K3PGP Experimenter's Corner
Lunar Prospector Status Report #25
March 18, 1998 - 7:00 p.m. EST (4:00 p.m. PST)
The Lunar Prospector spacecraft continues to perform nearly flawlessly, and all instruments are collecting good data, according to Mission Control at NASA's Ames Research Center. On the evening of Thurs., March 12 (PST), the Moon (and the spacecraft circling it) were participant to a penumbral lunar eclipse, in which sunlight (shining on the Moon) was partially blocked by the positioning of the Earth relative to the Sun. In general, a lunar eclipse takes place when the Moon falls -- either partially or entirely -- into the Earth's shadow. However, during what's called a "penumbral" lunar eclipse, in which the Moon enters an only partially darkened region of the Earth's shadow (called the penumbra), the entire Moon is thus only partially shadowed, dimming the lunar surface, and consequently, diminishing solar array current on an orbiting spacecraft. This is in contrast to a "partial total" lunar eclipse, when only a section of the Moon falls into the Earth's completely shadowed region (called the umbra), causing that wedge of the Moon to be completely blocked from sunlight. (See Lunar Prospector Status Report #21 for a diagram depicting penumbral lunar eclipses). During the March 12 penumbral eclipse, mission controllers noted a predictable decrease in solar array current. In addition, Prospector's battery showed a slight discharge before the spacecraft entered its usual 46-minute eclipse while traversing the night side of the Moon. Recharge following the eclipse was nominal.
The current state of the vehicle (as of 4:00 p.m. (PST) on Wed., March 18, 1998), according to Mission Operations Manager Marcie Smith, is as follows:
General
Spacecraft Orbit Number: 797
Data Downlink Rate: 3600 bps
Spin Rate: 11.95 rpm
Spin Axis Altitude: (see below)
Trajectory
Periselene: 84 km
Aposelene: 115 km
Period: 118 minutes
Inclination: 90.4 degrees
Occultations: 40 minutes in duration
Eclipses: 46 minutes in duration
On Fri., March 13 (PST), mission controllers reset the spacecraft's spin axis approximately 2 degrees, to position the Sun just above the top part of the spacecraft, for reasons of thermal control (since the booms are located on the bottom part of the spacecraft, they could potentially shadow some of the solar panels if the Sun were shining on the bottom of the spacecraft).. Also on that day, controllers trimmed the spin rate, in order to correct for small changes which had perturbed it during the orbit trim maneuver performed a week earlier. The precise command timeline was as follows:
Fri., March 13, 12:00 p.m. (PST)
Thruster heaters on
Fri., March 13, 12:04 p.m. (PST)
Maneuver parameters loaded
Fri., March 13, 12:26 p.m. (PST)
Thruster A1 and A4 fired (13 pulses) to
adjust spin axis 1.7 degrees
Fri., March 13, 12:31 p.m. (PST)
Thruster heaters on
Fri., March 13, 12:40 p.m. (PST)
Maneuver parameters loaded
Fri., March 13, 12:50 p.m. (PST)
Thruster T1 fired for 0.81 seconds
Fri., March 13, 12:51 p.m. (PST)
Thruster parameters resest
Mission controllers are still investigating the results of the spin axis re-orientation by reviewing spacecraft attitude (positioning) data both before and after the maneuver. The spin trim was exactly on target, adjusting the spin rate from 12.17 rpm to 11.95 rpm.
Alison Davis
Lunar Prospector Mission Office
NASA Ames Research Center
Moffett Field, Calif. 94035
Contents of this website are ©1997-98 of K3PGP and of the originating authors.