Lunar Prospector Status Report #94

July 30 , 1999 - 10:20 p.m. EST (7:20 a.m. PST)

The Lunar Prospector is now ready for the final burn to target it for impact in a permanently shadowed crater near the south pole of the moon. Yesterday, 7/29/99, LP was spun up to 23.4 rpm. This provided centrifugal force so that the propellant will stay over the lines feeding the thrusters during the large burns required for impact targeting. Today, the spacecraft was placed into an elliptical orbit in preparation for targeting to impact tomorrow night.

The spin up timeline is below. All times are GMT on DOY 210 (July 29).

15:07 T2 thruster heater commanded on
15:10 MAG/ER configuration change
15:15 Maneuver parameter file loaded and verified
15:32 T2 fired for 36.2 sec; spin rate changed from 12.2 to 23.4 rpm;
0.22 kg of propellant used
15:35 Spacecraft safed

12 commands were sent with no anomalies. The MAG/ER instrument was reconfigured to allow data collection at the higher spin rate. It continues to collect its highest priority data, near full moon, when the earth blocks the solar magnetic field, allowing high resolution measurements of the moon's field.

The velocity change maneuver timeline is below. All times are GMT on DOY 211 (July 30).

06:48 A3/A4 thruster heaters commanded on
06:56 Maneuver parameter file loaded and verified
06:58-07:45 occultation
07:55 Maneuver parameter configuration completed
08:03 Execute command sent (file built with 10 minute delay)
08:13 A3/A4 fired for 256 sec; velocity changed 40.5 m/s; 3.18 kg of prop used)
08:20 Spacecraft safed.

When the spacecraft came out of occultation at 07:45, the ground stations locked up on the telemetry as expected but the data could not flow to the LP Mission Control Center due to an equipment failure at the Data Control center at JPL. So the maneuver commands were sent in the blind, without telemetry verification. Telemetry flow was reestablished less than one minute before the start of the burn. The burn was confirmed in telemetry and by the navigation team. Preliminary estimates are that the burn performance was within 1% of predicted.

This maneuver changed the orbit from an almost circular orbit with an average altitude of 30 km above the surface, to a somewhat elliptical orbit, with an apoapsis of 230 km altitude. A nice side effect of this maneuver was that it raised the orbit over the nightside, and the spacecraft will not see eclipses again before impact.

Raising apoapsis allows us to fire tomorrow's maneuver from a greater height, so that the spacecraft can come in steeper, hitting the crater floor without hitting the crater rim on the way in. But it takes a lot of energy to change the orbit that much, and even using all of the usable fuel left, the trajectory will still be shallow, only 6.3 deg below the horizon. Also, the uncertainty in the terrain profile in the region near the south pole is fairly high, so there still is a chance the rim will be higher than expected and the spacecraft will hit it.

Current spacecraft state (0900 GMT 7/30/99, DOY 211):

Orbit: 7046
Downlink: 3600 bps
Spin Rate: 23.8 rpm

Spin Axis Attitude (ecliptic):
Latitude: -88.59 deg
Longitude: 51.71 deg

Trajectory:
Periapsis Alt: 17.1 km
Apoapsis Alt: 234.0 km
Period: 120 min

Occultations: 41 minutes
Eclipses: none
Propellant Remaining: 5.98 kg

Tomorrow night the final burn will be performed. The commands will be loaded an hour before actual burn, since the burn will occur on the far side of the moon, out of radio communication. The burn is scheduled to occur at 09:15 GMT 7/31/99 for impact at 09:51 GMT (02:51 am PDT).