By Charlene Porter
An astronaut crew aboard the space voyage that took the rover Curiosity to Mars would have been exposed to an excessive level of radiation, NASA scientists announced May 30. Analysis of data collected by a Radiation Assessment Detector (RAD) on board the flight is giving NASA scientists greater insight into the systems that will be required to protect a human crew on a Mars mission.
“The radiation environment in deep space is several hundred times more intense than it is on Earth,” said Cary Zeitlin, a principal scientist at Southwest Research Institute in San Antonio, Texas, a NASA partner in this research. “That is even inside spacecraft,” where RAD took its measurements in the instrument array of the Mars Science Laboratory.
Zeitlin is a principal author of the findings as published in the journal Science on May 31.
RAD data give scientists more information than ever before on a spacecraft environment comparable to that which will one day carry a human crew to Mars.
“Curiosity’s RAD instrument is giving us critical data we need so that we humans, like the rover, can dare mighty things to reach the Red Planet,” said William Gerstenmaier, NASA’s associate administrator for human exploration and operations.
With current technologies, radiation exposure exceeds the safe career limit already established by NASA of 1 sievert, the internationally agreed-upon unit for measurement of the biological risk of radiation exposure. RAD showed that Curiosity was exposed to an average of 1.8 millisieverts per day. The spacecraft took eight months to reach Mars, so on the first leg of the mission, astronauts’ cumulative radiation exposure would be nearly half the career acceptable level. With time spent on the planet and a return trip, astronauts on a Mars mission would be exposed to a level of radiation that would increase their risk of cancer.
Chris Moore, NASA’s deputy director of advanced exploration systems, said polyethylene shielding protects the crew on board the International Space Station. Another method, filling the space capsule’s walls with water, would offer some protection for the astronauts, because water’s hydrogen molecules will weaken the radiation entering the ship.
NASA research on better shielding technologies is underway. Developing better methods to detect an oncoming blast of radiation is another possible means to warn a crew, allowing them to take protective measures.
Another way to approach the problem is to make the spacecraft go faster. If a human crew can reach Mars more rapidly, the amount of time spent exposed to space radiation is reduced. Moore said nuclear-thermal propulsion of spacecraft is likely the best option for making substantial gains in the speed of the trip, but NASA has not made a lot of progress in developing that technology.
“It’s a long-range technology development activity, and it will probably be many years before that is ready,” Moore said. He said the use of nuclear rockets to close the distance between Earth and Mars might reduce the travel time down to about six months each way.
Space travel endangers the health of human astronauts with two forms of radiation. The sun emits solar energetic particles (SEPs) in solar flares and coronal mass ejections. Galactic cosmic rays (GCRs) come from beyond our solar system, with origins in supernova explosions and other high-energy events.
Current spacecraft shielding technology is more effective against SEPs than GCRs. NASA describes the GCRs as “highly energetic, highly penetrating particles” that would require shielding meters thick to protect space travelers, so thick and heavy that such shielding would be impossible to propel into space.
“We need to get there faster to reduce the impact of the galactic cosmic rays,” said Eddie Semones, the spaceflight radiation health officer at NASA’s Johnson Space Center, “but we need to have local shielding on board to eliminate the effect of solar particles, so it is hand in hand.”
Shielding technologies that would safely protect a crew from radiation exposure are currently available, while adequate propulsion technologies remain years away.
“But we have a path forward, we’re working on it,” Semones said.