Sky falls above the moon !

"Every day, more than a metric ton of meteoroids hits the Moon," says Bill Cooke of the Marshall Space Flight Center's Meteoroid Environment Office. They literally fall out of the sky, in all shapes and sizes, from specks of comet dust to full-blown asteroids, traveling up to a hundred thousand mph. And when they hit, they do not disintegrate harmlessly in the atmosphere as most would on Earth. On the airless Moon, meteoroids hit the ground.

Clues to how often and how hard the Moon is hit lie in data from four seismometers placed on the Moon by the Apollo 12, 14, 15, and 16 missions during 1969-72. They operated until NASA turned them off in 1977. For years, the seismometers recorded all manner of tremors and jolts, including almost 3000 moonquakes, 1700 meteoroid strikes, and 9 spacecraft deliberately crashed into the Moon. All these data were transmitted to Earth for analysis.

Cooke and Diekmann are now loading the old seismic data into machines at the MSFC where they can perform digital calculations at speeds impossible 30 years ago, rapidly trying new algorithms to find previously unrecognized impacts.

Critical to the analysis are nine man-made impacts. "NASA deliberately crashed some spacecraft into the Moon while the seismometers were operating," he explains. "They were the empty ascent stages of four lunar modules (Apollo 12, 14, 15 and 17) and the SIV-B stages of five Saturn rockets (Apollo 13, 14, 15, 16 and 17)." Their seismic waveforms tell researchers what an impact should look like.


According to the Standard Model, such meteoroids hit the Moon approximately 400 times a year—more than once a day. (Picture a map of Africa stuck with 400 pushpins.) The Apollo seismic dataset can test that prediction and many others.

The analysis is just beginning. "We hope to find many impacts," he says. Regardless of the final numbers, however, their work will have value. "We're developing new algorithms to find meteoroid impacts in seismic data." Eventually, Cooke believes, next-generation seismometers will be placed on the Moon and Mars to monitor quakes and impacts, and when the data start pouring in, "we'll be ready."