Astronomers are looking forward to July 31. That's when NASA's Lunar Prospector probe, currently orbiting the Moon, is scheduled to crash into a crater near its south pole.
Lunar Prospector was launched in January of 1998 and has been orbiting the Moon since. But now that it's life is nearing its end it will crash into the crater to try and find out if there really is frozen water on the moon.
Lunar Prospector detected hydrogen -- a possible sign of frozen water -- at the Moon's north and south poles last year. However, it remains unclear whether the hydrogen is bound up in water molecules. The upcoming crash may resolve that issue. If water ice is present at the impact site, water vapor will be released into space and detected by ground-based and space-based instruments.
The idea for the crash came from Dr. David Goldstein of the University of Texas at Austin and his colleagues. They figure a crash into the moon, observed by space and land-based telescopes, could liberate up to 40 lb of water vapor.
Such confirmation could have far-reaching implications. Lunar ice deposits eventually may serve as a water supply for human colonies on the moon.
The July 31 impact will be observed by the Hubble Space Telescope, the McDonald Observatory in Texas, Hawaii's Keck Telescope, and other facilities. Amateur astronomers are expected to be watching the Moon's south pole region on that date as well, although it is unclear whether their instruments will be able to detect the crash.
There is also a good chance nothing will be discovered. "While the probability for such a bold undertaking is low, the potential science payoff is tremendous," says Dr. Guenter Riegler, from the Office of Space Science at NASA.
The plan is to have the 354 lb spacecraft hit the Moon's surface at 3,800 mph. The approach angle will be about 6 degrees from horizontal, meaning that the incoming craft will barely clear the crater's rim.
"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," says Goldstein of the University of Texas. "Much of the splash will be forward and to the sides. "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."
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."
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