Alcuni ricercatori della NASA dell’MSFC, in previsione delle prossime missioni umane verso la Luna, hanno deciso di utilizzare i dati raccolti fino al 1977 dai sismografi lasciati dalle missioni Apollo, al fine di stimare con precisione maggiore la frequenza degli impatti di meteoriti sulla superficie selenica.
[http://science.nasa.gov/headlines/y2006/28apr_skyisfalling.htm](http://science.nasa.gov/headlines/y2006/28apr_skyisfalling.htm) The Sky is Falling NASA Science News April 28, 2006NASA researchers are mining old Apollo seismic data for clues to lunar
meteoroid impactsApril 28, 2006: Up on the Moon, the sky is falling.
“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.Apollo astronauts were never bothered by these projectiles. The Moon
has a surface area roughly equal to the continent of Africa. “If you spread
the impacts over so much terrain, the probability of being hit is very
low,” says Cooke. It helped that the astronauts didn’t stay long:
Adding all Apollo missions together, they were on the lunar surface less than two weeks. “The odds of being hit during such a short time were, again, very low.”But what about next time? Following the Vision for Space Exploration,
NASA is sending astronauts back to the Moon to stay longer and build
bigger bases (read: bigger targets) than Apollo astronauts ever did.
The odds of something precious being hit will go up. Should NASA be
worried?That’s what Cooke and MSFC colleague Anne Diekmann are trying to find
out.The truth is, “we really don’t know how many meteoroids hit the Moon
every day,” he says. “Our best estimates come from the ‘Standard
Meteoroid Model,’ which NASA uses to evaluate hazards to the space
station and the space shuttle.” Problem: The Standard Model is based
mainly on Earth-data, e.g., satellite observations of meteoroids
hitting Earth’s upper atmosphere and human observations of meteors flitting across the night sky. “The Standard Model may not work well for the
Moon.”For lunar purposes, “we need more data,” says Cooke. Fortunately, there
are more data. It comes from Apollo: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.“Here’s what’s interesting,” says Cooke. “Of some 12,000 events
recorded by the seismometers, less than half have been explained by known phenomena. There are thousands of tremors caused by … no one knows what.”He has a hunch: “Many of them may be meteoroid impacts.”
“Apollo scientists were very bright,” says Cooke, “but they didn’t have
the benefit of modern computers. We do.” 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.Also, in 1972, a 1,100 kg (2,400 lb) asteroid hit the Moon just north
of Mare Nubium, the Sea of Clouds. It was a major impact recorded at all
four seismic stations. “When we look at the seismic waveform of that
asteroid,” says Cooke, “we see that it has the same characteristics as
the man-made impacts - good sign that we know what we’re doing.”Cooke and Diekmann will hunt for impacts in the Apollo seismic records
using these known waveforms as a reference. In theory, they should be
able to pick out tremors from objects as small as 10 centimeters (4
inches), weighing as little as 1 kg (2.2 lb). “Four inches doesn’t
sound like much, but traveling at cosmic velocities, a four-inch meteoroid can blast a crater as wide as your desk.”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.”
