Bracing for impact

July 21, 1999

In 1998, a spectrometer on board NASA's Lunar Prospector spacecraft discovered tantalizing signs of frozen water at the Moon's north and south poles. The ice deposits, presumably the residues of ancient comet and meteorite impacts, are located in places that are permanently shadowed from sunlight by tall crater rims. Project scientists estimate that at least 200 million metric tons of water in the form of ice crystals are mixed with the top 18 inches of soil near the poles. If these figures are correct, there might be enough water on the Moon to support substantial human colonies.

Not everyone is convinced that "Moon water" really exists. Skeptics note that what the spectrometer on Lunar Prospector actually detected was hydrogen, and that there is no guarantee that the hydrogen atoms are bound up in water molecules. The debate -- hydrogen vs. water -- is not merely an academic one. The course of human space exploration may ride on the answer.

Earlier this year Dr. David Goldstein of the University of Texas at Austin and colleagues suggested an unorthodox way to settle the question: Simply crash Lunar Prospector into a shadowed crater and see if any water flies out of the impact site. The Lunar Prospector mission is nearly over, they reasoned, and the spacecraft would eventually collide with the Moon anyway. A kamikaze-style crash into a polar crater could liberate up to 40 pounds of water vapor that might be detectable from ground- and space-based observatories. A positive detection of either water vapor or OH (the hydroxyl radical formed when solar ultraviolet radiation frees a hydrogen atom from water) would provide definitive proof that water ice exists.

In June 1999 NASA accepted their proposal and scheduled the spacecraft to plunge into a permanently shadowed crater near the Moon's south pole on July 31, 1999.

"While the probability of success for such a bold undertaking is low, the potential science payoff is tremendous," said Dr. Guenter Riegler, from the Office of Space Science at NASA Headquarters.

If all goes as planned, the 354 lb spacecraft will enter the unnamed crater on July 31, 1999, traveling at a speed of 3800 mph, and slam into the crater floor at 0952 UTC. The approach angle will be about 6 degrees from horizontal, meaning that the incoming craft will barely clear the crater's rim. Impact is slated to occur at night while the Moon is visible from Texas and Hawaii where important ground-based telescopes are located.

"In the best case scenario, the spacecraft will hit in a place where there's ice mixed with the lunar soil," says Lisa Chu-Thielbar, the Lunar Prospector Mission Office Outreach Coordinator. "In the first few seconds after impact there will be a plume of soil that might be seen by large telescopes. It depends on how much soil is ejected and whether it rises over the lip of the crater."

"You can think of Lunar Prospector hitting the crater floor as a person doing a running belly flop into a pool. Much of the splash will be forward and to the sides," says Goldstein. "When the spacecraft hits it could produce as much as 18 kg of water heated to 400 K. There will be a sort of splash that will distribute the mixture of soil and water over an area of several square kilometers around the impact site. Water vapor will then begin to rise off the surface and out of the crater, which is about 4 km deep. If the water molecules are moving at their thermal velocity, 1100 m/s, the vapor cloud will start to be visible above the crater's rim about 4 seconds after impact."

"Almost immediately, UV rays from the sun will begin to break up the water into H (hydrogen) and OH (hydroxyl)," Goldstein continued. "If the column density is high enough, solar fluorescence will cause the OH molecules to be visible to telescopes with UV spectrometers. The gaseous plume is going to rise up for about 16 minutes and then fall back to the lunar surface in the same amount of time. The material will hover near the apex of its trajectory for a little while and that's when we hope to catch some of the brightest emission lines. As the cloud falls back down to the surface it will still be predominantly water. It will then form a little atmosphere, or 'exosphere' 50 - 100 km high that will last for an hour or more."

Goldstein and collaborators have been granted time on the Hubble Space Telescope, the 107" telescope at the McDonald Observatory in Texas, and the Keck telescope in Mauna Kea to search for spectral lines from fluorescing OH just after the impact. NASA's Submillimeter Wave Astronomy Satellite (SWAS) will also be watching. Scientists using that satellite will attempt to detect water directly by looking for spectral line emission at a wavelength of 538.2 microns.

Mission scientists caution that any clouds of water vapor and hydroxyl will be very tenuous, and it may not be immediately obvious whether or not these gases were detected. Data analysis could take up to 3 months.

More information about the upcoming impact of Lunar Prospector may be found at UT Austin's Lunar Prospector Impact Page. The UT Austin impact team includes Dr. David Goldstein, Dr. Edwin Barker, Prof. Steven Nerem, Mr. J. Victor Austin.

