Determination of Masses of Six Asteroids from Close Asteroid—Asteroid Encounters

Abstract

New masses of six asteroids: (6) Hebe, (10) Hygiea, (15) Eunomia, (52) Europa, (511) Davida and (704) Interamnia were determined within the ongoing Asteroid Mass Determination Program, started in 1998 in the Wroclaw University Observatory. The masses were calculated by means of the least-squares method as weighted means of the values found separately from the perturbations on several single asteroids. Special attention was paid to the selection of the observations of the asteroids. For this purpose, a criterion based on the requirement that the post-selection distribution of the (O — C) residuals should be Gaussian, was implemented. Asteroid encounters enabling mass determinations were found as a result of an extensive search for large asteroidal perturbations. In the search, gravitational perturbations exerted by 250 most massive asteroids on 4500 numbered minor planets (called hereafter test asteroids) were determined. The perturbations were calculated as differences in right ascension between perturbed and not perturbed orbits of the test asteroids when integrated backward. As candidates for the mass determinations were used the test asteroids, for which the maximum perturbation in whole time interval covered by its observation was large (more than about 1 arcsecond). Most of them were never used before for this purpose. Details of the mass determinations are given in Table I. As an outcome of the search for possible perturbers among 912 asteroids with diameters larger than 50 km, for all test asteroids under consideration, correct dynamical models including important perturbers, were proposed. So far, the program gave also reliable mass estimates for the three largest asteroids: (1) Ceres, (2) Pallas and (4) Vesta. The mass of Ceres, (4.70 + 0.04) x 10−10 M⊙, was obtained from 25 individual solution, that of Pallas, (1.21 ± 0.26) x 10−10 M⊙, was determined from 2 new test asteroids, and for Vesta, (1.36 ± 0.05) x 10−10 M⊙, was obtained as sa weighted mean of 26 individual solutions (Michalak 2000, Astronomy and Astrophysics, in press). This work was supported by the Wroclaw University grant No. 2041/W/IAl2000.