The mission of NASA's Lunar Prospector will end on July 31, 1999, when ground controllers attempt to direct the spacecraft to impact the surface of the Moon inside a permanently shadowed crater near the lunar south pole.
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.
Scientists hope that the direct impact into a lunar crater will liberate 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 byproduct, OH, would provide definitive proof of what some scientists have long suspected -- the presence of water ice in the lunar polar regions.
"While the probability of success for such a bold undertaking is low, the potential science payoff is tremendous," said Dr. Guenter Riegler, Director of the Research Program Management Division in the Office of Space Science at NASA Headquarters, Washington, DC.
"External peer reviews of this plan have been very favorable, and we have concluded that it is both technically and operationally feasible," Riegler said. "Since the implementation costs are minimal and the mission is scheduled to end anyway, it seems fitting to give Lunar Prospector the chance to provide scientific data right up to the very end of its highly successful mission."
Lunar Prospector was launched on Jan. 6, 1998, with a one-year primary and six-month extended mission to explore the lunar surface remotely. In March 1998, mission scientists announced that science instruments aboard Lunar Prospector had detected sufficiently large quantities of hydrogen at the lunar poles to infer the presence of water ice. In September, scientists estimated that up to six billion metric tons of water ice may be buried in the permanently shadowed craters of the Moon's poles.
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.
"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.
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.
Goldstein and his team will present a detailed description of their proposal in the June 15 issue of "Geophysical Research Letters."
NASA Ames Research Center, Moffett Field, CA
The current plan calls for Lunar Prospector to impact in a permanently shadowed area of an unnamed polar crater at 09:51 UT (5:51 a.m. EDT) on July 31. See the July 28 NASA Press Release for more details.
Ice on the Moon
Lunar Prospector Impact Page - University of Texas
Geophysical Research Letters Article on the Impact Plan
Lunar Prospector Heads for a Watery Grave - NASA Space Science News
Radar Used to Find Best Crash Sites - Cornell Press Release
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