available for (21) Lutetia. Results. We find the spin axis of (21) Lutetia to lie within 5◦ of (λ = 52◦, β = −6◦) in the Ecliptic J2000 reference frame (equatorial α = 52◦, δ =+12◦), and determine an improved sidereal period of 8.168 270±0.000 001 h. This pole solution implies that the southern hemisphere of Lutetia will be in “seasonal” shadow at the time of the flyby. The apparent cross-section of Lutetia is triangular when seen “pole-on” and more rectangular “equator-on”. The best-fit model suggests there are several concavities. The largest of these is close to the north pole and may be associated with strong impacts. Key words. minor planets, asteroids: individual: (21) Lutetia – methods: observational – techniques: high angular resolution – instrumentation: adaptive optics 1. Introduction planetary science. Unlike terrestrial planets, which have expe- rienced significant mineralogical evolution through endogenic The origin and evolution of the Solar System and its impli- activity since their accretion, small Solar System bodies have cations for early planetesimal formation are key questions in remained essentially unaltered. Thus, a considerable amount of information regarding the primordial planetary processes Based on observations collected at the W. M. Keck Observatory that occurred during and immediately after the accretion of and at European Southern Observatory Very Large Telescope (program the early planetesimals is still present among this population. ID:079.C-0493, PI: E. Dotto). The W. M. Keck Observatory is operated Consequently, studying asteroids is of prime importance in un- as a scientific partnership among the California Institute of Technology, derstanding the planetary formation processes (Bottke et al. the University of California, and the National Aeronautics and Space 2002) and, first and foremost, requires reliable knowledge of Administration. The Observatory was made possible by the generous their physical properties (size, shape, spin, mass, density, in- financial support of the W. M. Keck Foundation. ternal structure, etc.) in addition to their compositions and dy- Tables 1, 2, 4 and Figs. 3–5 are only available in electronic form at namics. Statistical analyses of these parameters for a wide http://www.aanda.org Article published by EDP Sciences Page 1 of 19 A&A 523, A94 (2010) range of asteroids can provide relevant information about inter- on Lutetia’s taxonomy and composition in a companion paper relationships and formation scenarios. (Drummond et al. 2010). In this respect, our observing program with adaptive optics, Thermal infrared observations used to determine the size and allowing diffraction-limited observations from the ground with albedo of Lutetia were initially inconsistent, with discrepancies 10 m-class telescopes, has now broken the barrier that separated in diameters and albedos at visible wavelengths reported (e.g., asteroids from real planetary worlds (e.g., Conrad et al. 2007; Zellner & Gradie 1976; Lupishko & Belskaya 1989; Belskaya Carry et al. 2008; Drummond et al. 2009; Carry et al. 2010; & Lagerkvist 1996; Tedesco et al. 2002; Mueller et al. 2006; Drummond et al. 2010). Their shapes, topography, sizes, spins, Carvano et al. 2008). Mueller et al. (2006)andCarvano et al. surface features, albedos, and color variations can now be di- (2008), however, interpreted these variations as an indication rectly observed from the ground. This opens these objects to geo- of surface heterogeneity, inferring that the terrain roughness of logical, rather than astronomical-only, study. While such surface Lutetia increased toward northern latitudes1, that the crater dis- detail is only possible for the largest asteroids, our main focus tribution is different over the northern/southern hemispheres, and is on determining accurately the size, shape, and pole. Among includes a possibility of one or several large craters in Lutetia’s them, we have observed (21) Lutetia, an asteroid that will be northern hemisphere. Indeed, the convex shape model derived observed in-situ by the ESA Rosetta mission. from the inversion of 32 optical lightcurves (Torppa et al. 2003) The Rosetta Mission will encounter its principal target, the displays a flat top near the north pole of Lutetia. Kaasalainen comet 67P/Churyumov-Gerasimenko, in 2014. However, its in- et al. (2002) have shown that large flat regions in these convex terplanetary journey was designed to allow close encounters with models could be a site of concavities. The southern hemisphere two main-belt asteroids: (2867) Steins and (21) Lutetia. The is not expected to be free from craters however, as Perna et al. small asteroid (2867) Steins was visited on 2008 September 5 (2010) detected a slight variation of the visible spectral slope, at a minimum distance of about 800 km (Schulz et al. 2009)and possibly due to the presence of large craters or albedo spots in (21) Lutetia will be encountered on 2010 July 10. Knowing the the southern hemisphere. geometry of the flyby (e.g., visible hemisphere, sub-spacecraft In this paper, we present simultaneous analysis of adaptive- coordinates as function of time, and distance) before the en- optics images obtained at the W. M. Keck and the European counter is crucial to optimize the observation sequence and Southern Observatory (ESO) Very Large Telescope (VLT) ob- schedule the on-board operations. The diameter of Lutetia (es- servatories, together with lightcurves, and we determine the timated at ∼100 km, see Tedesco et al. 2002, 2004; Mueller shape and spin state of Lutetia. In Sect. 2, we present the obser- et al. 2006) allows its apparent disk to be spatially resolved from vations, in Sect. 3 the shape of Lutetia, and finally, we describe Earth. Our goal is therefore to improve knowledge of its physical the geometry of the upcoming Rosetta flyby in Sect. 4. properties to prepare for the spacecraft flyby. Lutetia, the Latin name for the city of Paris, is a main-belt 2. Observations and data processing asteroid (semi-major axis 2.44 AU) that has been studied exten- sively from the ground (see Barucci et al. 2007, for a review, pri- 2.1. Disk-resolved imaging observations marily of recent observations). Numerous studies have estimated indirectly its spin (by lightcurve, e.g., Lupishko et al.