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June 3, 1999: NASA's first mission to the Moon
in 25 years may end with a splash on July 31, 1999,
when ground controllers deliberately attempt to crash
the diminutive Lunar Prospector
spacecraft into a permanently shadowed crater near the
lunar south pole.
Above: Artist's conception of ice in a crater on the Moon. Credit NASA/Ames. Officials at NASA/Ames announced the upcoming maneuver yesterday. Scientists estimate that a direct impact into a lunar crater could dislodge up to 40 pounds of water vapor that may be detectable from ground- and space-based observatories. A positive detection of water vapor or its photo-dissociated byproduct, OH, would provide definitive proof that water ice exists in the lunar polar regions.
Since its launch in January 1998, Lunar Prospector has scored one scientific triumph after another including a precision gravity map of the lunar surface, global maps of elemental composition, the detection of mini-magnetospheres related to large impact sites, and evidence pointing toward a small iron-rich lunar core. Undoubtedly the most tantalizing finding has been the possible detection of substantial water-ice deposits at both the North and South lunar poles. An analysis of data collected so far is consistent with nearly pure water ice deposits - the residue of cometary impacts - buried beneath as much as 18 inches of dry dusty regolith. If initial estimates of the polar water content are correct, then there may be enough water on the Moon to support substantial human colonies provided that it can be efficiently extracted from the regolith. By crashing into one of the shadowed craters at the Moon's south pole, Lunar Prospector will hopefully provide the first incontrovertible proof that water is really there and, perhaps, give some additional indications of its quantity. 
Left: Lunar Prospector uses a method
called neutron spectroscopy to look for water on the
Moon. Neutrons are subatomic particles that are continually ejected
from
the lunar soil by cosmic rays.
In this graphic, the coincident dips in medium-energy neutrons at both lunar poles (see
arrows) is a definitive signature for water.
Based on the extent of the dips, mission scientists
estimate that the total
amount of water on the Moon could be anywhere from 10 million to 6 billion metric tons (2.6 to 26 billion gallons).
More information.
Credit NASA/Ames.
This effort to gain additional science data about the Moon's composition was proposed to NASA by an external team of scientists led by Dr. David Goldstein of the University of Texas in Austin. Although the Lunar Prospector spacecraft will weigh only 354 pounds (161 kilograms) at mission end, the energy at impact will be the equivalent of crashing a two-ton car at more than 1,100 miles per hour.
The current plan calls for a controlled impact of the Lunar Prospector spacecraft in the early morning hours of July 31 directly into a small crater, located at the southern lunar pole. This crater is ideal for the proposed experiment. It is only 31 to 38 miles (50 to 60 kilometers) across and has a rim which is high enough to provide a permanent shadow, yet it is low enough to provide for a suitable spacecraft impact trajectory. Data from other observations suggest that the crater could contain a high concentration of water ice. Finally, the crater is observable at impact time from Earth-based observatories and orbiting platforms. 
Right: This artist's conception of
a simulated cometary impact on a lunar polar region may foreshadow the fate of Lunar Prospector in July.
Credit NASA/Ames.
Observing time has been granted at the University of Texas McDonald Observatory and on the Hubble Space Telescope. It is also being sought at other sites from which the Moon is clearly visible in the early morning hours of July 31. "A positive spectral detection of water vapor or its photo-dissociated byproduct, OH, would provide definite proof of the presence of water ice in the lunar regolith," Goldstein said. However, scientists warn that the failure to observe the desired signal does not mean that water ice is not present. The model could be wrong, the spacecraft may not impact the desired region or the impact energy may be insufficient to liberate an observable plume of water vapor or OH. The overall probability of success is estimated to be about 10 percent. Goldstein and his team will present a detailed description of their proposal in the June 15 issue of "Geophysical Research Letters." Further information about Lunar Prospector can be obtained at the project website at: http://lunar.arc.nasa.gov Lunar Prospector was the first of NASA's Discovery class of "faster, better, cheaper" space exploration missions. The $63 million mission is managed by NASA Ames Research Center, Moffett Field, CA. |
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Lunar Prospector set to make science "splash" -- NASA/Ames press release Lunar Prospector Home Page -- from NASA/Ames Ice on the Moon -- informative article about lunar water -- where it is and how to find it. Lunar Prospects -- Astronomy Picture of the Day, Sep. 18, 1998 Impact Moon -- Astronomy Picture of the Day, Mar. 26, 1999 The Nine Planets: the Moon -- from SEDS |
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