How much radiation awaits lunar colonists? A new NASA mission
aims to find out.
by Patrick L Barry
On the Moon, many of
the things that can kill you are invisible: breathtaking vacuum,
extreme temperatures and space radiation top the list.
Vacuum and temperature
NASA can handle; spacesuits and habitats provide plenty of air and
insulation. Radiation, though, is trickier.
The surface of the
Moon is baldly exposed to cosmic rays and solar flares, and some
of that radiation is very hard to stop with shielding. Furthermore,
when cosmic rays hit the ground, they produce a dangerous spray
of secondary particles right at your feet. All this radiation penetrating
human flesh can damage DNA, boosting the risk of cancer and other
of the Moon is exposed to space radiation.
According to the Vision
for Space Exploration, NASA plans to send astronauts back to the
Moon by 2020 and, eventually, to set up an outpost. For people to
live and work on the Moon safely, the radiation problem must be
"We really need
to know more about the radiation environment on the Moon, especially
if people will be staying there for more than just a few days,"
says Harlan Spence, a Professor of Astronomy at Boston University.
To carefully measure
and map the Moon's radiation environment, NASA is developing a robotic
probe to orbit the Moon beginning in 2008. Called the Lunar Reconnaissance
Orbiter (LRO), this scout will pave the way for future human missions
not only by measuring space radiation, but also by hunting for frozen
water and mapping the Moon's surface in unprecedented detail. LRO
is a key part of NASA's Robotic Lunar Exploration Program, managed
by the Goddard Space Flight Center.
One of the instruments
onboard LRO is the Cosmic Ray Telescope for the Effects of Radiation
"Not only will
we measure the radiation, we will use plastics that mimic human
tissue to look at how these highly energetic particles penetrate
and interact with the human body," says Spence, who is the
Principal Investigator for CRaTER.
telescope consists of silicon radiation detectors (red)
mounted on detector boards (green), separated by pieces
of "tissue-equivalent" plastic (tan).
By placing the radiation
detectors in CRaTER behind various thicknesses of a special plastic
that has similar density and composition to human tissue, Spence
and his colleagues will provide much-needed data: Except for quick
trips to the Moon during the Apollo program, most human spaceflight
has occurred near Earth where our planet's magnetic field provides
a natural shield. In low-Earth orbit, the most dangerous forms of
space radiation are relatively rare. That's good for astronauts,
but it leaves researchers with many unanswered questions about what
radiation does to human tissue. CRaTER will help fill in the gaps.
Out in deep space,
radiation comes from all directions. On the Moon, you might expect
the ground, at least, to provide some relief, with the solid body
of the Moon blocking radiation from below. Not so.
When galactic cosmic
rays collide with particles in the lunar surface, they trigger little
nuclear reactions that release yet more radiation in the form of
neutrons. The lunar surface itself is radioactive!
So which is worse for
astronauts: cosmic rays from above or neutrons from below? Igor
Mitrofanov, a scientist at the Institute for Space Research and
the Russian Federal Space Agency, Moscow, offers a grim answer:
"Both are worse."
Credit Lunar Prospector
of ground-level neutron radiation around the Moon's south
pole. "Hot spots" are red; cool spots, blue.
Mitrofanov is Principle
Investigator for the other radiation-sensing instrument on LRO,
the Lunar Exploration Neutron Detector (LEND), which is partially
funded by the Russian Federal Space Agency. By using an isotope
of helium that's missing one neutron, LEND will be able to detect
neutron radiation emanating from the lunar surface and measure how
energetic those neutrons are.
The first global mapping
of neutron radiation from the Moon was performed by NASA's Lunar
Prospector probe in 1998-99. LEND will improve on the Lunar Prospector
data by profiling the energies of these neutrons, showing what fraction
are of high energy (i.e., the most damaging to people) and what
fraction are of lower energies.
With such knowledge
in hand, scientists can begin designing spacesuits, lunar habitats,
Moon vehicles, and other equipment for NASA's return to the Moon
knowing exactly how much radiation shielding this equipment must
have to keep humans safe.