Lunar Prospector Mission

Date January 6, 1998-July 31, 1999

The Lunar Prospector mission was developed as part of NASA’s Discovery program to launch frequent, low-cost spacecraft to the Moon and to other planets. This reconnaissance mission was designed to locate water ice and other natural resources that could be used to sustain a future human lunar base.

Locale Polar orbit around the Moon

Key Figures

• G. Scott Hubbard (b. 1948), NASA mission manager at the NASA Ames Research Center
• Alan Binder (b. 1940), project principal investigator at the Lunar Research Institute

Summary of Event

Following the historic Apollo manned lunar landings from 1969 to 1972, only four unmanned Soviet probes were sent to the Moon between 1973 and 1976. Three were landers, with the Luna 24 mission returning a small sample of lunar regolith to Earth, and the fourth was a lunar orbiter. With no hope in sight of returning to the Moon, scientists had to be content with analyzing the data returned from these missions. Then, in 1989, U.S. president George H. W. Bush announced plans for a return to the Moon and for the human exploration of Mars. This sparked interest in lunar exploration and identified the need for more detailed information on lunar surface materials. The Lunar Orbiter and Apollo missions primarily obtained data from areas close to the lunar equator, which had been selected as potential landing sites. As there was no immediate need to study other areas, very little attention was given to the higher latitudes.

Although the Apollo manned missions to the Moon provided scientists with more than 380 kilograms (840 pounds) of moon rocks and dust samples to analyze, their studies raised more questions than they answered. It quickly became clear that scientific interest in the Moon would not end with Project Apollo. Lunar exploration had to continue, and the National Aeronautics and Space Administration (NASA) created the Lunar Exploration Science Working Group to develop a list of the most important unanswered scientific questions about the Moon. In 1992, the group produced a document titled “A Planetary Science Strategy for the Moon.” The document raised various questions: How did the Earth-Moon system form? How did the Moon evolve? What is the impact history of the Moon’s crust? Based on questions like these, a set of lunar science objectives was generated, and mission designers were charged with the development of instrumentation to collect the necessary data to answer these questions. As a result, two new lunar orbiter missions, Clementine and Lunar Prospector, were designed to provide that data.

Lunar Prospector was a relatively small spacecraft that, when fully fueled, weighed only 295 kilograms (650 pounds). It was also a remarkable spacecraft since it moved from the design and development stage to the completion and testing phase in only twenty-two months. It proved to be a relatively inexpensive design and was simple to operate. One of the many factors that made this possible was the fact that it was manufactured from “off-the-shelf” hardware that had been flown on previous missions.

Lunar Prospector was launched on January 6, 1998, and entered lunar orbit on January 11, beginning an eighteen-month mission. Its primary objectives were to map lunar resources, measure magnetic and gravitational fields, search for possible outgassing events, and locate water and ice deposits in lunar surface materials. To acquire the necessary data, Lunar Prospector carried five scientific instruments that include a gamma-ray spectrometer, an alpha particle spectrometer, a neutron spectrometer, a magnetometer, and an electron reflectometer. The spacecraft was spin-stabilized and placed into polar orbit with a period of 118 minutes at an altitude of 100 kilometers (about 63 miles). Lunar Prospector would pass over a lunar pole every hour, and it would completely cover the lunar surface twice each month as the Moon slowly rotated beneath its orbit. Lunar Prospector was thus able to map the entire lunar surface.

While in orbit, Lunar Prospector diligently performed its various experiments. Perhaps its most exciting discovery was the identification of the presence of water ice at the lunar poles. This data was obtained from the neutron spectrometer that can sense water ice by first identifying and then calculating the amount of hydrogen present in the lunar regolith. During a press conference on March 5, 1998, the Lunar Prospector mission team announced its findings and suggested that as much as 10 to 300 million tons of water ice may be scattered across areas near the lunar poles. It was anticipated that water ice would be found in the Aiken Basin of the lunar south pole, but finding water ice at the lunar north pole came as somewhat of a surprise. In fact, Lunar Prospector detected 50 percent more water ice at the north pole than at the south pole. Although the water ice experiment gave a positive result, it was not designed to determine in what form the water ice is present in the lunar regolith. The most likely guess is that the water ice exists as very tiny crystals dispersed over a very large area.

The end of the Lunar Prospector mission came on July 31, 1999. In an attempt to resolve the question of the presence of water ice, mission controllers decided to direct the Lunar Prospector to crash into one of the deeper craters at the lunar south pole. It was hoped that this impact would melt any water ice present and release a water vapor cloud that could be detected directly from Earth-based telescopes or a orbiting satellites. A positive result would have provided conclusive evidence that water ice does exist deep within craters at the lunar poles. The impact occurred exactly as planned, but the anticipated result did not happen; definitive proof would have to wait for another mission. Scientists did not see this as a negative result that suggests that water ice does not exist in the polar regions of the Moon. It simply showed that in a particularly ambitious experiment with a low probability of success, the experiment did not provide scientists with the necessary data to draw a conclusion. In any case, the Lunar Prospector mission was a complete success and paved the way for scientists to plan for future human expeditions to the Moon and beyond.

Significance

The Lunar Prospector mission was part of NASA’s new Discovery-class space missions. These low-cost spacecraft were designed to provide scientists with a greater number of missions that addressed a few basic questions, in contrast to the more ambitious and very expensive Viking or Voyager missions. The Lunar Prospector mission was competitively selected and was the third mission to fly in the Discovery series. Lunar Prospector was the only NASA-funded space project that was conceived and managed by a scientist from outside the agency. Dr. Alan Binder was the principal investigator and, for thirteen years, was personally involved in every aspect of the mission.

Scientifically, the Lunar Prospector mission returned data on the chemical composition of lunar surface material and measured the Moon’s magnetic and gravitational fields. It also looked for the presence of gases that may be emitted from fissures in the crust or from possible recent volcanic activity. From its polar orbit, Lunar Prospector was able to construct a more complete and detailed map of the lunar surface. Its most significant result was the indication that water ice may be present at both the north and south poles. This is significant for human occupation of the Moon since the water ice could be used as a source of drinking water, oxygen to breathe, or hydrogen for use as a fuel for spacecraft.

Ultimately, the success of the Lunar Prospector mission was not just measured by the return of valuable data on lunar surface materials. It also proved that when missions are properly managed, the cost of space exploration could be cut by a factor of ten. Much to the dismay of many NASA administrators, the success of Lunar Prospector brought to light an inherent incompetence and the self-serving activities of NASA bureaucracy and the various aerospace industries involved in the design and construction of its spacecraft.

Bibliography

Binder, Alan B. Lunar Prospector: Against All Odds. Tucson, Ariz.: Ken Press, 2005. A firsthand account from the mission’s principal investigator, the book sheds light on the politics and bureaucracy behind NASA’s space projects.

‗‗‗‗‗‗‗. “Lunar Prospector Overview.” Science 281 (September, 1998): 1475-1476. Overview article that summarizes the mission objectives and provides comments on the significance of the initial data returned from the Moon.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2005. Offers a detailed examination of the various objects in Earth’s solar system and how they relate to one another.

Schmitt, Harrison H. Return to the Moon. New York: Copernicus Books, 2006. Written by one of the twelve astronauts who walked on the Moon, the book offers a comprehensive study of the Moon’s potential energy resources and the future of lunar exploration.

Wilhelms, Don E. To a Rocky Moon: A Geologist’s History of Lunar Exploration. Tucson: University of Arizona Press, 1993. Although written before the Lunar Prospector mission, offers insight into the geological nature of the Moon and discusses goals for future lunar missions.