Settling alien worlds is thirsty work
- especially on the lunar surface.
by Patrick L Barry
The next time you look at the
Moon, pause for a moment and let this thought sink in: People have
actually walked on the Moon, and right now the wheels are in motion
to send people there again.
The goals this time around are
more ambitious than they were in the days of the Apollo program.
NASA's new Vision for Space Exploration spells out a long-term strategy
of returning to the Moon as a step toward Mars and beyond. The Moon,
so nearby and accessible, is a great place to try out new technologies
critical to living on alien worlds before venturing across the solar
Whether a moonbase will turn
out to be feasible hinges largely on the question of water. Colonists
need water to drink. They need water to grow plants. They can also
break water apart to make air (oxygen) and rocket fuel (oxygen+hydrogen).
Furthermore, water is surprisingly effective at blocking space radiation.
Surrounding the 'base with a few feet of water would help protect
explorers from solar flares and cosmic rays.
- Artwork by Pat Rawlings for Nasa
is gearing up to send humans back to the Moon
The problem is, water is dense
and heavy. Carrying large amounts of it from Earth to the Moon would
be expensive. Settling the Moon would be so much easier if water
were already there.
It's possible: Astronomers
believe that comets and asteroids hitting the Moon eons ago left
some water behind. (Earth may have received its water in the same
way.) Water on the Moon doesn't last long. It evaporates in sunlight
and drifts off into space. Only in the shadows of deep cold craters
could you expect to find any, frozen and hidden. And indeed there
may be deposits of ice in such places. In the 1990s two spacecraft,
Lunar Prospector and Clementine, found tantalizing signs of ice
in shadowed craters near the Moon's poles - perhaps as much as much
as a cubic kilometer. The data was not conclusive, though.
To find out if lunar ice is
truly there, NASA plans to send a robotic scout. The Lunar Reconnaissance
Orbiter, or "LRO" for short, is scheduled to launch in
2008 and to orbit the Moon for a year or more. Carrying six different
scientific instruments, LRO will map the lunar environment in greater
detail than ever before.
"This is the first in a
string of missions," says Gordon Chin, project scientist for
LRO at NASA's Goddard Space Flight Center. "More robots will
follow, about one per year, leading up to manned flight" no
later than 2020.
LRO's instruments will do many
things: they'll map and photograph the Moon in detail, sample its
radiation environment and, not least, hunt for water.
For example, the spacecraft's
Lyman-Alpha Mapping Project (LAMP), will attempt to peer into the
darkness of permanently shadowed craters at the Moon's poles, looking
for signs of ice hiding there.
How can LAMP see in the dark?
By looking for the dim glow of reflected starlight.
LAMP senses a special range
of ultraviolet light wavelengths. Not only is starlight relatively
bright in this range, but also the hydrogen gas that permeates the
universe radiates in this range as well. To LAMP's sensor, space
itself is literally aglow in all directions. This ambient lighting
may be enough to see what lies in the inky blackness of these craters.
artist's concept of the Lunar Reconnaissance Orbiter,
which is still being designed
"What's more, water ice
has a characteristic spectral 'fingerprint' in this same range of
ultraviolet light, so we'll get spectral evidence of whether ice
is in these craters," explains Alan Stern, a scientist at the
Southwest Research Institute and principal investigator for LAMP.
The spacecraft is also equipped
with a laser that can shine pulses of light into dark craters. The
main purpose of the instrument, called the Lunar Orbiter Laser Altimeter
(LOLA), is to produce a highly accurate contour map of the entire
Moon. As a bonus, it will also measure the brightness of each laser
reflection. If the soil contains ice crystals, as little as 4%,
the returning pulse would be noticeably brighter.
LOLA by itself can't prove that
ice is there. "Any kind of reflective crystals could produce
brighter pulses," explains David Smith, principal investigator
for LOLA at NASA's Goddard Space Flight Center. "But if we
see brighter pulses only in these permanent shadows, we'd strongly
One of LRO's instruments, named
Diviner, will map the temperature of the Moon's surface. Scientists
can use these measurements to search for places where ice could
exist. Even in the permanent shadows of polar craters, temperatures
must be very low for ice to resist evaporation. Thus, Diviner will
provide a "reality check" for LRO's other ice-sensitive
instruments, identifying areas where positive signs of ice would
not make any sense because the temperature is simply too high.
Another reality check will come
from LRO's Lunar Exploration Neutron Detector (LEND), which counts
neutrons spraying out of the lunar surface. Why does the Moon emit
neutrons? And what does that have to do with water? The Moon is
constantly bombarded by cosmic rays, which produce neutrons when
they hit the ground. Hydrogen-bearing compounds like H2O
absorb neutrons, so a dip in neutron radiation could signal an oasis
... of sorts. LEND is being developed by Igor Mitrofanov from the
Institute for Space Research, Federal Space Agency, Moscow.
Pat Rawlings - NASA
out in the cosmos.
"There's a strong synergy
between the various instruments on LRO," notes Chin. "None
of these instruments alone could provide definitive evidence of
ice on the Moon, but if they all point to ice in the same area,
that would be compelling."
Chin also points out another
reason that finding ice near the Moon's poles would be exciting:
Not far from some permanently
shadowed craters are mountainous regions in permanent sunlight,
known romantically as "peaks of eternal sunshine." Conceivably,
a moonbase could be placed on one of those peaks, providing astronauts
with constant solar power - not far from crater-valleys below, rich
in ice and ready to be mined.
Wishful thinking? Or a reasonable
plan? Lunar Reconnaissance Orbiter will beam back the answer.