Estimating the Mass of Asteroid 433 Eros During the NEAR Spacecraft Flyby
D.K. Yeomans, P.G. Antreasian, A. Cheng, D.W. Dunham, R. W. Farquhar, R.W. Gaskell, J.D.
Giorgini, C.E. Helfrich, A.S. Konopliv, J.V. McAdams, J.K. Miller, W.M. Owen, Jr., P.C. Thomas,
J. Veverka, B.G. Williams
D.K. Yeomans, P.G. Antreasian, R. W. Gaskell, J.D. Giorgini, C.E. Helfrich, A.S. Konopliv, J.K.
Miller, W.M. Owen Jr., and B.G. Williams are with the Navigation and Flight Mechanics Section at
the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91 109. A. Cheng,
D.W. Dunham, R.W. Farquhar, and J.V. McAdams are with the Applied Physics Laboratory, Johns
Hopkins University, Laurel, MD 20723. P.C. Thomas and J. Veverka are with Cornell University,
Ithaca, NY, 14853.
The terminal navigation of the Near-Earth Asteroid Rendezvous (NEAR) spacecraft during its
flyby of asteroid 433 Eros on December 23, 1998 involved coordinated efforts to determine the
heliocentric orbits of the spacecraft and Eros and then to determine the relative trajectory of the
spacecraft with respect to Eros. Although the gravitational perturbation on the NEAR spacecraft
from the nearby Eros was not nearly as obvious as during the asteroid Mathilde flyby in June 1997
(l), this perturbation was evident in the spacecraft tracking data. Using ground-based Doppler and
range tracking of the spacecraft as well as spacecraft images of the asteroid's center and surface
features, the mass and rotation pole of Eros could be determined. The mass value for Eros was
determined to be (7.2 +/- 1 .8) x lo** 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 g/cm**3. The rotation pole
position determined for the asteroid is compatible with ground-based results as well as those
determined by the NEAR imaging team.
After a successful main engine firing on January 3, 1999, the NEAR spacecraft is currently
scheduled for an Ero