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[cdn-nucl-l] Space station radiation shields 'disappointing'
Posted in New Scientist on October 23, 2002 and at:
Application of Linear No-Threshold theory to radiation exposure in
Space station radiation shields 'disappointing'
19:00 23 October 02
Exclusive from New Scientist Print Edition
Radiation levels on the International Space Station are as high as they
were on the antiquated Russian space station Mir, in spite of NASA's
attempts to clad the ISS with better shielding. If NASA can't protect
astronauts, its vision of sending a crew into deep space may come to
Data collected by NASA and a Russian-Austrian collaboration show that
astronauts on the ISS are subjected to about 1 millisievert of radiation
per day, about the same as someone would get from natural sources on
Earth in a whole year. Spending three months in these conditions
translates into about one-tenth the long-term cancer risk incurred by
While this may be an acceptable risk, sending astronauts beyond the
Earth's protective magnetic field will vastly increase their exposure.
"If you sent two people to Mars, one of them would die," says Marco
Durante of the Federico II University in Naples, who has studied the
health effects of radiation in Mir astronauts for ESA.
Radiation inside the ISS, and the now defunct Mir, is caused when the
fast, heavy ions that make up cosmic rays collide with the aluminium
hull, releasing a shower of secondary particles into the living
To mitigate this effect, the ISS has been fitted with additional
polyethylene shielding that contains lighter atomic nuclei, which are
less likely to throw out neutrons when hit by cosmic rays. The data
shows this lowers astronaut exposure by a few per cent, but this is not
as much as was hoped, says Thomas Berger of the Austrian Universities'
Atom Institute in Vienna.
Entirely new technologies
The shielding could be scaled up to cut out up to 30 per cent of the
dose at the Lagrangian point, where NASA envisages setting up a space
station (see NASA prepares to boldly go). But that is not enough, says
Frank Cucinotta, head of NASA's ISS radiation group at the Jet
Propulsion Lab in Pasadena, California. "It doesn't get rid of the whole
exposure unless you put in so much it's incredibly heavy." Entirely new
shielding technologies will have to be developed, he says, and no one
knows how long that might take.
The effects of this kind of radiation on the body are not well
understood. NASA works with the same exposure limits as those set by the
US Environmental Protection Agency for radiation workers on Earth - no
one should receive a dose that increases their relative risk of dying of
cancer by over 15 per cent. Cucinotta says the radiation risk on the ISS
is 5 per cent.
But Durante disagrees. In a study of eight astronauts who had spent 70
days or longer on Mir, he found three with chromosomal abnormalities
that might be precancerous. From this he calculates that there is a 20
per cent higher risk of dying from cancer - above NASA's limits.
There is even less of a consensus on how to convert radiation dose into
cancer risk for the three to fourfold increase in radiation levels that
exist beyond low-Earth orbit. That makes it impossible for authorities
or the astronauts to make an informed decision about what they are
letting themselves in for.
And while it might be worth taking a higher risk for a one-off mission,
it's not acceptable to send dozens of astronauts routinely into
high-risk situations, like those that could exist at the Lagrangian