NASA researchers are mining old Apollo seismic data for clues
to lunar meteoroid impacts
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April
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.
Right:
The Moon's surface is peppered with impact craters. [More]
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.
![](images/skyisfalling/aldrinseismometer_apollo11_strip3.jpg)
Above:
Buzz Aldrin deploys a seismometer in the Sea of Tranquillity.
[More]
"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.
![see caption](images/skyisfalling/LM12impact.jpg)
Above:
A seismic waveform recorded when Apollo 12's lunar ascent
module crashed into the Moon on Nov. 20, 1969. [Larger
image]
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—a 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."
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Authors: Dr. Tony
Phillips and Dave Dooling | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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