Living for two to four years outside Earth's protective atmosphere and magnetosphere will expose astronauts to much higher radiation doses than those on Earth. The radiation also may come in different forms. One type of radiation that is poorly understood but potentially very hazardous is neutron radiation -- something Maurer's neutron spectrometer is designed to characterize. It is relatively harmless on Earth because the atmosphere absorbs much of its energy, but in space it could be quite dangerous, Maurer said.

When high-energy cosmic rays reach Earth, they collide with particles in the atmosphere, producing lower energy neutrons. By the time the cosmic particles and the neutrons they produce reach humans on the surface, they possess relatively low energy and rarely penetrate deeper than the outer layers of skin.

For astronauts on Mars, the situation would be much different.

Instead of hitting the buff atmosphere of Earth, cosmic rays slip through Mars' thin carbon dioxide covering relatively unimpeded. A high-energy cosmic ray ion could smash into an aluminum beam, for instance, ejecting high-energy neutrons. Even the neutrons unleashed when cosmic rays hit atoms in the atmosphere could pose a health threat.

"These neutrons will penetrate farther into the body of the astronaut than they penetrate in our bodies at sea level," Maurer said. "Since they're higher energy to start with, when they finally interact and deposit a large percentage of their radiation dose, it will be deeper in the body hence, more likely in a blood-forming organ such as the liver or the spleen or the pancreas or something like that."

The result is likely to be an increase in cancer risk for astronauts, but since scientists know nothing about the neutron radiation environment of Mars, it needs be measured, he said.

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