| N EAR E ARTH A STEROID R ENDEZVOUS | |
Radio Science:
Determining the Mass of Asteroid 253 Mathilde
For three days before and after the asteroid encounter, the spacecraft will
be tracked using radiosignals that will be sent to the spacecraft and then
returned. Slightchanges in the spacecraft's velocity can be determined from
these tracking data.
When the asteroid's mass has been determined, a
combination of the spacecraft's images of the asteroid, together with
Earth-based optical and radar data taken during the asteroid's approach
to Earth in late 1997 will be used to determine the asteroid's shape,
and hence its volume. By dividing the asteroid's mass by its volume,
its bulk density can be determined. The determined bulk density will
provide a valuable clue as to whether the asteroid is solid rock or
whether the asteroid might have the lower density expected from a loose
collection of rocky material with internal voids.
During the NEAR spacecraft's encounter with asteroid 253 Mathilde on
June 27, 1997, the spacecraft will pass within 1200 km of the asteroid.
During this fast flyby (10 km/second), Mathilde's gravitational
attraction will exert a slight tug on the passing spacecraft, changing
its velocity by about 3 millimeters per second. The size of this tug will be directly proportional to the asteroid's mass. The spacecraft's
flight path will be altered by about 0.00002 degrees so that one hour
after the flyby when the spacecraft has traveled another 36,000 km, it
will have deviated only 12 meters from the path it would have taken had
it not passed by Mathilde one hour earlier.
Don Yeomans - JPL
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