Bracing for Impact
Astronomers prepare to observe Lunar Prospector's
crash into the Moon's south pole
Above: A possible impact trajectory for Lunar Prospector. The pictured terrain is a radar backscatter image, and the target crater is circled. Ground zero is is at lunar latitude 87.7o S and longitude 42o E. Credit: UT Austin Lunar Prospector Impact Page [larger image].
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.
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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 0951 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.
Left: One of several possible Lunar Prospector impact trajectories. Some of the topography is unknown because a portion of the crater is permanently in shadow and optical telescopes cannot see what is inside the darkened area. Similarly, Earth-based radar cannot be used to map the entire area because the tall rim on the Earth facing side of the crater shields the interior from view. The dotted line is an estimate of the crater's assumed symmetric shape where no data are available. Image Credit: UT Austin Lunar Prospector Impact Page [larger image].
"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 or minutes 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."
Right: Click to view a 450 kB computer-simulated animation of the OH vapor plume that might be created by the impact of Lunar Prospector. Credit: UT Austin Computational Fluid Physics Lab.
"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.
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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 ObservationsUpdate: The planned impact time has been revised to 0951 UT on July 31 because of HST scheduling constraints.
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 or minutes 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)."
Left: The nearly-full Moon as it will appear on on the eastern seaboard of the USA at the time of impact on July 31, 1999. Based on lunar phases generated by the US Naval Observatory.
"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: 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.
Destined for a Watery Grave -- NASA scientists have decided to send Lunar Prospector crashing into the Moon's south pole in search of water, June 4, 1999, NASA Science News
Zeroing in on Lunar Ice -- Astronomers explore the Lunar Prospector crash site using radar, June 4, 1999, NASA Space Science News
Lunar Prospector set to make science "splash" -- NASA/Ames press release
NASA Press Release (3 September 1998) -- announcing enhanced estimate of quantity of water on the Moon
NASA Press Release (5 March 1998) -- announcing the detection of ice on the Moon
Lunar Prospector Home Page -- from NASA/Ames
Ice on the Moon -- informative article about lunar water -- where it is and how to find it.
SWAS home page -- from Harvard
McDonald Observatory home page -- University of Texas, Austin
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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