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Lunar Prospector Status Report #26

March 25, 1998 - 7:00 p.m. EST (4:00 p.m. PST)

The spacecraft continues to perform very well and all instruments continue to collect good data. The Lunar Prospector orbit is evolving consistently with predictions from the latest gravity model (LP75D) supplied by Dr. Konopliv/JPL, which has been in use since March 3, 1998 for Lunar Prospector orbit determination product generation. The accuracy of orbit determination solutions has improved dramatically over pre-launch models. Figure 1 shows definitive solutions plotted against propagations of the Lunar Prospector orbit since the beginning of the mission and following the first mapping orbit maneuver on March 8. On this figure, both the apoapsis (maximum) and periapsis (minimum) altitudes of each orbit are shown. As planned before launch, definitive orbit solutions for the time span prior to March 3 are being re-computed by Goddard Space Flight Center using the updated gravity model and will be made available by mid-April.

Orbital history of apoapsis and periapsis altitude

A plan for conducting Lunar Prospector orbit maneuver has been implemented which calls for maneuvers approximately every two months. These maneuvers are timed to coincide with periods when the orientation of the Lunar Prospector orbit plane is such that continuous ground station coverage is available (when orbit normal is aligned with the Earth-moon line every 14 days). This guarantees coverage of the maneuvers, which consist of two burns spaced close to 180 degrees apart to adjust orbit eccentricity and the location of periapsis. The next orbit maneuver is tentatively planned on or about May 1st. Figure 2 contains a long-term propagation of the LP orbit which shows how the orbit would evolve if no more maneuvers were conducted. Next week, a small attitude re-orientation will likely be scheduled to trim out the attitude.

orbit history and perdicted evolution apoapsis and periapsis altitude vs. days

The current state of the vehicle ( as of 4:00 p.m. (PST) on Wed., March 25, 1998), according to Mission Operations Manager Marcie Smith, is as follows:

Spacecraft
Orbit Number: 895
Data Downlink Rate: 3600 bps
Spin Rate: 11.96 rpm

Spin Axis Attitude
Longitude: 280 degrees
Latitude: 86.4 degrees

Trajectory
Periselene: 89 km
Aposelene: 109 km
Period: 118 minutes
Inclination: 91 degrees

Occultations: 45 minutes in duration
Eclipses: 47 minutes in duration

Ken Galal
Lunar Prospector Trajectory Engineer
NASA Ames Research Center
Moffett Field, Calif. 94035