Amateur Observations

The Association of Lunar and Planetary Observers (ALPO), a group of amateur and professional astronomers, has issued a call for all lunar enthusiasts to monitor the south polar region of the Moon on July 31 for visible signs of Prospector's impact. Observations of all types are invited - written, sketched, photographic, and electronic - and observers are encouraged to report their results to ALPO.

Will amateur astronomers really be able to observe the crash?

"It's doubtful," says Lunar Prospector's principal investigator Dr. Alan Binder, "but I would encourage anyone to try."

Few, if any, non-professionals have the spectroscopic hardware required to detect water vapor at infra-red wavelengths or hydroxyl in the ultraviolet band. However, there is a chance that amateurs with large telescopes might be able to see a visible debris plume for a few seconds just after impact. The plume, consisting of lunar soil possibly mixed with rapidly vaporizing ice, would come into view for just a few seconds above the crater's rim.

For such observations to have scientific value, they must be recorded photographically or with a CCD, and the image frames should be accurately time-tagged. A suitable recording setup would consist of an 8 inch or larger telescope equipped with a CCD video camera such as the Astrovid 2000. The camera's output should be recorded on the video track of a VCR. The audio track can be used to record time information. The easiest way to do this is to tune a shortwave receiver to WWV, which transmits time signals at 2.5, 5, 10, 15, and 20 MHz, and route the audio output of the receiver to the audio input of the video recorder. To hear what a WWV signal sounds like, you can phone (303) 499-7111. Another good source of time information is the Canadian radio station CHU which broadcasts at 3.330 MHz and 7.335 MHz.

"If an impact plume is detectable by amateurs, I suspect that it will be recorded by those equipped with astro-video equipment," says Bill Dembowski, the ALPO Coordinator for Lunar Topographical Studies. "Not only are these cameras quite sensitive, but they allow the recording of hundreds of images in a short space of time which can later be searched and enhanced to show very faint details."

The picture above, captured by ALPO lunar observer Charles Shirk of Dayton, Ohio, shows what the Moon's south pole will look like to observers on Earth on July 31, 1999.

"At the end of this month the Moon will be higher in the sky than it was on August 15, 1994 when I took that picture," says Shirk, "so viewing access to the impact site is slightly more favorable in spite of the differences in the libration values. I do not expect to see the actual impact of Lunar Prospector, but I will still be watching for any rising debris plumes tangent to the lunar limb."

Shirk, an experienced lunar observer, offers these words of advice to novice Moon watchers on July 31st:

"From the Eastern Daylight time zone, the impact time places the Moon in the SW-WSW sky with its polar axis rotated clockwise by approximately 30 degrees from vertical (see below)."

"Depending on the telescope used, some images will be reversed left-to-right or top-to-bottom. Therefore, users of alt-azimuth mounted scopes which invert the image (top-to-bottom) should look along the 11 o'clock position on the limb through the eyepiece. Operators of alt-azimuth mounted scopes which reverse the image (left-to-right) should look along the 5 o'clock position on the limb through the eyepiece."

"Operators using equatorial mounted scopes should move the scope southward to the farthest southern limb, and they will be close enough to the target area to enclose the environs of of the impact crater even at magnifications of 200x."

"For others having Moon maps or charts," concludes Shirk, "simply draw a line between the lunar-eastern rims of craters Maginus and Moretus and where that line crosses the limb of the Moon is very close to the location of the impact site."

"Those of us in the east will be at a distinct disadvantage," adds Dembowski, "because the anticipated time of impact will occur at 5:52 am EDT which places the Moon in a relatively bright sky. The farther west an observer is, the darker the skies and the higher the Moon will be in the sky (which makes for steadier viewing). Eastern observers may benefit from the use of a red filter to darken the sky where necessary."

A fitting end

Mission scientists emphasize that the failure to observe a plume, by professionals or amateurs, does not signify a lack of water on the Moon.

"There could be water there but in the form of hydrated minerals," says Goldstein, "in which case it would be much harder to extract. Our impact would have enough energy to vaporize water ice, but it shouldn't have enough energy to separate the water from minerals."

According to the UT Austin Lunar Prospector Impact Web Site: "A negative result will tell us nothing one way or the other about potential lunar water resources. The spacecraft could miss the crater entirely; it could impact high up the inner rim; it could miss a water deposit. Many things could go wrong. Still, it is befitting of this extremely productive little spacecraft, that even in its final act, Lunar Prospector may serve yet once more as a source of knowledge about our Moon."
Further information about Lunar Prospector can be obtained at the project website at:

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.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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