http://www.newscientist.com/article/mg20327266.100-too-much-radiation-fo
r-astronauts-to-make-it-to-mars.html

FORGET the risk of exploding rockets or getting sideswiped by a wayward
bit of space junk. Radiation may be the biggest hurdle to human
exploration beyond low-Earth orbit and could put a damper on a recently
proposed mission to Mars orbit.

A panel tasked by the White House with reviewing NASA's human space
flight activities (New Scientist, 22 August, p 8) suggests sending
astronauts to one of Mars's moons, Phobos or Deimos, among other
possibilities raised in its report released last week
(http://tinyurl.com/mbajav).

>From such a perch, astronauts could use remote-controlled robots to
explore the Martian surface and retrieve samples - from the planet as
well as the moon itself - for later close-up study on Earth. This would
avoid the need to develop expensive hardware to land humans on a body
with substantial gravity, like Mars.

"I, for one, would go to Phobos or Deimos in a heartbeat, even without
any hope of landing on Mars," says planetary scientist Pascal Lee of the
Mars Institute, a California-based research organisation.

But the insidious threat of space radiation in the form of galactic
cosmic rays could keep astronauts confined much closer to home.

The rays are actually speeding protons and heavier atomic nuclei that
rain onto our solar system from all directions. They can slice through
DNA molecules when they pass through living cells and the resulting
damage can lead to cancer.

People on the ground are protected by our planet's atmosphere and
magnetic field, which also provide some protection to astronauts on the
International Space Station. Lunar missions are short enough to keep
radiation risks low, and the moon itself blocks half of the incoming
particles. Crews on long journeys beyond low-Earth orbit would have no
such protection.

Relatively lightweight aluminium or plastic shielding can block charged
particles from the sun. But it would take impractically thick and heavy
shields to stop the higher-energy galactic cosmic rays. "Shielding is
not a solution to the risk problem," says Frank Cucinotta, chief
scientist for radiation studies at NASA's Johnson Space Center in
Houston, Texas.

Alternative technologies - which would generate bubbles of plasma that
could protect spacecraft without adding much weight - are still at an
early stage of development.

So how dangerous would a trip to Mars orbit be? Estimates of how much a
given dose of space radiation increases the risk of cancer are fraught
with uncertainty. But calculations by Cucinotta and his colleagues
suggest the trip would not meet NASA's existing rules, which aim to keep
each astronaut's lifetime risk of fatal cancer from space radiation
below 3 per cent.

For journeys outside Earth's magnetic field, astronauts could reach that
limit in less than 200 days in a spacecraft with aluminium walls nearly
4 centimetres thick, according to worst-case scenario estimates
(Radiation Measurements, DOI: 10.1016/j.radmeas.2006.03.011).

But the White House panel expects a round-trip mission to a Martian moon
would take four times as long, lasting 750 days. Since such trips would
expose astronauts to more radiation than is currently allowed, the panel
asked NASA if it would consider simply accepting higher risks for the
missions. Steven Lindsey, head of NASA's astronaut office, thinks most
astronauts would probably be open to the idea. "It depends on the
individual," he says. "I've got crew members that will fly on anything."
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