Lunar Prospector Status Report #92

July 23 , 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.

There was no commanding this week.

Current spacecraft state (0000 GMT 7/23/99, DOY 204)

Orbit:6951
Downlink:3600 bps
Spin Rate:12.12 rpm

Spin Axis Attitude (ecliptic):
Latitude: -88.7 deg
Longitude: 055 deg

Trajectory:
Periapsis Alt:20.4 km
Apoapsis Alt: 39.6 km
Period: 111 min

Occultations:46 minutes
Eclipses: 30 minutes
Propellant Remaining:9.51 kg

Next week, the last week of operations, there will be several critical activities.

On July 26 at 14:02 GMT, a small orbit correction maneuver will be fired to raise periapsis up just enough (about 5 km) to last through the end of the week. The velocity will be changed only 1.44 m/s and only 0.12 kg of propellant will be used. Also on July 26, the MAG/ER instrument will be commanded to get better resolution data during the full moon period.

On July 28, there will be a lunar eclipse: the moon goes into the earth's shadow. This means that for several hours, the spacecraft does not see full sunlight even when its on the sun side of the moon, and so it will not be able to fully recharge the battery between nightside passes. Plans are to turn off non-critical subsystems as well as cycle the transmitter (the biggest power user on the spacecraft) as required to keep the battery from discharging below critical levels. The spectrometer instruments will be powered off before the eclipse and will be left off for the last 3 days of the mission. The moon begins to enter Earth shadow at 08:56 GMT and exits Earth shadow at 14:11 GMT. During that interval, as the spacecraft orbits the moon, it goes in and out of moon shadow as well as in and out of earth shadow.

On July 29, the spacecraft will be spun up to 23.7 rpm (from nominal 12 rpm) at 15:30 GMT. This is in preparation for the velocity change maneuvers later in the week which will target the spacecraft for impact. Because these maneuvers will use the remaining usable fuel, the spacecraft must be spun up to allow the fuel to reach the fuel lines to the jets. Without the spin up, the acceleration from the thruster firing would move the last pools of fuel away from the fuel lines, cutting the maneuver short. The spin up will use 0.22 kg of propellant.

On July 30 at 08:15, a maneuver will be fired to raise apoapsis, putting the spacecraft into a more elliptical orbit. Almost half of the remaining propellant will be used to do this maneuver. This will be done so that when we target for impact on July 31, we come in as steeply as possible to hit the crater floor after passing over the rim of the crater. The spacecraft velocity will be increased by 40 m/s and 3.00 kg of propellant will be used.

On July 31 at 09:16, the final maneuver will be fired to lower periapsis beneath the lunar surface such that it will impact at the target point of -87.7 deg latitude, 42 deg longitude. The velocity will be decreased by 45 m/s, using 3.32 kg of propellant. The remaining 2.85 kg of propellant is unusable, trapped in the propellant tank and lines. Because this maneuver must be fired when the spacecraft is on the far side of the moon as seen from earth, these commands will be loaded about an hour earlier with a time delay built in so that they will occur at the proper time. If all goes well, when the spacecraft passes behind the moon at 08:59 GMT, that will be the last we hear from it. The impact site is so close to the pole, and the spacecraft impact angle is so low (6.3 deg), the spacecraft will never be in view of the tracking station before impact. The impact will occur at 09:52 GMT (2:52 am PDT).

Ground based and space based telescopes, however, will be able to observe the impact plume cloud.