Prospector's Grand Finale

by John E. Gruener
Hernandez Engineering

As Lunar Prospector's primary mapping mission begins to wind down, Dr. Alan Binder and his co-investigators are already in the final stages of planning the second phase of this already very successful mission (see the September 4 issue of Science for the latest scientific results). This second phase, sometimes referred to as the 'extended mission,' will involve lowering the orbital altitude of Lunar Prospector to obtain data with better spatial resolution and higher sensitivity. However, before this second phase begins, Prospector will be commanded to perform several maneuvers.

Lunar Prospector continues to orbit the Moon in a 100 km circular polar orbit, with the spacecraft spin axis roughly perpendicular to the ecliptic plane. Prospector's current attitude, and the attitude it has been in throughout the mission, is such that when it is over the Moon's north pole, the white conical communications antenna points away from the lunar surface (in the 'plus Z' direction), and the blunt end, or bottom, of the spacecraft faces the Moon. On October 5, a two step process will flip Prospector 180 degrees. First, the spin axis will be rotated 90 degrees such that Prospector's three booms will 'roll' around the Moon, as if they were spokes on a wheel. Then on October 12, Prospector will be rotated another 90 degrees and it's spin axis will once again be perpendicular to the ecliptic, though the spacecraft will now be 'upside down.'

This is being done so the Prospector science team will better understand the response function of the science instruments. Though the instruments don't need any pointing, hence their compatibility with a spinning spacecraft, their data do exhibit non-symmetric responses which vary with latitude. Computer modeling can account for these responses, but the science team would like the chance to verify the modeling with data collected from a different spacecraft-instrument-Moon geometry. Also, the south-facing, or 'minus Z,' face of the alpha particle spectrometer (APS) was damaged before reaching the Moon (probably during separation from the trans-lunar injection stage), and thus the high northern latitudes and polar region of the Moon have less 'alpha coverage' than the rest of the lunar surface. Flipping the spacecraft will allow the opposite, or 'plus Z,' face of the APS to better map the alpha particles coming from the northern regions. Finally, during the Leonid meteor shower, the bottom of the spacecraft will be pointed in the direction of the oncoming particles to protect the electronics and solar arrays from critical damage.

On December 19, Prospector will perform several maneuvers that will lower its orbit to a circular altitude of 40 km above the lunar surface. It will remain in this orbit for about a month so that Binder and the others can be certain that the updated lunar gravity model (made with Prospector data collected earlier in the mission) accurately predicts Prospector's orbital motion. Once comfortable with the spacecraft's performance and the gravity model's accuracy, Prospector will be commanded in mid-January to again lower it's orbit, this time down to a final altitude of 25 km above the lunar surface. At this altitude, the spatial resolution of the neutron, gamma ray, and alpha particle spectrometers will be approximately 40 km (as compared to the 150 km resolution during the primary mapping altitude of 100 km). Also, the sensitivity of the gravity and magnetic experiments will increase with the square and the cube of the spacecraft's altitude, respectively. So, when Prospector's altitude decreases from 100 km to 25 km (a four fold decrease), the sensitivity in the gravity data will increase by 16 times, and the magnetic data will have 64 times better sensitivity.

Of course, at an altitude of only 25 km above the lunar surface, the gravitational perturbations to Prospector's orbit will be great and the circular orbit will quickly degenerate into an elliptical one. Current modeling of the orbit suggests that within two weeks the orbit should be a 10 x 40 km ellipse. At that time, orbital adjustment maneuvers will be performed, raising Prospector's orbit back to a circular altitude of 25 km. This two week cycle will continue until all of the onboard fuel is spent (or until the mission runs out of money, which ever comes first). Once Lunar Prospector loses the ability to reboost its orbit or control its attitude, the inevitable impact into the lunar surface will quickly follow. Current predictions have this impact occurring sometime in July 1999. When all is said and done, Lunar Prospector will have orbited the Moon more than 5000 times.

For the engineers and politicians in the space exploration community, Lunar Prospector will be remembered for many things; simple design, low cost, quick production schedule, small work force and streamlined management style, and maps of potential lunar resources for use in future explorations of the Moon. However, in the planetary science community, the real legacy of Lunar Prospector will be its scientific data sets, which will continue to be refined and studied for many years. Correlations between the lunar samples gathered during the Apollo program and Prospector's remote sensing data will play an integral role in the efforts to better understand our nearest planetary neighbor. One can only hope that the recent success of the Lunar Prospector and Clementine missions will rekindle interest in the Moon, and set the stage for our return to the lunar surface in the 21st century.

You can follow the Lunar Prospector Mission at: http://lunar.arc.nasa.gov