* Astronomy

Blobrana · L

RE: Asteroid (21) Lutetia

Blobrana · L

Rosetta triumphs at asteroid Lutetia

Lutetia at Closest approach

Asteroid Lutetia has been revealed as a battered world of many craters. ESA's Rosetta mission has returned the first close-up images of the asteroid showing it is most probably a primitive survivor from the violent birth of the Solar System.
The flyby has been a spectacular success with Rosetta performing faultlessly. Closest approach took place at 18:10 CEST, at a distance of 3162 km.

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Blobrana · L

The Asteroid Lutetia will become the largest space rock to be visited by a probe when the European Rosetta mission flies past it on Saturday (GMT).
The 100km-wide mass of rock will be the last encounter in Rosetta's tour of the Solar System before it sweeps out to meet up with a comet in 2014.
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Blobrana · L

On July 10, 2010, the European spacecraft Rosetta will fly the asteroid 21 Lutetia at a distance of about 3000 km. At this moment, Lutetia is at a distance from Earth of 450 million km. It is located in the Main-Belt between Mars and Jupiter which is thought to be constituted by the building blocks of a planet prevented from forming by the mighty planet Jupiter. This small object whose size is estimated at a hundred kilometres owes its name to the Latin name of Paris where it was discovered in 1852. Seen from Earth, it appears as small as a one euro coin placed at a distance of 11km. Its telescopic observation is consequently very challenging. It requires the use of larger Earth-based telescopes if one wishes to know better the object, its shape, its own rotation before its close approach with Rosetta. This has been achieved by an international team led by Benoit Carry of Paris Observatory. With a two-year work on the 10-m telescope of the W.M. Keck Observatory and on the 8m VLT (ESO), it has been possible to reconstruct the three-dimensional shape of 21 Lutetia.
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Blobrana · L

Title: The triaxial ellipsoid dimensions, rotational pole, and bulk density of ESA/NASA Rosetta target asteroid (21) Lutetia
Authors: J. D. Drummond, A. Conrad, W. J. Merline, B. Carry, C. R. Chapman, H. A. Weaver, P. M. Tamblyn, J. C. Christou, C. Dumas

We seek the best size estimates of the asteroid (21) Lutetia, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid, and to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101 x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics, with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions of 1 x 1 x 8 km, but it is evident, both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia has significant departures from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of 124 ±5 x 101 ±4 x 93 ±13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC] = [52, +12]. Using two separately determined masses and the volume of our hybrid model, we estimate a density of 3.5 ±1.1 or 4.3 ±0.8 g cm^-3 . From the density evidence alone, we argue that this favours an enstatite-chondrite composition, although other compositions are formally allowed at the extremes (low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We discuss this in the context of other evidence.

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Blobrana · L

Title: Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by
Authors: I.N.Belskaya, S.Fornasier, Yu.N.Krugly, V.G.Shevchenko, N.M.Gaftonyuk, M.A.Barucci, M.Fulchignoni, R.Gil-Hutton

A wide observational campaign was carried out in 2004-2009 aimed to complete the ground-based investigation of Lutetia prior to the Rosetta fly-by in July 2010. We have obtained BVRI photometric and V-band polarimetric measurements over a wide range of phase angles, and visible and infrared spectra in the 0.4-2.4 micron range. We analysed them together with previously published data to retrieve information on Lutetia's surface properties. Values of lightcurve amplitudes, absolute magnitude, opposition effect, phase coefficient and BVRI colours of Lutetia surface seen at near pole-on aspect have been determined. We defined more precisely parameters of polarization phase curve and showed their distinct deviation from any other moderate-albedo asteroid. An indication of possible variations both in polarization and spectral data across the asteroid surface was found. To explain features found by different techniques we propose that (i) Lutetia has a non-convex shape, probably due to the presence of a large crater, and heterogeneous surface properties probably related to surface morphology; (ii) at least part of the surface is covered by a fine-grained regolith with particle size less than 20 microns; (iii) the closest meteorite analogues of Lutetia's surface composition are particular types of carbonaceous chondrites or Lutetia has specific surface composition not representative among studied meteorites.

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