Physics – Optics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13c1403c&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P13C-1403
Physics
Optics
[6000] Planetary Sciences: Comets And Small Bodies, [6205] Planetary Sciences: Solar System Objects / Asteroids
Scientific paper
We report the results of our campaign to improve our understanding of the physical characteristics of asteroid (21) Lutetia ahead of the Rosetta flyby in 2010 July. This included measurements of shape, size, pole, density, and a search for satellites. We utilized primarily adaptive optics (AO) on large ground-based telescopes (Keck, Gemini, and VLT). We coordinated these efforts with HST observations (Weaver et al. 2010, A&A 518, A4), made in support of Rosetta’s ALICE UV spectrometer. Preliminary results were supplied to Rosetta mission teams in fall of 2009 to assist in planning for the mission. Observations and analyses were complete and submitted for publication before the flyby (Drummond et al. 2010, A&A, in press; Carry et al. 2010, A&A, in press). Using more than 300 AO images of Lutetia, which subtended only slightly more than two resolution-elements (0.10”) for these large telescopes, we were able to derive accurate size and shape information, as well as a pole and spin period. We modeled the size and shape using both a triaxial-ellipsoid model and a 3D radius-vector model. The radius-vector model used our new technique of multi-dataset inversion, called KOALA (for Knitted Occultation, Adaptive optics, and Lightcurve Analysis), in which we utilized not only our AO imaging, but also 50 lightcurves spanning 48 years. We combined the best aspects of each model to produce our best-estimate 3D shape model, a hybrid having ellipsoid-equivalent dimensions of 124 x 101 x 93 km (± 5 x 4 x 13 km) and effective diameter 105 ± 7 km. We found the spin axis of Lutetia to lie within 5 deg of [long, lat (52,-6)] or [RA DEC (52,+12)] and determined an improved sidereal period of 8.168270 ± 0.000001 h. We predicted the geometry of Lutetia during the flyby and showed that the southern hemisphere would be in seasonal shadow at that time. The model suggested the presence of several concavities and irregularities that may be associated with large impacts. The model matches remarkably the Rosetta imaging in both shape and absolute size, and we take this as a validation of our technique. The overall size-scaling required of our model to match best the Rosetta images appears to be only 2.0%, while the RMS deviation of points on our model contours from the Rosetta contours is about 2.7 km (about 2.5%). The detailed shape, as well as the a- and b-dimensions of the object, will ultimately be better determined from the Rosetta flyby imaging; but knowledge of the pole, spin period, and c-dimension may still depend heavily on our ground-based solutions. Using two separately determined (pre-Rosetta) masses and the volume from our hybrid shape model, we estimate a density of 3.5 ± 1.1 or 4.3 ± 0.8 g/cc, favoring an enstatite-chondrite (EC) composition for this large M-type asteroid (Chapman et al. 2010, DPS), although other compositions are formally allowed.
Carry Benoit
Chapman Clark R.
Christou Julian C.
Conrad Albert
Drummond Jack D.
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