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A Hole In One At 3,800MPH


Greenbelt - July 28, 1999 -
Numerous ground and space-based observatories will assist NASA in a search for water in material hurled upward from the planned impact of the Lunar Prospector (LP) spacecraft in the early morning hours of July 31.

Flight dynamics controllers at NASA's Goddard Space Flight Center, Greenbelt, Md., are guiding the spacecraft to the designed crash inside a crater near the south pole of the Moon. Scientists believe there may be water ice near the surface in the crater bottom, which is in permanent shadow.

Energy from the impact, equivalent to crashing a two-ton car at more than 1,100 miles per hour, may liberate up to 40 pounds of water in vapor form, experts predict.

"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, D.C.

"This impact is unlike the other Moon missions of the 1960's and 70s' where the incoming trajectory was almost perpendicular to the lunar surface," said David Folta, the Goddard Lunar Prospector Team Lead in the Guidance, Navigation, and Control center (GN&C).

The difficulty of this maneuver is that Lunar Prospector is already in orbit about the Moon and the angle of impact is only 6.5 degrees. Achieving this grazing impact is similar to hitting the middle of a pie pan laid on home plate with a baseball thrown from second base at over 100 miles per hour.

In this case, LP is the baseball and the rim of the pie pan represents the rim of the crater, almost four kilometers above the floor of the crater. The tolerance for error in the final trajectory is plus or minus three percent.

Any greater error in the negative direction will result with an impact with the crater rim before reaching its destination. A larger error in the positive direction will result in a complete miss of the targeted crater."

"The Hubble Space Telescope's STIS (Space Telescope Imaging Spectrograph) instrument will monitor the impact site, searching for a glow of ultraviolet light emitted by the OH molecule (which consists of one atom each of oxygen and hydrogen)," said Dr. David Leckrone, senior scientist for Hubble at Goddard.

The OH would be produced by the breakup of water molecules (H2O) by ultraviolet sunlight, which is intense in the vacuum of space. This is the same technique astronomers use to look for the signature of water in comets. The OH emission tends to be bright and easily detected, compared to other indicators of water that would be available to HST.

From its vantage point above the Earth's atmosphere, and with its unsurpassed image clarity and sensitivity, HST is well suited for this job. It will be a difficult observation, but we've taken extra care to assure that it is executed correctly. It's hard to predict what to expect, explained Leckrone.

"But the idea of performing a "controlled" experiment on the surface of the Moon, monitored by an array of earth and space-based telescopes, is exciting in its own right. Certainly, the direct verification of the existence of water on the Moon would be an important milestone in the exploration of the solar system," added Leckrone.

NASA's Submillimeter Wave Astronomy Satellite (SWAS) will join the hunt for water. "SWAS is the only observatory that can directly confirm the presence of complete water molecules by detecting characteristic microwave radiation from water vapor," said Dr. Gordon Chin, SWAS Project Scientist at Goddard.

"The observations will be challenging. We can't follow our normal procedure of pointing the spacecraft at its target using guide stars because the Moon is too bright - it would blind the star sensor. Instead, we must rely on the spacecraft's gyroscopes and a detailed knowledge of the Moon's position relative to SWAS. However, SWAS is extraordinarily sensitive to water. If water is present, and there are no unusual complications, we should detect it even in the small amounts predicted," added Chin.

To reach the crater target, two maneuvers are used, one to raise the highest part of the orbit to increase the incoming angle into the crater and to adjust the time of impact, and another that targets the exact latitude, longitude, and angle of impact.

As with the primary and extended mission, the Goddard GN&C is providing the trajectory design, maneuver generation, and navigation for this event. Folta has only several hours to re-target the final inbound flight before final impact July 31 at 5:51 a.m. EDT using the navigation data provided by Mark Beckman, also of the GN&C.

This effort to gain additional science data about the Moon's composition was proposed to NASA by an external team of engineers and astronomers led by Dr. David Goldstein of the University of Texas at Austin.

"We have had an interest in thin planetary atmospheres for a while and how comet impacts might have led to the accumulation of water ice at the lunar poles," said Goldstein.

"We were thinking about how the Leonid meteorite impacts on the Moon might be seen when we realized that since the Lunar Prospector mission was scheduled to end anyway, Lunar Prospector could become an artificial meteorite in which the time, position and energy of impact would be precisely known. "Our team at the U. of Texas suggested the idea to the Lunar Prospector team, and they were enthusiastic about the opportunity to produce significant scientific results right up until the very end of the mission," added Goldstein.

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