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The spacecraft: SMART-1, a lunar orbiter belonging to the European Space Agency (ESA).

The impact site: Lacus Excellentiae (The Lake of Excellence), an ancient, 100-mile wide crater in the Moon's southern hemisphere.

The time to watch: Saturday, September 2nd at 10:41 p.m. PDT (Sept. 3rd, 0541 UT).

Right: The impact site, with current moon phase shown. Updated daily. Image courtesy: ESA. [More]

Why is SMART-1 crashing? There's nothing wrong with the spacecraft, which is wrapping up a successful 3-year mission to the Moon. SMART-1's main job was to test a European-built ion engine. It worked beautifully, propelling the craft in 2003 on a unique spiral path from Earth to the Moon. From lunar orbit, SMART-1 took thousands of high-resolution pictures and made mineral maps of the Moon's terrain. One of its most important discoveries was a "Peak of Eternal Light," a mountaintop near the Moon's north pole in constant, year-round sunlight. Peaks of Eternal Light are prime real estate for solar-powered Moon bases.

But now SMART-1 is running low on fuel. It has to come down sometime—and soon—so ESA mission scientists decided to crash it in a place where the crash can be seen from Earth and studied.

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When SMART-1 hits the ground, it will explode in a flash of light. This won't be the sort of explosion we'd see on Earth. The Moon has no oxygen to support fire or combustion. Instead, the flash will be caused by rocks and soil made so hot by the impact that they suddenly glow.

The area will be in complete darkness at the moment of impact, so much the better to see the flash. How bright will it be? No one knows. Estimates range from 7th to 15th magnitude. In other words, it might be bright enough for backyard telescopes--or so dim that even big professional observatories won't see a thing. The only way to find out is to look. Observing tips may be found here (ALPO), here (ESA) and, in many languages, here (REA Brazil).

"We'll be watching," says Bill Cooke, the head of NASA's Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Alabama. "Measuring the brightness of SMART-1's impact is important to our research."

His group at the Marshall Space Flight Center has spent the last year watching things hit the Moon—not spacecraft, but meteoroids. "The Moon is under constant bombardment from meteoroids," says Cooke. "They hit the ground and explode just like SMART-1 will do." The Moon actually sparkles, slowly and faintly, as one space rock after another hits the ground.

Above: Possible SMART-1 impact times and coordinates. Image courtesy: ESA. [More]

Cooke's group has a knack for this kind of work: Using only two small telescopes, they've spotted eight meteoroid impacts this year, almost doubling the number of confirmed sightings in all of the history of astronomy before 2006. Cooke attributes their success to improvements in digital video cameras, which they use to record the brief flashes.

Lunar impacts interest NASA greatly. Astronauts are going back to the Moon and "we need to know what kind of danger meteoroids pose to both people and Moon bases," explains Cooke. How often do they hit? And what kind of damage do they do?

Think of SMART-1 as a controlled, man-made meteoroid impact, he says. "We know exactly how much kinetic energy SMART-1 packs. And, if all goes well, we're going to see how bright a flash it makes. This will help us interpret our meteoroid data."

When SMART-1 hits, it won't plunge straight into the ground. "The spacecraft will enter Lacus Excellentiae at a shallow angle, only a few degrees from horizontal," notes Cooke. For this reason, it will gouge a long, narrow crater, about a meter wide and many meters long. The grazing impact should kick up a plume of debris—no one knows how high. If it rises high enough, the plume might catch some sunlight and become visible to telescopes on Earth. The chances of this, however, are slim. The main event is the flash of heat and light at the "point" of impact.

Another side-effect of the shallow approach is uncertainty about when, exactly, SMART-1 will strike. The spacecraft is due to glide low over the floor of Lacus Excellentiae several times on Sept. 3rd. Mission controllers believe it will hit on orbit number 2890 at 0541 UT. But it could equally well hit one orbit earlier or one orbit later. Possibilities are summarized in the table, above. The nominal impact time favors observers in western parts of North America and across the Pacific Ocean. Depending on when SMART-1 hits, however, almost anyone could catch the flash.

Visit the SMART-1 home page for updates and more information.

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Author: Dr. Tony Phillips | Production Editor: Dr. Tony Phillips | Credit: Science@NASA