THE ORBIT OF (2) PALLAS

A comparison between recent astrometric observations of the asteroid ( 2) Pallas made at Bordeaux Observatory with the positions given in the Ephemerides of Minor Planets (EMP) published by the Institute of Theo retical Astronomy at St Petersburg shows systematic offsets in right a scension, about 0.6 ''. The orbital elements of Pallas given in the EM P since 1986 are based on a set of orbital elements of Pallas publishe d in 1984 on Minor Planet Circular (MPC) 8665. We give in this paper s ome arguments which prove that the offsets between the observed and th e calculated positions can be explained as follows: the orbital elemen ts of Pallas given on MPC 8665 were considered to be expressed in the usual (FK4, B1950.0) system when introduced in the EMP for 1986, where as we prove that they are expressed in the non-standard (FK5, B1950.0) system. We then make a new determination of the orbital elements of P allas directly in the (FK5, J2000.0) system, and of the mass of (1) Ce res, adding to the data found in Landgraf (1987) and covering the peri od 1802 - 1978, the positions obtained with the Bordeaux and La Palma automatic meridian circles from 1984 to 1993. The result obtained for the mass of Ceres is (5.0 +/- 0.2) 10(-10) solar masses, which is in a greement with the average of the values obtained by other authors.