Radiation might be the biggest hurdle to human space
exploration beyond low-Earth orbit and could stall the recently
proposed mission to Mars.
A panel tasked by the White House with reviewing NASA's human
space flight activities 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.
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.
It would take impractically thick and heavy shields to stop
high-energy galactic cosmic rays
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."