NASA’S LUNAR PROSPECTOR CONFIRMS
PRESENCE OF ICE ON THE MOON

by Brian Ibbotson (Ibbotson@physics.arizona.edu)

On March 5, 1998, scientists at NASA’s Ames Research Center in California announced that the ongoing Lunar Prospector mission confirmed the existence of water ice on the lunar surface. The scientists, led by principal investigator Alan Binder, estimate the find at between 10 and 300 million metric tons of ice. The ice seems to be in the form of small crystals similar to frost intermixed with the lunar regolith, the fine rock dust that covers the surface of the moon. The discovery confirmed both long-standing theories about the presence of lunar ice and recent experimental data that suggested such a presence. Scientists were surprised, however, by the fact that the ice was found at both the North and South poles of the Moon. Previous data had suggested that the Aitken Basin, which encompasses the lunar South pole, might have water ice, but there was no expectation of a discovery at the lunar North pole. Not only was ice found in both locations, but Prospector’s data indicates nearly fifty percent more ice in the North over the South

Amount of Ice Uncertain, For Now
This discovery was made using data collected by Prospector’s neutron spectrometer. This instrument is used to measure the energy levels of neutrons emitted from the lunar surface, which can be classified into three ranges of energies. As these neutrons pass through the material on the lunar surface, ions within the material slow down these neutrons in characteristic ways. Thus, observed differences in the ratios between these three energy ranges provide a ‘signature’ that indicates the passage of the neutrons through a specific substance. While in a North-South lunar orbit, Prospector mapped a 150 km by 175 km footprint of the surface at each of the poles. In each footprint, the spectrometer recorded declines in the frequency of epithermal neutrons - neutrons occupying the middle of the three ranges - emitted, consistent with the presence of small crystals of water ice. These declines further suggest that the ice is spread over a wide area of the lunar surface and is not concentrated. The amount of lunar ice present is not precisely known at this point, however. The wide estimate of 10 to 300 million metric tons arises from a combination of the Prospector data and scientists’ assumptions regarding the origin of the ice. Much of the ice present in the lunar regolith is assumed to have been deposited through meteorite impacts over several billion years, and previous research has suggested that such impacts have changed the nature of the lunar regolith through a depth of 2.0 meters. Thus, while the neutron spectrometer is sensitive only to a depth of 0.5 meters, scientists suspect that the lunar ice may be intermixed in the regolith down to 2.0 meters. Further observations using the neutron spectrometer combined with new data from the use of additional instruments, such as Prospector’s gamma ray spectrometer, will allow scientists to confirm data already collected and further test conclusions already drawn. Higher-quality data will be available in early 1998, when the mission will enter an extremely low orbit-only 10 km above the surface-that will allow highly-detailed measurements to be made.

Implications of Lunar Ice
The presence of water ice on the Moon was first proposed in 1961 by Kenneth Watson, Bruce C. Murray, and Harrison Brown at the California Institute of Technology. Since that time, many scientists (and science fiction writers) have speculated about how such a presence would impact a manned space program. The most common and best-grounded claims are that the presence of a large quantity of water on the moon would allow a manned station to reclaim potable drinking water and to electrolyze the water into hydrogen and oxygen: oxygen for breathing and hydrogen and oxygen together for use in rocket fuel. Thus, the availability of oxygen and fuel supplies on the Moon would open the possibility of using the Moon as a siting station for the launch of manned or unmanned missions. Given the 85 percent lower gravity on the lunar surface, such launches could be achieved at a far lower cost in fuel than from the Earth’s surface. However, any attempt to move from a proof-of-concept demonstration of the utility of lunar ice for the above purposes to the implementation of a full-scale mining of the surface and actual use of the water would face tremendous logistical and political hurdles. The logistical problems are certainly formidable, as the location of ice at the lunar poles would necessitate building a system to transport either the raw ice or its reclaimed products across large distances, as any base or colony would likely be situated nearer the Moon’s equator. The political hurdles are enormous as well. The Outer Space Treaty, which is governed by the United Nations and to which the United States and the other space-faring states are signatories, would prohibit any state from asserting a claim on the use of the lunar ice. Any change to this policy that would allow NASA to proceed with mining would be difficult for the United States to negotiate. Domestically, environmental organizations would certainly be opposed to any large-scale mining operation on the lunar surface. Internationally, the increasing frustration of Third World countries over their lack of access to orbit would likely lead them to vote against such an authorization for any developed nation.

Renewed Emphasis on Lunar Missions
Prospector is the first NASA mission to the Moon since the termination of the Apollo program in the mid-1970s. During the ‘golden age’ of lunar exploration and observation, both the United States and the Soviet Union launched a variety of manned (Apollo) and unmanned (Luna, Ranger, Surveyor and Lunar Orbiter ) spacecraft. For nearly two decades, however, public attention has been focused elsewhere: on the Space Shuttle, missions to Mars and intrasolar probes such as the Voyager series. In recent years the Moon has once again caught the attention of the public and of policymakers. In 1994 the Celementine spacecraft, launched by the Department of Defense’s Ballistic Missile Defense Organization, visited the Moon and conducted radar experiments that suggested that the lunar South pole might contain ice. In the wake of such evidence, Lunar Prospector was launched in January 1998, as part of NASA’s Discovery Program of ‘faster, better, cheaper’ spacecraft. Several other missions to the Moon are in the planning stages, including a Japanese mission currently in development for the year 2004. There has been talk of NASA proposing a manned mission to the Moon for sometime after the year 2020, but along with a manned mission to Mars this will likely remain mere speculation until after the International Space Station is complete in 2005. It will be interesting to see over the next few years how the discovery of lunar ice by Prospector will impact on the discussion of a manned return to the Moon.-