Ken Tapping, September 09, 2009

At 4:40 AM PDT on October 9, amateur astronomers with telescopes with objectives larger than about 20 cm may have the chance to observe a very rare event, the deliberate crashing of a large spacecraft onto the Moon's surface. This act is part of our ongoing search for water on the Moon.

The Moon's is an incredibly dry place, far dryer than any place on Earth. It's easy to guess why. The Moon has almost no atmosphere, so during the lunar day the surface layers are heated to 120 C or more and then cooled to far below zero during the lunar night. Liquid water cannot exist in the more-or-less vacuum at the Moon's surface; it is either ice or water vapour. So the temperature changes evaporate any ice in the surface layers, and the water vapour escapes and is blown away by the solar wind. Moreover, with no plate tectonics recycling the surface layers, this drying-out process has been going on for billions of years.

The churning of the surface layers by meteorite impacts has just helped things along. The only places there could be water on the Moon today are either deep underground, or in places where the Sun never shines and any surface ice never melts. Interestingly there might well be such places on the Moon, in deep craters close to the Moon's poles, where the Sun never gets high above the horizon, and the temperature hovers around -170 C.

In 1998 it was announced that NASA's Clementine spacecraft had detected ice near the lunar poles. To check this, when Clementine's work was done, the spacecraft was crashed onto the lunar surface to see whether telescopes on Earth could detect water vapour from ice vaporized by the impact. None was found. However, the search goes on, because knowing whether there is water on the Moon is important for understanding the Moon's history, and also, having some local source of water will make it a lot easier to set up a base on the Moon. Shipping it from Earth could cost $10,000 per kilogram!

The project came about because the Atlas rocket which launched the Lunar Reconnaissance Orbiter spacecraft had some spare carrying capacity; so an additional spacecraft, called the Lunar Crater Observation and Sensing Satellite - LCROSS, was put on board too. On October 9, one of the booster stages of the Atlas rocket will be put into a vertical dive to the Moon's surface near the South Pole, so it will impact at many kilometres a second. It will blow a hole in the ground and vaporize any buried ice it encounters. 

LCROSS will be following close on the tail of the booster, so that in the few moments between the impact of the booster and being destroyed itself, it will fly through the debris plume thrown up by the booster, looking for water. The Lunar Reconnaissance Orbiter will be looking too, from a safe distance. In addition, the impact has been set for a time the Moon is high in the sky in Hawaii, where large telescopes will be looking on. This means the impact will be observable from Western Canada. It's not clear how bright the flash will be, but it could be observable by amateurs. It might be a challenge, but opportunities like this don't come up often.