While professionals watch for a nearly transparent cloud of water vapor, amateurs will monitor the Moon's south pole for visible signs of Lunar Prospector's crash.

 

July 30, 1999: This Saturday morning at 2:52 a.m. Pacific Daylight Time (09:52 UT), Lunar Prospector will plunge into a crater near the Moon's south pole in search of frozen water. The Hubble Space Telescope, the Keck telescope in Hawaii, and many others will be trained on the Moon as astronomers look for signs of water vapor expelled from the crash site. These observations require large telescopes equipped with sensitive spectrometers. Although amateurs won't be able to see the tenuous vapor cloud -- ultraviolet and infrared spectrometers are required for that -- there's a slim chance that a plume of lunar soil and spacecraft debris might be visible through amateur telescopes just after Lunar Prospector slams into the Moon. If the debris makes it over the crater's obscuring rim, the plume would briefly shine by means of reflected sunlight (just like the rest of the Moon) and it could persist for as long as several minutes glowing faintly against the black sky just above the lunar limb.

Right: On February 20th, 1965, the Ranger 8 spacecraft crashed into the Moon. Its camera recorded this picture at an altitude of about 11 kilometers, 5 seconds before striking the lunar surface. No credible report of an impact sighting was made.
 

 

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Dr. David Goldstein, who leads the University of Texas team that proposed the impending crash of Lunar Prospector, estimates that if 10 kg of typical lunar soil are ejected from the crater into a dust cloud 20 km in diameter, then the plume will be 10,000 to 100,000 times fainter than the surface brightness of the sunlit lunar limb. The total brightness of Goldstein's most optimistic theoretical plume would be roughly equivalent to an 8th magnitude star.

"The faintest stars you can see very close to the limb of a full moon are 3rd or 4th magnitude," says David Dunham of the International Occultation Timing Association. "An 8th magnitude plume is about 100 times too faint even for a good telescope."

Will amateur astronomers really be able to see anything?

"It's doubtful," says Lunar Prospector's principal investigator Dr. Alan Binder, "but I would encourage anyone to try."
  According to Goldstein, theoretical calculations of the debris plume's brightness shouldn't be taken too seriously. It's difficult to estimate precisely how much debris will be in the plume, how high the plume might rise, and what the reflectivity and sizes of the dust particles might be. It's conceivable that the debris plume could be much dimmer or much brighter than expected.

Left: A frame from a 2MB MPG animation of the scheduled crash from the UT Austin Lunar Prospector Impact Page

Undaunted by the slim prospects for a detection, hundreds of amateur astronomers plan to monitor the Moon for signs of impact early Saturday morning. The Association of Lunar and Planetary Observers (ALPO), the American Association of Variable Star Observers (AAVSO), and members of the International Occultation Timing Association (IOTA) are participating in the observing campaign. Many amateurs are highly skilled and respected observers. With fast-improving CCD and video imaging technology at affordable prices, they have access to equipment which rivals that found in many professional observatories. If a unexpectedly bright plume arises, amateurs somewhere will almost certainly record it.

Observing tips for amateurs

 

Questions about how to observe the Moon?   ASK A LUNAR OBSERVER   (Sponsored by the Association for Lunar & Planetary Observers)

The impact will not be visible to the naked eye. A telescope -- the bigger the better -- is a necessity for serious impact observing. (If you simply want to marvel at the beauty of the nearly full moon, the naked eye is fine.) Dry air observing sites are best. If the air is humid it will scatter the moonlight, greatly increasing the brightness of the sky, and making a faint plume that much more difficult to see.

The moment of impact, 0952 UT on July 31, was chosen so that the Moon would be visible at night from Hawaii and Texas where important telescopes are located. The best potential viewing sites for the collision range eastward from the mid-Pacific to central North America. The table below shows the local time of impact and where the Moon will be in the sky for representative cities from Australia to New York.
 

location	local time	where to look for the Moon
Sydney, Australia	19:52	the Moon rises 6 minutes after impact
Christchurch, NZ	21:52	nearly due east, 17o above the horizon
Mauna Kea, Hawaii	23:52 on 7/30/99	37o above the southeastern horizon
San Francisco, CA	02:52	almost due south, 42o above the horizon
Austin, TX	04:52	48o above the south-western horizon
Huntsville, AL	04:52	40o above the south-western horizon
New York, NY	05:52	28o above the south-western horizon, with a rapidly brightening dawn sky

For amateur observations to have lasting 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. 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.
  Right: A CCD image of the Moon's south polar region obtained by ALPO member Charles Shirk of Dayton, Ohio on August 15, 1994 using a 10 inch Schmidt-Cassegrain telescope. The lunar libration angles when this picture was taken were +1^o in longitude and -1.6^o latitude. On July 31, 1999 the values will be -5^o and +3^o, respectively.

"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 altazimuth 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 altazimuth 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."

Note to Observers: All forms of amateur data including images and video may be sent to Dr. Tony Phillips, who will forward them to the appropriate scientists at NASA and the University of Texas at Austin.

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. 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."

Please visit the Lunar Prospector project web site from NASA/Ames and the Lunar Prospector impact page from UT Austin for more information about the Lunar Prospector mission and its impending crash into the Moon. See also: LunarImpact.com for science news, images and impact observing hints for amateurs.

Lunar Prospector was the first of NASA's competitively selected "faster, better, cheaper" Discovery-class missions. The $63 million mission is managed by NASA Ames Research Center, Moffett Field, CA.

 

Web Links

 

A stay of execution for Lunar Prospector -- Lunar Prospector survived the July 28, 1999 lunar eclipse and is on track for a July 31, 1999 collision with the Moon , July 28, 1999, NASA Science News

Lunar Prospector in Eclipse -- The July 28, 1999 partial lunar eclipse poses a last-minute threat to Lunar Prospector. , July 26, 1999, NASA Space Science News

Bracing for Impact -- Astronomers prepare to observe the crash of Lunar Prospector on July 31, 1999. Includes observing hints for amateurs, July 21, 1999, NASA Science News

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

More NASA Science News

 

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For more information, please contact: Dr. John M. Horack , Director of Science Communications

Author: Dr. Tony Phillips Curator: Linda Porter NASA Official: M. Frank Rose