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Lunar Splashdown
July 29, 1999

Sudden Impact

image: NASA

In a radical experiment to determine whether water exists on the moon, NASA will crash a dying satellite, the Lunar Prospector, into a crater at the moon’s south pole. Scientists will focus their telescopes on the scattered debris from the impact, searching for proof of water in the resulting vapor cloud.

While other man-made objects have struck the moon, this will be the first time that anything has been specifically directed to do so. David Goldstein of the University of Texas at Austin, a designer of the experiment, admits the chances of detecting water are small (estimated to be less than 10 percent). But that’s hardly a reason not to try. "The Lunar Prospector mission is going to end at the end of July," he says, when the satellite runs out of fuel. "The spacecraft is going to crash on the moon one way or another, so we might as well crash it in a useful spot... and try to detect water vapor." Prospector will be crashed on July 31, exactly 30 years after Neil Armstrong’s first steps on the moon.

Moon Mist

image: NASA

NASA launched the Lunar Prospector in January 1998 to scan the surface of the moon using six different instruments. It was the first U.S. moon mission since the Apollo 17 in 1972. The satellite is currently in a polar orbit around the moon, which means that it passes over each pole every orbit.

On July 31, as it passes over the lunar south pole, researchers will fire rockets on the far side, thrusting the Lunar Prospector into a collision course with the moon. The impact will be equivalent to crashing a two-ton car at more than 1,100 miles per hour.

If water exists on the moon, then the kinetic energy from the explosion will potentially evaporate the water ice, which will subsequently rise above the site of impact in a gaseous plume. As the water molecules are broken into OH and H by sunlight, the OH molecules will glow and be detectable by several telescope facilities here on earth. The Hubble Space Telescope will also observe the event during the three orbits granted to the project. The water vapor may be detectable for several hours, but data analysis could take days or even weeks if the signal is faint.

Water, Water, Everywhere?

image: NASA

Last spring, the neutron spectrometer aboard the Lunar Prospector detected a significant amount of hydrogen in the dark craters at the moon’s poles. Because hydrogen is a component of water, its detection is believed to indicate that water is present. Data recorded by the Clementine spacecraft, launched in 1994, have suggested that water ice deposits, as deep as 16 feet (4.9 m), may exist at the lunar south pole. But scientists are not certain.

None of the Apollo missions have detected water ice on the moon’s surface. But this might be explained by noting that the Apollo had missions landed in areas where the soil is constantly exposed to sunlight. Data collected from the Arecibo Observatory did not indicate water ice, but the resolution of the equipment may not have allowed the detection of the water ice if it is only present on the surface as small
chunks or crystals.

Stopping on a Dime

"We chose to crash it into the south pole for two reasons," explains Goldstein. First, only the poles are thought to have water ice in craters. The poles are in "cold traps" where the sun never shines and the temperature remains around 100 degrees Kelvin (-280 F; -173 C).

The second reason, Goldstein continues, "is that we don’t think it’s likely that we’ll be able to hit a smaller target, and the north pole has mostly small craters." Flight engineers will aim the Lunar Prospector to crash into what scientists consider the likeliest place for water ice: the huge Mawson crater, which is 51 km (32 miles) in diameter, 2.5 km (1.5 miles) deep, and approximately 60 km (38 miles) from the south pole.

A group of scientists from Cornell University and NASA’s Jet Proulsion Laboratory have developed a new technique, known as radar interferometry, to produce the first three-dimensional topographic images of the lunar polar regions. This work, reported in the journal Science (June 4, 1999) is expected to provide essential data for choosing the optimal crash site.

Moon Base Alpha

image: NASA

Discovering water on the moon "would be really significant for future human explorations of space," according to Goldstein, particularly if NASA were to create a moon base. "A potential moon base would have to be at the poles.... Water there would be helpful for producing rocket fuel, for building a moon base, for providing liquid water for people to consume."

The moon would also be a good launch platform to explore other points in the universe. Because of the moon’s lower gravity and atmospheric drag, as compared with the earth, far less energy would be required to launch the same vehicle from the moon.

A Fitting Tribute

Aboard Prospector is a capsule containing the ashes of the late Eugene M. Shoemaker, a pioneering planetary geologist famous—along with his wife, Carolyn—for the study and discovery of comets. After Shoemaker was killed in a July 1997 auto accident in Australia, University of Arizona Lunar and Planetary Laboratory professor Carolyn C. Porco conceived, designed and produced this memorial honoring him. "It was legend among planetary scientists that Gene’s life-long dream was to go to the moon and study its geology firsthand," Porco said. "At his journey’s end—thirty years to the month after humans first set foot on the moon—Eugene M. Shoemaker will become the first inhabitant of Earth to be sent to rest on another celestial body." Even the crash landing serves as a fitting tribute: Shoemaker was also famous for his work on extraterrestrial impacts.

Elsewhere on the Web

The Lunar Prospector web page at NASA’s Ames Research Center

Be sure to check out the informative tour of the project at NASA’s Observatorium

Keep up to date on observations by the Hubble Space Telescope

Collaborators on the Lunar Prospector Impact Mission include:

At the University of Texas at Austin:
David B. Goldstein; R. Steven Nerem; J. Victor Austin
Edwin S. Barker of the McDonald Observatory

At NASA Ames Reseach Center:
Alan B. Binder, Principal Investigator, Lunar Prospector Mission, and Director, Lunar Research Institute

At Los Alamos National Laboratory:
William C. Feldman

by STN2

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