The Mass of Asteroid 253 Mathilde from near Tracking Data
D.K. Yeomans, D.J. Scheeres, A.S. Konopliv, J.D. Giorgini, P.W. Chodas, A.B. Chamberlin (JPL/Caltech), J.-P. Barriot (CNES)
The first asteroid mass determination using spacecraft tracking data
will be made as a result of the NEAR spacecraft flying past asteroid 253
Mathilde on June 27, 1997. The NEAR spacecraft is scheduled to pass
within 1200 km of Mathilde at a relative velocity of 10 km/s. Doppler
X-band tracking data will be analyzed to determine the gravitational
perturbation upon the spacecraft due to Mathilde's mass. The spacecraft
will be continuously tracked on either side of the encounter - except
for a period of about one hour at closest approach when imaging
experiments will be conducted. An error analysis by one of us (DJS)
studied the sensitivity of the mass determination accuracy to the amount
of continuous tracking time available before and after the encounter.
The asteroid's diameter and bulk density were assumed to be 61 km and
2.6 g/cc respectively. Taking into account the uncertainties in the
tracking station locations, the spacecraft's orientation, the Earth's
timing and tropospheric/ionospheric effects, it appears likely that
Mathilde's mass can be determined to the 10% level if the spacecraft is
tracked continuously for three days on either side of the close
approach. An analysis of Mathilde's shape by the NEAR imaging team and
the expected long-term radar imaging of the asteroid during the second
half of 1997 by Steve Ostro and his colleagues should allow a volume
(and hence a bulk density) determination to a comparable level of
accuracy.