Galactic cosmic rays and high-energy solar particles, recorded aboard NASA's Curiosity rover during its 8½-month interplanetary cruise, will give future astronauts a significant — but not excessive — dose of radiation on a round-trip journey to Mars.
Ever since Victor Hess discovered cosmic rays in 1912, scientists have come to realize that space radiation is one of the most formidable hazards during long-duration space travel. A typical astronaut aboard the International Space Station, even while protected by the craft's outer hull and Earth's magnetosphere, absorbs as much radiation in a six-month stay as we ground-dwellers do in 20 years. Head deeper into space — say, on a mission to a nearby asteroid or to Mars — and the risks are magnified considerably.
Plenty of craft have monitored space radiation over the years, but those detectors were completely unprotected in order to get "raw" measurements. Now, however, researchers finally have an idea of how much an astronaut would get zapped inside a reasonably well-shielded spacecraft cruising through the inner solar system.
The findings come from the Radiation Assessment Detector (RAD) that took an 8½-month ride to Mars aboard NASA's Curiosity rover. During that long cruise, RAD was installed inside Curiosity, which in turn was sandwiched inside a coccoon-like aeroshell with the rocket-propelled descent stage over it and a thick heat shield below it. This gave RAD shielding from space radiation much like what NASA engineers are building into the forthcoming Orion crew capsule.
Weighing just 3.8 pounds (1.7 kg), the instrument has an upward-looking "telescope" that lets radiation enter through a 60°-wide cone, and it registers hits from two kinds of high-energy particles. There's a constant background of galactic cosmic rays (GCRs), stripped-down nuclei of various atoms, that arrive from the depths of interstellar space at relativistic speeds and can pack a punch of 500 to 600 million electron volts (MeV) or more. The other major source comes from the Sun in the form of high-energy protons, helium ions, and a few heavier ions. Sporadic flares and coronal mass ejections accelerate these solar energetic particles (SEPs) to energies of up to a few hundred MeV. Both types are harmful because they ionize the atoms in any tissue they're passing through.
As reported by Cary Zeitlin (Southwest Research Institute) and others in Science for May 31st, the RAD instrument recorded the equivalent of 0.466 sievert (a unit of measurement for tissue exposure to ionizing radiation) while en route to Mars. Almost all of that came from cosmic rays: although RAD recorded five distinct pulses of SEPs between early December 2011 and mid-July 2012, those accounted for only about 5% of all the particles detected. These results are actually quite close to researchers' previous estimates of radiation exposure.
"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," Zeitlin notes in a SwRI press release. To put this in perspective, NASA draws the line at 1 sievert of accumulated radiation exposure over an astronaut's entire career, a dose that statistically increases the chance of cancer-induced death by 3%.
Based on the RAD results, a round-trip mission to Mars involving about 360 days of interplanetary travel would expose the crew to about 0.6 sievert of radiation. That's a lot of radiation, though under the established lifetime limit.
But predicting the harm from space radiation is an inexact science. For example, the Sun was relatively quiet during Curiosity's long cruise — yet much more potent solar flares can and do occur. Also, women are more susceptible than men to a given radiation dose because they have lower body masses. And no one really knows how much tissue damage the highest-energy cosmic rays can cause, making assumptions about exposure uncertain.
Still, it's progress. "Scientists need to validate theories and models with actual measurements, which RAD is now providing," notes principal investigator Donald Hassler "These measurements will be used to better understand how radiation travels through deep space and how it is affected and changed by the spacecraft structure itself."