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The Near Earth Asteroid Rendezvous (NEAR) spacecraft flew within 3830 kilometers of asteroid 433 Eros on 23 December 1998. The gravitational perturbation on NEAR was evident in the spacecraft tracking data. Ground-based Doppler and range tracking of the spacecraft as well as spacecraft images of the asteroid's center of figure and surface features were used to determine the mass and rotation pole of Eros. The mass of Eros is (7.2 +/- 1.8) X 10^sup 18^ grams and, coupled with a volume estimate provided by the NEAR imaging team, this mass suggests a bulk density of 2.5 +/- 0.8 grams per cubic centimeter. The rotation pole position is 15.6 (+/-3.7) degrees in right ascension and 16.4 (+/-1.8) degrees in declination, which is consistent with ground-based and NEAR imaging team observations.
Asteroid 433 Eros is second only to 1036 Ganymede as the largest near-Earth asteroid, and its orbital characteristics make it an accessible target for a spacecraft rendezvous mission. Its general spectral classification is a so-called S-type, the most common type in the inner belt of asteroids between Mars and Jupiter. S-type asteroids may be the parent bodies of the ordinary chondrites, the most common type of meteorite (1). To constrain the processes by which the asteroids in the inner belt formed, it is important to understand the bulk densities of various asteroid types. By comparing the bulk density of a parent asteroid with the corresponding density of its most likely meteorite analog, a porosity estimate can be made for the asteroid. Bulk density measurements require knowledge of the object's mass and volume; masses and volumes determined from spacecraft observations exist only for asteroids 243 Ida (2), 253 Mathilde (3, 4), and now Eros.
NEAR is currently scheduled for an Eros rendezvous in mid-February 2000 (5); an aborted main engine firing on 20 December 1998 delayed the planned rendezvous with Eros originally scheduled for 10 January 1999 (Fig. 1). NEAR flew by Eros on 23 December 1998 at a close approach distance of 3827 km at a relative velocity of about 965 m/s. Although the main engine burn was aborted on 20 December 1998, there were unscheduled attitude control jet firings that changed the velocity of the spacecraft. As a result, our efforts to determine the...