Lunar Prospector Status Report #6

January 11, 1998 - 10:00 a.m. EST (7:00 a.m. PST)

The Lunar Prospector spacecraft has successfully entered orbit around the Moon, reports Mission Operations Manager Marcie Smith. Following a highly successful first lunar orbit insertion (LOI) burn early this morning, an absolutely critical maneuver for mission success, all indications are that we have a very healthy spacecraft and that everything is right on track for a long and productive science mission, according to Mission Director Alan Binder of the Lunar Research Institute, Gilroy, CA.

At 2:45 a.m. PST (5:45 a.m. EST), the maneuver command to begin the first LOI burn was loaded into the spacecraft register and began timing down. At precisely 3:45 a.m. PST (6:45 a.m. EST), the two thrusters on the underside of the spacecraft were fired for 32.2 minutes continuously to slow the vehicle and put it into lunar orbit. That maneuver was completed at 4:17 a.m. PST (7:17 a.m. EST). Preliminary analysis of the tracking data shows that the spacecraft is in an 11.8-hour period orbit, very close to the target 12-hour orbit.

Today's lunar orbit insertion maneuver was a critical event for the Lunar Prospector spacecraft. If, for whatever reason, the engines could not have been commanded to fire within about 2 hours of the time of closest approach, the mission would have ended in a flyby of the Moon. Now, the spacecraft is safely in a stable orbit. Although second and third LOI burns are scheduled for tomorrow and Tuesday, they are not time critical.

The latest pre-LOI trajectory showed that Lunar Prospector flew by the Moon at an altitude of 71 km (about 45 miles), about 11 km below the original target. The inclination of the orbit was about 89.7 degrees, just below the 89.9 degree target. The time of closest approach was estimated to be 33 seconds earlier than the original target.

The spacecraft is currently spinning at 13.2 rpm, and all science instruments are on and working. Although mission science data collection has not officially begun, the spectrometer scientist was thrilled to see his first lunar data during the time of closest approach.

The initial estimate of the orbit period at 11.8 hours suggests that the orbit will reach an altitude of just over 8,500 km at its furthest distance from the Moon just under six hours after the burn, according to missions operations personnel. A contingency spin trim maneuver scheduled for after the LOI burn was cancelled, as there was very little change in the spin rate during the maneuver and it was not deemed necessary.

The spacecraft performed perfectly throughout the LOI maneuver, even though some minor data outages were encountered. The Deep Space Network (DSN) stations lost lock on the uplink and downlink signal from the spacecraft during the burn as a result of problems experienced in getting the trajectory predictions to the stations before the maneuver. As a result, about 5 minutes of data was lost. Analysis of today's event has ensured that the prediction problem is now well understood; similar outages are not anticipated during the second and third LOI burns tomorrow and Tuesday mornings.

A capture orbit correction maneuver is scheduled for this morning, Jan. 11 at 9:40 a.m. PST (12:40 p.m. EST), if necessary. This will allow correction of the inclination if it was perturbed sufficiently during today's first LOI burn. The next major activity will be initiated at about 3:15 a.m. PST (6:15 a.m. EST) on Monday morning, Jan. 12 when a second 30-minute LOI burn will be fired resulting in a predicted reduction of the orbit period of the spacecraft to only 3.5 hours at the conclusion of LOI 2. The third and final LOI burn is scheduled for early Tuesday morning, Jan. 13, 1998.

David Morse
Ames Research Center
Moffett Field, CA 94035