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An Explosion on the Moon

12.23.2005

So you thought
nothing ever happens on the moon?

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December
23, 2005: NASA scientists have observed an explosion
on the moon. The blast, equal in energy to about 70 kg of TNT,
occurred near the edge of Mare Imbrium (the Sea of Rains) on
Nov. 7, 2005, when a 12-centimeter-wide meteoroid slammed into
the ground traveling 27 km/s.

"What a
surprise," says Marshall Space Flight Center (MSFC) researcher
Rob Suggs, who recorded the impact's flash. He and colleague
Wes Swift were testing a new telescope and video camera they
assembled to monitor the moon for meteor strikes. On their
first night out, "we caught one," says Suggs.

Right: The red dot marks the location
of the Nov. 7, 2005, meteoroid impact. Credit: NASA/MSFC/Bill
Cooke. [Larger
image]

The object that hit the moon was "probably a Taurid,"
says MSFC meteor expert Bill Cooke. In other words, it was
part of the same meteor shower that peppered Earth with
fireballs in late October and early November 2005. (See "Fireball
Sightings" from Science@NASA.)

The moon was peppered, too, but unlike Earth, the moon
has no atmosphere to intercept meteoroids and turn them into
harmless streaks of light. On the moon, meteoroids hit the
ground--and explode.

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"The flash we saw," says Suggs, "was about as bright as
a 7th magnitude star." That's two and a half times dimmer than
the faintest star a person can see with their unaided eye, but
it was an easy catch for the group's 10-inch
telescope.

Cooke estimates that the impact gouged a crater in the
moon's surface "about 3 meters wide and 0.4 meters deep." As
moon craters go, that's small. "Even the Hubble Space
Telescope couldn't see it," notes Cooke. The moon is 384,400
km away. At that distance, the smallest things Hubble can
distinguish are about 60 meters wide.

This isn't the first time meteoroids have been seen
hitting the moon. During the Leonid meteor storms of 1999 and
2001, amateur and professional astronomers witnessed at least
half-a-dozen flashes ranging in brightness from 7th to 3rd
magnitude. Many of the explosions were photographed
simultaneously by widely separated observers.

Above: The Nov. 7th lunar Taurid
explosion, shown as a sequence of 6 false-color video frames.
Credit: Wes Swift/NASA.

Since the Leonids of 2001, astronomers have not spent
much time hunting for lunar meteors. "It's gone out of
fashion," says Suggs. But with NASA planning to return to the
moon by 2018, he says, it's time to start watching
again.

There are many questions that need answering: "How
often do big meteoroids strike the moon? Does this happen only
during meteor showers like the Leonids and Taurids? Or can we
expect strikes throughout the year from 'sporadic meteors?'"
asks Suggs. Explorers on the moon are going to want to
know.

"The chance of an astronaut being directly hit by a big
meteoroid is miniscule," says Cooke. Although, he allows, the
odds are not well known "because we haven't done enough
observing to gather the data we need to calculate the odds."
Furthermore, while the danger of a direct hit is almost nil
for an individual astronaut, it might add up to something
appreciable for an entire lunar outpost.

Of greater concern,
believes Suggs, is the spray—"the secondary meteoroids
produced by the blast." No one knows how far the spray reaches
and exactly what form it takes.

Right: An artist's concept of the Nov.
7, 2005, explosion. Credit: NASA/MSFC. [Larger
image]

Also, ground-shaking impacts could kick up moondust,
possibly over a wide area. Moondust is electrostatically
charged and notoriously clingy. (See "Mesmerized
by Moondust" from Science@NASA.) Even a small amount of
moondust can be a great nuisance: it gets into spacesuit
joints and seals, clings to faceplates, and even makes the air
smell when it is tramped indoors by moonwalkers. Could
meteoroid impacts be a source of lunar "dust storms?" Another
question for the future....

Suggs and his team plan to make more observations.
"We're contemplating a long-term monitoring program active not
only during major meteor showers, but also at times in
between. We need to develop software to find these flashes
automatically," he continues. "Staring at 4 hours of tape to
find a split-second flash can get boring; this is a job for a
computer."

With improvements, their system might catch lots of
lunar meteors. Says Suggs, "I'm ready for more
surprises."

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Author: Dr. Tony Phillips
| Production Editor: Dr. Tony
Phillips | Credit: http://science.nasa.gov/

More
Information

As far as they know, Suggs and Swift were the
only ones who recorded the impact of Nov. 7th—"probably
because we were the only ones looking," says Suggs. So,
unlike the lunar Leonids of 1999 and 2001, the lunar
Taurid of 2005 was not confirmed by a second or third
observer.

Nevertheless, "we are 99% sure it was real," says
Suggs.

Other possibilities include

a satellite passing in front of the moon,
glinting in sunlight;

a cosmic ray hitting the video camera's CCD
chip;

a meteor in Earth's atmosphere, directly
between Earth and the Moon.

"We don't believe it was a satellite," says Cooke
who, together with aerospace engineer Heather McNamara,
searched through NORAD's catalogue of 8363 "trackable
objects" in Earth orbit. "There was no unclassified
satellite or piece of space debris in the right place at
the right time to cause the flash."

It couldn't have been a cosmic ray. "We observed
the lunar explosion in five consecutive video frames
(total time span: 150 msec). A cosmic ray would have
caused a flash in only one frame," explains
Suggs.

Right: The light curve of the
flash observed by Suggs and Swift on Nov. 7, 2005.
Credit: NASA/MSFC

And finally, it almost certainly couldn't have
been a meteor in Earth's atmosphere. "To masquerade as a
lunar impact, a meteor in Earth's atmosphere would have
to be heading directly toward our observing site at the
Marshall Space Flight Center, head on, so that it looked
like a point rather than a streak of light," says Suggs.
"A meteoroid hitting the moon is more plausible.
Furthermore," he says, "the light curve of our Nov. 7th
Taurid has the same shape as light curves of lunar
Leonids observed in 1999 and 2001. Also, it doesn't
match the light curve of a 'point meteor.'"

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NASA Official: John M. Horack
Last
Updated: June 9, 2005
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