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May
21, 2008: Not so long ago, anyone claiming to see
flashes of light on the Moon would be viewed with deep suspicion
by professional astronomers. Such reports were filed under
"L" … for lunatic.
Not
anymore. Over the past two and a half years, NASA astronomers
have observed the Moon flashing at them not just once but
one hundred times.
"They're
explosions caused by meteoroids hitting the Moon," explains
Bill Cooke, head of NASA's Meteoroid Environment Office at
the Marshall Space Flight Center (MSFC). "A typical blast
is about as powerful as a few hundred pounds of TNT and can
be photographed easily using a backyard telescope."
As
an example, he offers this video of an impact near crater
Gauss on January 4, 2008:
Above:
A lunar impact on Jan. 4, 2008. This is number 86 on the
list of 100 impacts recorded by the MEO team since their
survey began in 2005. Larger movies: 0.8
MB gif, 5.9
MB avi.
The
impactor was a tiny fragment of extinct comet 2003 EH1. Every
year in early January, the Earth-Moon system passes through
a stream of debris from that comet, producing the well-known
Quadrantid meteor shower. Here on Earth, Quadrantids disintegrate
as flashes of light in the atmosphere; on the airless Moon
they hit the ground and explode.
"We
started our monitoring program in late 2005 after NASA announced
plans to return astronauts to the Moon," says team leader
Rob Suggs of the MSFC. If people were going to be walking
around up there, "it seemed like a good idea to measure
how often the Moon was getting hit."
"Almost
immediately, we detected a flash."
That
first detection—"I'll never forget it," he says--came
on Nov. 7, 2005, when a piece of Comet Encke about the size
of a baseball hit Mare Imbrium. The resulting explosion produced
a 7th magnitude flash, too dim for the naked eye but an easy
target for the team's 10-inch telescope.
A
common question, says Cooke, is "how can something explode
on the Moon? There's no oxygen up there."
These
explosions don't require oxygen or combustion. Meteoroids
hit the moon with tremendous kinetic energy, traveling 30,000
mph or faster. "At that speed, even a pebble can blast
a crater several feet wide. The impact heats up rocks and
soil on the lunar surface hot enough to glow like molten lava--hence
the flash."
During
meteor showers such as the Quadrantids or Perseids, when the
Moon passes through dense streams of cometary debris, the
rate of lunar flashes can go as high as one per hour. Impacts
subside when the Moon exits the stream, but curiously the
rate never goes to zero.
"Even
when no meteor shower is active, we still see flashes,"
says Cooke.
Above:
A map of the 100 explosions observed since late 2005. A complete
list with lunar coordinates is available here.
These
"off-shower" impacts come from a vast swarm of natural
space junk littering the inner solar system. Bits of stray
comet dust and chips off old asteroids pepper the Moon in
small but ultimately significant numbers. Earth gets hit,
too, which is why on any given night you can stand under a
dark sky and see a few meteors per hour glide overhead—no
meteor shower required. Over the course of a year, these random
or "sporadic" impacts outnumber impacts from organized
meteor showers by a ratio of approximately 2:1.
"That's
an important finding," says Suggs. "It means there's
no time of year when the Moon is impact-free."
Fortunately,
says Cooke, astronauts are in little danger. "The odds
of a direct hit are negligible. If, however, we start building
big lunar outposts with lots of surface area, we'll have to
carefully consider these statistics and bear in mind the odds
of a structure getting hit."
Secondary
impacts are the greater concern. When meteoroids strike the
Moon, debris goes flying in all directions. A single meteoroid
produces a spray consisting of thousands of "secondary"
particles all traveling at bullet-like velocities. This could
be a problem because, while the odds of a direct hit are low,
the odds of a secondary hit may be significantly greater.
"Secondary particles smaller than a millimeter could
pierce a spacesuit," notes Cooke.
Right:
A simulated meteoroid explodes on impact at the NASA/Ames
Vertical Gun Range. This is a genuine photo showing the spray
of secondary particles: more.
At
present, no one knows how far and wide secondary particles
travel. To get a handle on the problem, Cooke, Suggs and colleagues
are shooting artificial meteoroids at simulated moon dust
and measuring the spray. This work is being done at the Vertical
Gun Range at NASA's Ames Research Center in Mountain View,
CA: full
story.
Meanwhile,
back at the observatory, the team has upgraded their original
10-inch (25 cm) telescope to a pair of telescopes, one 14-inch
(36 cm) and one 20-inch (51 cm), located at the Marshall Space
Flight Center in Alabama. They've also established a new observing
site in Georgia with a 14-inch telescope. Multiple telescopes
allow double- and triple-checking of faint flashes and improve
the statistical underpinnings of the survey.
"The
Moon is still flashing," says Suggs. Indeed, during the
writing of this story, three more impacts were detected.
New
title: 103 Explosions on the Moon.
Stay
tuned to Science@NASA for a follow-up story describing how
amateur astronomers can participate in this research.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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NASA
Meteoroid Environment Office -- home of the lunar
impact monitoring program
EXTRA!
TEACHER WORKSHOP: Teachers in the Southeast
U.S. are invited to apply to attend a workshop entitled
"Paving the Way to the Moon and Beyond," held
June 12 – 14, 2008 in Huntsville, Alabama. The two day
workshop, sponsored by the Lunar Precursor Robotic Program
(LPRP), will focus on content that will explain the
who, the what, and the why of lunar exploration. Beginning
with the Lunar Reconnaissance Orbiter (LRO) and the
Lunar Crater Observation and Sensing Satellite (LCROSS),
teachers will research mission design and scientific
goals and experiments through hands on activities. Included
in the workshop will be a tour of NASA science labs
and an opportunity to talk with scientists involved
in exploration related activities. All materials will
be appropriate for elementary and middle school preservice
and inservice teachers and will be aligned with national
standards. Example activities: Earth-Moon comparisons
and motions, craters and lunar soils, solar influences
on the Moon. Most activities will take place at the
Educator Resource Center, located at the Space and Rocket
Center. Housing costs for three nights will be provided
(at the University of Alabama in Huntsville). Stipends
also will be provided. To attend, contact Mitzi Adams
256 961 7626 or mitzi dot adams @ nasa dot gov.
NASA's
Future: US
Space Exploration Policy |
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