Session 71 - Comets & Asteroids.
Display session, Friday, January 09
Exhibit Hall,

## [71.06] New Masses and Densities for 1 Ceres, 2 Pallas, and 4 Vesta

J. L. Hilton (U. S. Naval Observatory)

As part of providing new ephemerides for the Astronomical Almanac, new masses were determined for 1 Ceres, 2 Pallas, and 4 Vesta, the three largest asteroids. These masses are: Ceres = (4.35 \pm 0.05) \times 10^-10M_\sun, Pallas = (1.60 \pm 0.04) \times 10^-10 M_\sun, Vesta = (1.52 \pm 0.09) \times 10^-10 M_\sun. The mass for Ceres is smaller than most previous determinations of its mass. This smaller mass is a direct consequence of the fact that the mass determined for Pallas is at the upper one sigma bound of previous determinations for its mass. The mass for Vesta is in good agreement with previous determinations of its mass. The uncertainty in the mass of Pallas is 1/5 that of the most recent determination of its mass and the uncertainty in the mass of Vesta is 1/3 that of the most recent determination of its mass.

Using the best available determinations for the mean radii of these asteroids, the densities found are Ceres \rho = 1.98 \pm 0.03 gm cm^-3, Pallas \rho = 4.2 \pm 0.2 gm cm^-3, and Vesta \rho = 3.9 \pm 0.3 gm cm^-3. The density of Ceres is fairly close to that of the similar C-type asteroid 253 Mathilde (\rho = 1.3 \pm 0.2 gm cm^-3; Veverka, J. 1997, BAAS, 29, 958). However, the density of Pallas suggests that it is not related to the C-type asteroids, as a simple two-color to eight-color analyses would indicate. The similarity between its density and that of Vesta supports the 28-color analysis of M. Birlan, et al. 1996, Aston. Astrophys., 305, 984, that they are members of the same class of asteroids.