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Superflares could kill unprotected astronauts

A study of the most powerful solar flare of the past 500 years suggests that another like it would carry enough punch to kill astronauts in a poorly shielded spacecraft. The crew of a future mission to Mars might be at risk unless their craft is made of the right materials.

Solar flares send high-energy protons streaming through the solar system, and the radiation is sometimes intense enough to endanger the health of astronauts. In January, the two men on the International Space Station had to shelter in the bulkier Russian side of the station during a particularly powerful series of flares.

Scientists have only been able to directly measure the radiation from solar eruptions for the past four decades. The sun probably produces flares bigger than any seen during this time, so to estimate the worst-case event, they used to simply add together the intensities of two known events, or combine the properties peculiar to each.

Now, Lawrence Townsend of the University of Tennessee in Knoxville, US, and his colleagues have calculated the radiation effects of the most powerful solar flare ever recorded, based on evidence that the flare left behind in Greenland's ice nearly 150 years ago.

The event was seen by the British astronomer Richard Carrington in September 1859 (New Scientist print edition, 4 September 2004). It easily surpassed the monster eruption of March 1989, which knocked out the power grid in Quebec, Canada.

High-energy protons

The radiation from the Carrington flare created nitrates and beryllium-10 in Earth's upper atmosphere. These ended up frozen into Greenland ice, allowing scientists today to measure their levels in ice cores.

The amount of nitrates implies that Earth's atmosphere was hit by about 20 billion high-energy protons per square centimetre, more than in any other event of the past 500 years. And the beryllium reveals that the energy spectrum of these protons was roughly equivalent to that of a flare recorded in August 1972.

Townsend's team used this information to calculate the radiation doses astronauts would receive from a Carrington-type flare, given different levels of shielding. They found that astronauts behind only a few centimetres of aluminium - the shielding found in the average spacecraft - would suffer a dose that could cause acute radiation sickness and possibly even death.

This should not alarm astronauts on the space station, says Townsend. Parts of the station have enough shielding even for this worst-case event, and the Sun's activity is closely monitored, so the crew should get several hours' warning to hide if a huge flare erupts.

But it may have implications for the design of long-duration interplanetary manned missions. Conventional aluminium might not be the best choice of building material for a spacecraft for Mars. "Aluminium is not a good radiation shield," says Townsend. "We are looking at alternative materials, such as polyethylene and carbon foams impregnated with hydrogen. A worst-case event would probably be survivable if you use some other material than aluminium."

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