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Lunar Impact

Week of July 26, 1999

August 5, 9:00 a.m. EDT: Prospector definitely impacted the Moon! As of right now, there has still been no confirmed plume sighting. The big 'scopes have taken observations, but will require many days of analysis to be able to figure out if they saw anything (after all, the Moon is a very bright object and it interferes with observations of the much fainter plume).

Also, please read my webpage about a possible devastating budget cut for NASA. I'm asking U.S. readers to notify their Representatives and Senators if you think this is as unfair as I do. Thanks.

Just 100 hours after its launch on January 6, 1998, the small piece of metal called Lunar Prospector began silently orbiting the Moon (small indeed: it's drum shaped, about 1.3 meters across). As its name implies, Prospector was built to examine the mineral content of our nearest neighbor, and to learn more about the Moon's structure.

On March 5th, 1998, however, the Prospector scientific team announced their big result: they had measurements that indicated there might be frozen water on the Moon. I say ``might be'' because what they really found was the presence of hydrogen, and the only way we know to get hydrogen to stick around on the Moon is to have it in the form of water (each molecule of water is made up of two hydrogen atoms and one oxygen atom). The current theory is that somehow, perhaps from millennia of comet bombardment, water has been deposited on the Moon. Usually, it evaporates quickly away in the two week long lunar day, but at the poles there are deep craters that never see sunlight. Water that accumulates there will always be frozen.

However, it's not certain that the hydrogen detected is in the form of water. Scientists want more proof. So someone on the project got a bright idea: they decided to crash Prospector onto the surface.

But not just anywhere on the surface. They wanted to smash it into a crater near the South Pole. Hopefully, if luck is with them, it will hit an ice deposit. The energy of the impact will melt the ice into water and make it splash in a plume high off the surface, where it will be in direct sunlight. The ultraviolet light from the Sun will then interact with the water in a way that might make it possible to see from the Earth. Many telescopes (including STIS, the camera on board Hubble I work with) will be pointed that way to see if they can detect the telltale signs of water. If they can, it will be an exciting observation: the presence of water on the Moon makes it much easier to establish colonies there. Water is very heavy and difficult to move; so it's very costly to bring along. But if water is already there it's far easier to move it. Even a small amount can greatly ease the cost (both in money and labor) to get a foothold on the Moon.

The problem is, this is a very difficult observation. The estimated amount of water that will be in the plume is less than 20 kilograms! While most people (including me) think that impacts are usually very high energy (no doubt from watching too many asteroid impact movies) this event will be relatively low energy: the impact speed will be about 1700 meters per second or about 6000 kilometers per hour. That's fast, but nothing like a typical meteor impact!

Many of us have our doubts this is enough to be detected, and even the Prospector team admits this is a (literally!) last ditch effort. However, it costs very little to try. As it happens, the impact is timed to be the last day of the funded mission anyway! So why not go out with a bang?

The obvious question is: what will we on Earth see? The impact is set for 9:52 Universal Time on Saturday, July 31st (Universal Time is defined as the time at Greenwich, England; Eastern Time for the United States is four hours earlier, so impact for me will be at 5:52 in the Saturday morning). The Moon will be two to three days past full, and will rise around 10:00 p.m. local time wherever you are (note: this is for the Unites States, currently on Daylight Savings Time). The impact was timed so that it could be easily seen from Texas; those of you on the other side of the world will miss it. The impact site is a crater very near the South Pole of the Moon. The crater itself is just past the southern limb of the Moon, so it will not be directly visible. The plume of water may be visible, but models show that it will be about 10,000 times fainter than the limb of the Moon itself. That's faint!

This cannot be seen with the naked eye. Your best bet to see it is to use a telescope, and the bigger the better. Dry air is a big help; humid air tends to scatter light, making the sky background brighter from the moonlight. The edge of the Moon should be blocked or just off the edge of the view from the eyepiece. If you plan on taking images (with a camera or CCD) then check out a website asking for amateur images of the impact!

I'll add this: while we expect this event to be very faint, we might be wrong. I know some very smart people that predicted that the Shoemaker-Levy 9 comet would have make any visible effect when it impacted Jupiter, but those impacts were actually very bright in infrared and the aftermath was easily seen afterwards by even small telescopes (I saw the black marks myself with a 15 centimeter telescope!). So the Prospector impact might make a splash big enough to see. The only way to know is to go out and look! If it's clear here in DC, I'll be up looking for it. If I see anything, I'll post it here.

For more info, there are several web pages.

  1. The horse's mouth: The Lunar Prospector Impact Page, provided by the impact science team. This is loaded with info, including animations, images, text, and an HTML slideshow of important events too.

  2. Another horse's mouth: This has good info for us ground-based observers of the impact.

  3. The very nice Lunar Prospector Home Page, care of the NASA Ames Research center. This has links about the Moon too: history, science, landings, and more.

  4. Regular readers know what's next: the ubiquitous link to the Nine Planets website. Lots of Moon info there.

    See the Pantry arranged by topic, or use the menu below to see it chronologically.

    ©2008 Phil Plait. All Rights Reserved.

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