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Lunar Prospector Status Report #63

December 30, 1998 - 1:00 p.m. EST (10:00 a.m. PST)

The Lunar Prospector spacecraft is operating well and all instruments continue to collect good data.

There was no commanding this week.

Current spacecraft state (0000 GMT 12/30/98):

Orbit: 4300
Downlink: 3600 bps
Spin Rate<: 12.00 rpm

Spin Axis Attitude (ecliptic):
Latitude:-88.0 deg
Longitude: 242 deg

Trajectory:
Periapsis Alt: 29 km
Apoapsis Alt: 52 kmPeriod: 112 min

Occultations: 46 minutes
Eclipses: none
Propellant Remaining: 17.45 kg


The spacecraft has now been in the 40 km average altitude orbit for over 10 days. Doppler data is being collected to verify that the lunar gravity model is accurate at this low altitude. The tracking schedule has been changed many times this last week due to problems with other spacecraft, but in general we have maintained good coverage and should have an updated gravity model in about a week. Data so far indicates that the gravity model we have used is very good.

Data is currently being reviewed from the Clementine project to ensure adequate clearance over the lunar terrain in the extended mission orbit. Whatever orbit the spacecraft is placed in, the asymmetries in the lunar gravity field cause the orbit to become elliptical, so that over a two week period, the altitude will vary by +/- 15 km. For highest science resolution, it is desired to place the spacecraft in an orbit with an average altitude of 25 km, but this means that the spacecraft will get to within 10 km of the average lunar surface during the two weeks between maneuvers.

The moon is not flat, however, and there are several high mountains on the surface. The project is currently getting the latest data on the terrain from the Clementine project to run the orbit profiles over and ensure at least a 6 km margin at all times. If the 25 km orbit does not provide this margin, the spacecraft will be placed in a 30 km average altitude orbit for the extended mission. The maneuver to lower the orbit for the extended mission is scheduled for January 16.