|
+ Play
Audio
|
+ Download Audio | +
Email to a friend | +
Join mailing list
August
11, 2008: There are places on the Moon where the
sun hasn't shined for millions of years. Dark polar craters
too deep for sunlight to penetrate are luna incognita,
the realm of the unknown, and in their inky depths, researchers
believe, may lie a treasure of great value.
NASA
is about to light one up.
Sometime
between May and August 2009, depending on launch dates, the
booster stage for NASA's LCROSS probe will deliberately crash
into a permanently-shadowed lunar crater at 9,000 km/hr, producing
an explosion equivalent to about 2,000 pounds of TNT (6.5
billion joules). The blast will jettison material out of the
crater into broad daylight where astronomers can search the
debris for signs of lunar water.
Water
is the treasure. NASA plans to send people back to
the Moon by 2020 and eventually set up a lunar outpost. Water
would be an invaluable resource for astronauts living and
working on the Moon. Not only could people drink it, but water
could be used to grow plants for food, or it could be split
into hydrogen for rocket fuel and oxygen to replenish the
outpost's air. It even could shield astronauts from dangerous
space radiation.
 Hence
the kamikaze mission, called the Lunar CRater Observation
and Sensing Satellite (LCROSS), to search for H2O
on the Moon. "If LCROSS's booster stage hits a patch
of lunar regolith that contains at least 0.5 percent water
ice, water should be detectable in the plume of ejecta,"
explains Anthony Colaprete, principal investigator for LCROSS
at NASA's Ames Research Center.
Right:
The LCROSS booster stage hurtles toward the Moon as the mission's
robotic satellite looks on. [more]
The
other half of the LCROSS mission, a robotic satellite, will
observe the impact and then itself crash into the Moon 4 minutes
later. Most of the Moon is bone dry, of course. With virtually
no atmosphere and 300° temperature swings between night and
day, most of the Moon's surface is a hostile place for water.
But there are a few cold, dark places where frozen water could
stay put. At the lunar poles, the sun is always low on the
horizon, so some crater ridges cast shadows that keep parts
of the crater floors in perpetual darkness. Temperatures in
the inky black shadows hover around 40° above absolute zero
(-233° Celsius), cold enough for water ice to survive indefinitely.
"There's
tantalizing evidence that water might be there," Colaprete
says. A lunar orbiter called Clementine detected hints of
water ice in some of these craters in 1994 and so did the
1999 Lunar Prospector mission, but unfortunately the data
were not conclusive.
That's
where LCROSS comes in. Ice blasted into the sunlight by the
impact would vaporize. Ultraviolet light from the sun would
then split the H2O molecules into H and OH. Mission
planners hope LCROSS's sensors will detect the fingerprint
of H20 in near-infrared light and also a characteristic
wavelength emitted by OH at 308 nanometers.
Above:
The "life cycle" of LCROSS's impact plume. Click
on the image to view a larger diagram and more
information.
Currently,
Colaprete's team is searching for the best impact sites inside
various shadowed craters. "The first and most important
criterion is that we think the impact area will be productive
from an ejecta standpoint," Colaprete explains. "If
we don't get ejecta into sunlight, it wouldn't matter if we
hit an iceberg because we would never know it." For example,
if the impact site is close to a high crater wall, the ejecta
would have to travel far to get out of the wall's shadow and
reach the sunlight above. And if the impactor hits a steep
slope in the bottom of a shadowed crater, much of the ejecta
would blast out sideways instead of upward toward the sunlight.
So a good site would be relatively flat-bottomed — less than
about 15° of slope — with a fluffy regolith free of large
boulders or rubble that would blunt the blow.
Colaprete says that, so far, one of the best sites appear
to be in a 17 km-across unnamed crater just west of Peary
crater (88.6° N, 33.0° E), near the Moon’s north pole. "We've
gone through essentially every possible launch date and picked
a crater [for each date]," he says.
Above:
The Moon's north pole. Each of the yellow dots marks a crater
with possibly permanent shadows. According to a 2003 study,
as much as 7,500 km2 around the lunar north pole
could lie in perpetual shadow. [more]
Choosing
impact sites must also take another factor into account: visibility
from Earth. Hundreds of amateur and professional astronomers
will join the LCROSS robotic orbiter in watching the crash.
The
explosion itself will probably be hidden by the walls of the
target crater. Instead, what astronomers will look for is
the impact plume. An expanding cone of ejecta will rise more
than 6 kilometers above the lunar surface and spread outward
for about 40 km in every direction. Glistening in the sunlight,
the debris is expected to shine like a 6th to 8th magnitude
star—invisible to the human eye but an easy target for backyard
telescopes.
Colaprete's
team will time the impact so that it happens while the Moon
is high in the sky at night in Hawaii. There, LCROSS scientists
will observe the ejecta plume with the powerful Infrared Telescope
Facility. But astronomers on the west coast of the U.S. and
in Japan could be able to see the impact as well, depending
on the precise impact time. "It really is going to turn
into an international event," Colaprete says. "Everyone's
going to be training their eyes on the impact to observe it."
Stay
tuned to Science@NASA to find out how amateur astronomers
can collaborate with LCROSS scientists to help make this historic
search for water on the Moon a smashing success.
SEND
THIS STORY TO A FRIEND
Editor: Dr.
Tony Phillips | Credit: Science@NASA
|
