"Lunar Prospector has performed flawlessly over the past year," said Dr. Alan Binder, Lunar Prospector principal investigator and director of the Lunar Research Institute in Gilroy, Calif. "The quality of the data we've gathered is, in some cases, a factor of ten better than that which we promised to NASA at the outset, and we fulfilled all of our science objectives long before this first anniversary." "Lunar Prospector has been an extraordinarily successful mission," said Scott Hubbard, NASA Mission Manager at the Ames Research Center in Mountain View, Calif. "This little spacecraft has returned wonderful science and has proved the concept of 'faster, better, cheaper'."

On December 19, 1998, the spacecraft was commanded into a 40 kilometer (25 mile) lunar polar orbit, down from its 100 kilometer (63 mile) mapping orbit, signaling the transition to the extended mission. The spacecraft will remain in the new 40-km orbit for about 4 weeks, and then be commanded to an even closer 25-30 km (approximately 15 to 19 mile) orbital path later this month. These actions will officially complete the end of the very successful primary mission, which began January 1998.

The extended mission is expected to continue through June 1999, during which time the five instruments onboard will gather additional science data at significantly higher resolutions. These higher resolutions will enable scientists to continue to refine their estimates concerning the concentration and form of hydrogen detected at the north and south lunar poles, which mission scientists interpret as deposits of water ice. Mapping of the Moon's magnetic and gravity fields will also benefit greatly from the lower orbit. Additionally, initial global maps of the Moon's elements will be confirmed with the close-up data.

Lunar Prospector was launched on Jan. 6, 1998, aboard a Lockheed Martin Athena 2 rocket and entered lunar orbit on Jan. 11, 1998. The Lunar Prospector mission is a joint effort of Lockheed Martin Missiles & Space, NASA Ames Research Center, and the Lunar Research Institute. Additional important contributions came from Los Alamos National Laboratory, the U.C. Berkeley Space Science Laboratory, and the NASA Goddard Space Flight Center. The $63 million mission is managed by the Ames Research Center.

Lunar Prospector Science Review

Water Ice at the Poles

The north and south poles of the Moon may contain up to six billion metric tons of water ice, a more than ten-fold increase over previous estimates, according to scientists working with data from NASA's Lunar Prospector mission.

Growing evidence now suggests that water ice deposits of relatively high concentration are trapped beneath the soil in the permanently shadowed craters of both lunar polar regions. The researchers believe that alternative explanations, such as concentrations of hydrogen from the solar wind, are unlikely.

In March of 1998, mission scientists reported a water signal with a minimum abundance of one percent by weight of water ice in rocky lunar soil (regolith) corresponding to an estimated total of 300 million metric tons of ice at the Moon's poles. "We based those earlier, conscientiously conservative estimates on graphs of neutron spectrometer data, which showed distinctive dips over the lunar polar regions," said Binder. "This indicated significant hydrogen enrichment, a telltale signature of the presence of water ice.

"Subsequent analysis, combined with improved lunar models, shows conclusively that there is hydrogen at the Moon's poles," Binder said. "Though other explanations are possible, we interpret the data to mean that significant quantities of water ice are located in permanently shadowed craters in both lunar polar regions.

"The data do not tell us definitively the form of the water ice," Binder added. "However, if the main source is cometary impacts, as most scientists believe, our expectation is that we have areas at both poles with layers of near-pure water ice." In fact, the new analysis "indicates the presence of discrete, confined, near-pure water ice deposits buried beneath as much as 18 inches (40 centimeters) of dry regolith, with the water signature being 15 percent stronger at the Moon's north pole than at the south."

How much water do scientists believe they have found? "It is difficult to develop a numerical estimate," said Dr. William Feldman, co-investigator and spectrometer specialist at the Department of Energy's Los Alamos National Laboratory, NM. "However, we calculate that each polar region may contain as much as three billion metric tons of water ice."

Elemental Composition Maps

In other results, data from Lunar Prospector's gamma ray spectrometer have been used to develop the first global maps of the Moon's elemental composition. The maps delineate large compositional variations of thorium, potassium and iron over the lunar surface, providing insights into the Moon's crust as it was formed. The distribution of thorium and potassium on the Moon's near side supports the idea that some portion of materials rich in these trace elements was scattered over a large area as a result of ejection by asteroid and comet impacts.

Lunar Magnetic Fields

Mission scientists also report the detection of strong, localized magnetic fields. While the magnetic fields are relatively weak and not global in nature like those of most planets, the Moon does contain magnetized rocks on its upper surface, according to data from Lunar Prospector's magnetometer and electron reflectometer. The resultant strong, local magnetic fields create the two smallest known magnetospheres in the Solar System.

These mini-magnetospheres are located diametrically opposite to large impact basins on the lunar surface, leading scientists to conclude that the magnetic regions formed as the result of these titanic impacts. One theory is that these impacts produced a cloud of electrically charged gas that expanded around the Moon in about five minutes, compressing and amplifying the pre-existing, primitive ambient magnetic field on the opposite side. This field was then "frozen" into the surface crust and retained as the Moon's then-molten core solidified and the global field vanished

Gravity Map of the Moon

Using data from Prospector's Doppler gravity experiment, scientists have developed the first precise gravity map of the entire lunar surface. In the process, they have discovered seven previously unknown mass concentrations, lava-filled craters on the lunar surface known to cause gravitational anomalies. Three are located on the Moon's near side and four on its far side. This new, high-quality information will help engineers determine the long-term, altitude-related behavior of lunar-orbiting spacecraft, and more accurately assess fuel needs for possible future Moon missions.

Iron Lunar Core

Finally, Lunar Prospector data suggests that the Moon has a small, iron-rich core approximately 186 miles (300 kilometers) in radius, which is toward the smaller end of the range predicted by most current theories. "This theory seems to best fit the available data and models, but it is not a unique fit," cautioned Binder. "We will be able to say much more about this when we get magnetic data related to core size later in the mission." Ultimately, a precise figure for the core size will help constrain models of how the Moon originally formed.

Lockheed Martin Missiles & Space, based in Sunnyvale, Calif., is a leading supplier of satellites and space systems to military, civil government and commercial communications organizations around the world. These spacecraft and systems have enhanced military and commercial communications; provided new and timely remote-sensing information; and furnished new data for thousands of scientists studying our planet and the universe.