Space radiation between Earth and
Mars poses a hazard to astronauts. How dangerous is it out there?
NASA scientists are working to find out.
by Dr Tony Phillips
NASA has a mystery
to solve: Can people go to Mars, or not?
"It's a question of
radiation," says Frank Cucinotta of NASA's Space Radiation Health
Project at the Johnson Space Centre."We know how much radiation
is out there, waiting for us between Earth and Mars, but we're not
sure how the human body is going to react to it."
NASA astronauts have
been in space, off and on, for 45 years. Except for a few quick
trips to the moon, though, they've never spent much time far from
Earth. Deep space is filled with protons from solar flares, gamma
rays from newborn black holes, and cosmic rays from exploding stars.
A long voyage to Mars, with no big planet nearby to block or deflect
that radiation, is going to be a new adventure.
NASA weighs radiation
danger in units of cancer risk. A healthy 40-year-old non-smoking
American male stands a (whopping) 20% chance of eventually dying
from cancer. That's if he stays on Earth. If he travels to Mars,
the risk goes up.
The question is, how
"We're not sure," says
Cucinotta. According to a 2001 study of people exposed to large
doses of radiation - e.g., Hiroshima atomic bomb survivors
and, ironically, cancer patients who have undergone radiation therapy - the
added risk of a 1000-day Mars mission lies somewhere between 1%
and 19%. "The most likely answer is 3.4%," says Cucinotta, "but
the error bars are wide."
The odds are even worse
for women, he adds. "Because of breasts and ovaries, the risk to
female astronauts is nearly double the risk to males."
Researchers who did the study assumed
the Mars-ship would be built "mostly of aluminium, like an old Apollo
command module," says Cucinotta. The spaceship's skin would absorb
about half the radiation hitting it.
An artist's concept of DNA battered by galactic cosmic rays.
"If the extra risk
is only a few percent‚€¶ we're OK. We could build a spaceship using
aluminium and head for Mars." (Aluminium is a favourite material
for spaceship construction, because it's lightweight, strong, and
familiar to engineers from long decades of use in the aerospace
"But if it's 19%‚€¶ our
40something astronaut would face a 20% + 19% = 39% chance of developing
life-ending cancer after he returns to Earth. That's not acceptable."
The error bars are
large, says Cucinotta, for good reason. Space radiation is a unique
mix of gamma-rays, high-energy protons and cosmic rays. Atomic bomb
blasts and cancer treatments, the basis of many studies, are no
substitute for the "real thing."
Astronauts have rarely
experienced a full dose of these deep space GCRs. Consider the International
Space Station (ISS): it orbits only 400 km above Earth's surface.
The body of our planet, looming large, intercepts about one-third
of GCRs before they reach the ISS. Another third is deflected by
Earth's magnetic field. Space shuttle astronauts enjoy similar reductions.
Apollo astronauts travelling to the
moon absorbed higher doses - about 3 times the ISS level - but only
for a few days during the Earth-moon cruise. GCRs may have damaged
their eyes, notes Cucinotta. On the way to the moon, Apollo crews
reported seeing cosmic ray flashes in their retinas, and now, many
years later, some of them have developed cataracts. Otherwise they
don't seem to have suffered much. "A few days 'out there' is probably
safe," concludes Cucinotta.
command modules were well-enough shielded for quick trips
to the Moon and back.
But astronauts travelling
to Mars will be "out there" for a year or more. "We can't yet estimate,
reliably, what cosmic rays will do to us when we're exposed for
so long," he says.
Finding out is the
mission of NASA's new Space Radiation Laboratory (NSRL), located
at the US Department of Energy's Brookhaven National Laboratory
in New York. It opened in October 2003. "At the NSRL we have particle
accelerators that can simulate cosmic rays," explains Cucinotta.
Researchers expose mammalian cells and tissues to the particle beams,
and then scrutinize the damage. "The goal is to reduce the uncertainty
in our risk estimates to only a few percent by the year 2015."
Once the risks are
known, NASA can decide what kind of spaceship to build. It's possible
that ordinary building materials like aluminium are good enough.
If not, "we've already identified some alternatives," he says.
How about a spaceship
made of plastic?
of reinforced polyethylene - Are these the building blocks
of future spacecraft?
"Plastics are rich
in hydrogen - an element that does a good job absorbing cosmic rays,"
explains Cucinotta. For instance, polyethylene, the same material
garbage bags are made of, absorbs 20% more cosmic rays than aluminium
A form of reinforced polyethylene developed at the Marshall Space
Flight Centre. is 10 times stronger than aluminium, and lighter,
too. This could become a material of choice for spaceship building,
if it can be made cheaply enough. "Even if we don't build the whole
spacecraft from plastic," notes Cucinotta, "we could still use it
to shield key areas like crew quarters." Indeed, this is already
done onboard the ISS.
If plastic isn't good
enough then pure hydrogen might be required. Pound for pound, liquid
hydrogen blocks cosmic rays 2.5 times better than aluminium does.
Some advanced spacecraft designs call for big tanks of liquid hydrogen
fuel, so "we could protect the crew from radiation by wrapping the
fuel tank around their living space," speculates Cucinotta.
Can people go to Mars?
Cucinotta believes so. But first, "we've got to figure out how much
radiation our bodies can handle and what kind of spaceship we need
to build." In labs around the country, the work has already begun.