Asteroid 21 Lutetia: Low Mass, High Density

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Science  28 Oct 2011:
Vol. 334, Issue 6055, pp. 491-492
DOI: 10.1126/science.1209389


Asteroid 21 Lutetia was approached by the Rosetta spacecraft on 10 July 2010. The additional Doppler shift of the spacecraft radio signals imposed by 21 Lutetia’s gravitational perturbation on the flyby trajectory were used to determine the mass of the asteroid. Calibrating and correcting for all Doppler contributions not associated with Lutetia, a least-squares fit to the residual frequency observations from 4 hours before to 6 hours after closest approach yields a mass of (1.700 ± 0.017) × 1018 kilograms. Using the volume model of Lutetia determined by the Rosetta Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera, the bulk density, an important parameter for clues to its composition and interior, is (3.4 ± 0.3) × 103 kilograms per cubic meter.

The Asteroid 21 Lutetia, discovered in 1852, is one of the larger main-belt asteroids. In 2004, it became the flyby target asteroid for the Rosetta spacecraft mission. An important characteristic of an asteroid is its bulk density, derived from its mass and its volume. Although there are a number of asteroid mass determination techniques, by far the most accurate is spacecraft tracking during a close flyby.

The velocity of a spacecraft flying by a body of sufficient size and at a sufficiently close distance is perturbed by the attracting force of that body. The perturbed velocity is estimated from the additional Doppler shift of the transmitted radio signal in comparison with the expected Doppler shift of an unperturbed trajectory (1).

The Rosetta spacecraft was tracked during the flyby of asteroid 21 Lutetia on 10 July 2010 with NASA’s Deep Space Network (DSN) 70-m antenna (DSS-63) near Madrid, Spain (2). The flyby distance was d = 3168 ± 7.5 km, the high relative flyby velocity was v0 = 14.99 km/s, and the projection angle between the relative velocity and the direction to Earth was α = 171.2°, all of which define the postencounter amplitude of the Doppler shift (3).

After correcting for contributions not associated with 21 Lutetia (2), the final Doppler frequency shift 6 hours after Rosetta’s closest approach to the asteroid was Δf = 36.2 ± 0.2 mHz (Fig. 1). The value of GM (gravitational constant G × body mass M) from a least-squares fitting procedure is GM = (11.34 ± 0.11) × 10−2 km3s−2, corresponding to a mass of (1.700 ± 0.017) × 1018 kg (error, 1.0%). The uncertainty in GM (2) considers the error from the least-squares fit mainly driven by the frequency noise (0.55%), the uncertainty in the 21 Lutetia ephemeris introduced by the uncertainty in the flyby distance of ±7.5 km (0.24%), and the considered uncertainty in the tropospheric correction introduced by the zenith delay model and the mapping function of the ground station elevation (0.8%). These contributions yield a total uncertainty of 1.0%. The values for GM and Δf agree within the error with the analytical solution (3). The derived mass is lower than other mass determinations of Lutetia from astrometry (fig. S7).

Fig. 1

Filtered and adjusted frequency residuals at X-band frequency from 4 hours before closest approach to 6 hours after closest approach. Two tracking gaps (light red shaded zones) are indicated from 5 min before closest approach to 45 min after closest approach as planned (7), and from 192 min to 218 min after closest approach when DSS 63 accidentally dropped the uplink. The red solid line is a least-squares fit to the data from which GM is determined.

One of the most important global geophysical parameters—which provides clues to the origin, internal structure, and composition of 21 Lutetia—is its mean (bulk) density, derived from the mass and the volume. Observations of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera and ground observations using adaptive optics were combined to model the global shape. The derived volume is (5.0 ± 0.4) × 1014 m3 (4). The volume leads to a bulk density of (3.4 ± 0.3) × 103 kg/m3. This high bulk density is unexpected in view of the low value of the measured mass. It is one of the highest bulk densities known for asteroids (5). Assuming that Lutetia has a modest macroporosity of 12%, it would imply that the bulk density of its material constituents would exceed that of stony meteorites. Unless Lutetia has anomalously low porosity compared with other asteroids in its size range, its high density likely indicates a nonchondritic bulk composition enriched in high atomic number like iron. It may also be evidence for a partial differentiation of the asteroid body (6).

Supporting Online Material

Materials and Methods

Figs. S1 to S7

References (926)

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. As shown in (7), the expected final postencounter Doppler shift of a two-way radio carrier signal is Δf(t)=4fXcGMdv0sinαcosβ , where α′ = 172.18° is the direction to Earth projected into the flyby plane and β = 3° is the direction angle to Earth above the flyby plane. Using the fit solution for Lutetia of GM = (11.34 ± 0.11) × 10−2 km3/s2, the analytical result of the relation above is 36.4 ± 0.4 mHz.
  3. Similar high bulk densities are known for the asteroids 4 Vesta, 16 Psyche, 20 Massalia, and 22 Kalliope, all of which are larger than Lutetia (8). Bulk densities of more primitive C-type asteroids are in the range 1200 kg/m3 to 2700 kg/m3.
  4. Acknowledgments: The Rosetta Radio Science Investigation experiment is funded by the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Bonn under grants 50QM1002 (T.P.A. and B.H.) and 50QM1004 (M.P., M.H., S.T., and M.K.B.), and under a contract with NASA (S.W.A. and J.D.A.). We thank T. Morley for valuable comments and all persons involved in Rosetta at the European Space Research and Technology Centre, European Space Operations Centre, European Space Astronomy Centre, Jet Propulsion Laboratory, and the European Space Tracking Network and Deep Space Network ground stations for their continuous support.
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