Going to Mars?

This self-portrait of NASA’s Mars rover Curiosity combines dozens of exposures taken by the rover’s Mars Hand Lens Imager. (NASA/JPL-Caltech/MSSS)

Some 18 months ago, the Mars Science Laboratory began a 253-day journey to Mars to deliver the Curiosity Rover to its ruddy surface. While that Rover has performed beautifully and gained headlines, equally important data have now been released about its eight-month odyssey through space that began on November 26, 2011.

En route, the first-ever onboard radiation detector – an instrument acronymically called RAD – measured the radiation environment inside the spacecraft. The results are of vital importance if humans are ever going to journey to the Red Planet.

First, however, one must know that outer-space radiation comes from cosmic sources beyond the solar system – mostly from supernova explosions in our galaxy and in others – and also from the Sun. The former creates steadier and more intense radiation, except when the Sun erupts with a coronal mass ejection or major flare, in which case solar radiation becomes the more intense source.

During RAD’s odyssey to Mars the Sun was quiet, so it could not assess how hazardous it would be for humans to be in space during a major solar storm. Rather, it mostly sampled so-called galactic radiation.

The results were not encouraging. No astronaut who might have made this trip could have been happy with the radiation received.

“In terms of accumulated dose, it’s like getting a whole-body CT scan once every five or six days,” said Dr. Cary Zeitlin, a principal scientist involved with the measurements. Put another way, it was like being a Hiroshima survivor a mile or so from Ground Zero – but having that radiation exposure three times a month, over and over again from January through August.

Zeitlin said, “Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself.”

Galactic Cosmic Rays tend to be highly energetic, highly penetrating particles that are not stopped by the modest shielding provided by a spacecraft. These high-energy particles include heavy ions, which are atomic nuclei without their usual complement of electrons. Heavy ions are known to cause more biological damage than other types of particles.

“A vehicle carrying humans into deep space would likely have a ‘storm shelter’ to protect against solar particles. But the Galactic Cosmic Rays are harder to stop, and even an aluminum hull a foot thick wouldn’t change the dose very much,” said Zeitlin.

The RAD data showed an average dose equivalent of about 0.66 Sievert for this particular round-trip to Mars, which would increase one’s fatal cancer risk by three or four percent. Unfortunately, this did not include solar radiation exposure, since no major solar storms erupted during this time. Moreover, said Zeitlin, “Time spent on the surface of Mars might add considerably to the total dose equivalent.”

More study is needed. But there seems no getting away from the fact that anyone going to Mars will receive dangerous amounts of radiation. “This issue will have to be addressed, one way or another, before humans can go into deep space for months or years at a time,” said Zeitlin.

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