A new set of ephemerides for 15 of the largest asteroids has been produced
for use in the Astronomical Almanac. The ephemerides cover the period from
1800 through 2100. The internal uncertainty in the mean longitude at epoch,
1997 December 18, ranges from 0".05 for 7 Iris through 0".22 for 65 Cybele
, and the uncertainty in the mean motion varies from 0".02 per century for
4 Vesta to 0".14 per century for 511 Davida. This compares very favorably w
ith the internal errors for the outer planets in recent Jet Propulsion Labo
ratory planetary ephemerides. However, because the asteroids have relativel
y little mass and are subject to perturbations by other asteroids, the actu
al uncertainties in their mean motions are likely to be a few tenths of an
arcsecond per century. As part of the improvement to the ephemerides, new m
asses and densities were determined for 1 Ceres, 2 Pallas, and 4 Vesta, the
three largest asteroids. These masses are as follows: Ceres = (4.39 +/- 0.
04) x 10(-10) M-circle dot, Pallas = (1.59 +/- 0.05) x 10(-10) M-circle dot
, and Vesta = (1.69 +/- 0.11) x 10(-10) M-circle dot. The mass for Ceres is
smaller than most previous determinations of its mass. This smaller mass i
s a direct consequence of the increase in the mass determined for Pellas. T
he densities found for these three asteroids are 2.00 +/- 0.03 g cm(-3) for
Ceres, 4.2 +/- 0.3 gm cm(-3) for Pallas, and 4.3 +/- 0.3 g cm(-3) for Vest
a. The density for Ceres is somewhat greater than that found for the taxono
mically similar 253 Mathilde.