Lunar Prospector Status Report #75

March 25, 1999 - 1:00 p.m. EST (10:00 a.m. PST)

The Lunar Prospector spacecraft continues to collect good science data in the extended mission orbit. Four of five science instruments are on: the Gamma Ray Spectrometer (GRS), the Neutron Spectrometer (NS), the Magnetometer (MAG) and the Electron Reflectometer (ER). Data continues to be collected for the Doppler Gravity Experiment (DGE) as well.

Cycling of the spacecraft transmitter continued through most of the period. The transmitter has been powered off for short periods when the propellant tank heater is on to reduce battery drain. As the propellant is used for orbit maintenance burns, the time the heater is on becomes shorter. After the orbit maintenance burn fired yesterday (DOY 083), the on-time has become short enough that cycling of the transmitter has been discontinued.

The orbit maintenance burn to maintain the extended mission 30-km average altitude orbit was successfully fired yesterday (DOY 083). The next orbit maintenance burn is scheduled for April 21. There were some anomalous commanding events during the maneuver burn, and the reorientation burn scheduled for after the orbit maintenance burn was postponed. During the maneuver sequence, three commands that were transmitted to the spacecraft were not properly received and did not execute. The first two were retransmitted, executed successfully, and the sequence continued. When the third command did not process correctly, the decision was made to discontinue the procedure until some investigation could be made. By then, the orbit maintenance burns had completed successfully and on time (with preliminary analysis from the navigation team showing a performance error of less than 1 percent.) The reorientation burns were not critical and were postponed.

So far, there has been no conclusion on what might have caused the commands to fail to execute. The DSN staff is looking into possible problems with the station (DSS 66) transmitter and the spacecraft team is reviewing all the spacecraft data.

The maneuver timeline is given below (times are GMT):

082/1640 Earth Moon Sensor ON; Pressure transducer ON

083/1704 A1/A2 thruster heaters ON [one cmd in this file had to be resent]

084/1710 load maneuver parameters 084/1730 fire part 1 - A1/A2 - 43.8 sec burn - 6.87 m/s - 0.55 kg propellant

084/1738 A1/A2 thruster heaters ON

084/1743 load maneuver parameters

084/1810 fire part 2 - A1/A2 - 47.1 sec burn - 7.39 m/s - 0.59 kg propellant

084/1828 A1/A4 thruster heaters ON (for reorientation maneuver) [one cmd in this file had to be resent]

084/1854 load maneuver parameter file (for reor) [one cmd did not execute and decision made to cancel reor] 084/1901 clear maneuver parameter registers

084/1908 turn thruster heaters OFF

The reorientation burn has been tentatively scheduled for March 29 (DOY 088).

Current spacecraft state (0000 GMT 3/25/99, DOY 084):

Orbit: 5397
Downlink: 3600 bps
Spin Rate: 12.09 rpm

Spin Axis Attitude (ecliptic):
Latitude: -89.8 deg
Longitude: 265 deg

Trajectory:
Periapsis Alt: 15.0 km
Apoapsis Alt: 45.0 km
Period: 111 min

Occultations: none
Eclipses: 49 minutes