Sending humans to Mars holds radiation risk, study shows
A radiation-monitoring device carried by NASA's Mars rover Curiosity took measurements during the trip to the red planet. A resulting study appears in Friday's issue of the journal Science.
For any astronaut tapped for a trip to Mars, a journey today would be a once-in-a-lifetime experience – in more ways than one.
With shielding comparable to the level built into NASA's new crew-exploration vehicle (CEV), just the trip to and from the red planet could expose travelers to an accumulated dose of radiation that would approach – and in some cases exceed – the maximum allowable career limits for a NASA astronaut.
The Europeans, Russians, and Canadians accept a somewhat more liberal exposure limit than the National Aeronautics and Space Administration does. Even at that limit, Mars would probably represent a memorable but one-and-done spaceflight career for any prospective astronaut.
Those are among the implications of measurements taken by a radiation-monitoring device that NASA's Mars rover Curiosity carries. The measurements were taken during Curiosity's 253-day, 347-million-mile trip to the red planet. The data were unveiled Thursday in a study set to appear in Friday's issue of the journal Science.
"NASA is very excited to get this new cruise data," said Eddie Semones, radiation health officer at NASA's Johnson Space Center in Houston during a briefing Thursday to discuss the results. The data also will help shape human-exploration missions other than to Mars, he adds, noting that missions to retrieve asteroids or even missions to the moon will benefit from the information the new results provide.
Scientists and engineers long have recognized radiation in space as perhaps the most significant challenge to exploration beyond low-Earth orbit, where Earth's magnetic field acts as a natural deflector shield.
The hazard has spawned a range of studies on how to deal with the risk, ranging from concepts for spacecraft that can generate protective magnetic fields to individual body armor for astronauts.
To date, researchers had gathered "outdoor" radiation measurements on a wide variety of robotic spacecraft visiting comets, asteroids, and other planets. The detectors on these missions were exposed to space by design, so they could measure radiation levels around a planet or in interplanetary space.
Curiosity's device, the Radiation Assessment Detector, is bolted to the rover's deck to measure surface radiation on Mars. So, like the rover, RAD was cocooned within the Mars Science Laboratory spacecraft during its trip. Thus, its en route measurements provide the first data from within an interplanetary spacecraft and with a level of shielding that the first Mars explorers might receive from their craft.
The study comes at a time when two private groups are aiming to send humans to Mars long before any NASA astronauts are sent – a prospect the Obama administration and the agency envision taking place after 2030.
In March, a group headed by Dennis Tito, the first space tourist to visit the International Space Station, announced the goal of sending two people on a 501-day fly-around mission to Mars, launching in 2018. The mission, Inspiration Mars, is a one-off attempt at inspiring a new generation of space explorers as well as providing NASA with information on technological, physiological, and psychological issues that no Earth-bound Mars analog station can deliver.
Earlier, a nonprofit group based in the Netherlands announced an effort to set up the first permanent human settlement on Mars in 2023.
The radiation that the new study examines – actually speedy, electrically charged particles such as protons and ions – comes from two sources.
The sun sends out a constant stream of protons in a feature called the solar wind. The wind is punctuated by solar storms, ranging from flares to enormous coronal-mass ejections – the most powerful eruptions the sun delivers. The protons from these storms travel with higher energies than the solar wind and so represent the biggest particle-radiation risk from the sun, the RAD researchers say.
The second source is the galaxy itself, which bathes a spacecraft in low but persistent levels of energetic particles known as galactic cosmic rays. These are generated when massive stars explode as supernovae or in other high-energy cosmic events. About 85 percent of the cosmic rays are protons, another 14 percent are helium ions, and the remaining 1 percent consists of heavier ions.
Heavy-ion collisions can have an influence larger than their small proportion would suggest because when these smack into a spaceship, they create a shower of secondary particles that can inflict biological damage. However, estimates of the level of damage carry large uncertainties.
RAD took its measurements between Dec. 6, 2011, and July 14, 2012, which was less than a month before Curiosity landed. During that period, it recorded protons from five energetic outbursts from the sun, which collectively delivered a level of radiation that matched 15 days' worth of exposure to galactic cosmic rays and their byproducts.
Using a unit of radiation exposure known as a sievert, the team estimates that the one-way trip recorded an exposure level from all sources of 466 millisieverts, give or take 84 millisieverts – or about 47 percent of the 1-sievert career limit that Canada, Europe, and Russia have for their astronauts.
For NASA, that level far exceeds the maximum career exposure for astronauts of 198 to 396 millisieverts, depending on gender and age. NASA takes a more conservative stance than its international partners in no small part because of the uncertainties attached to the secondary effects of the heavier ions among galactic cosmic rays.
Scaling the numbers down from Curiosity's trip to a 180-day, one-way excursion, the team calculates that a crew in a CEV-like spacecraft would be exposed to some 331 millisieverts from galactic cosmic rays alone, with additional exposure during solar outbursts. A round trip would yield an exposure level of 662 millisieverts, unless the sun unleashed some hefty burps to increase the level.
Mission planning can tweak these numbers in astronauts' favor, researchers say.
Sending explorers during a low point in the sun's 11-year sunspot cycle could reduce the risk from solar particles. But the sun's magnetic field would be weaker, allowing more galactic cosmic rays to buzz through the solar system. Likewise, a more active sun means a stronger magnetic field and fewer galactic cosmic rays – but greater chances of strong outbursts from the sun.
Since solar particles come from a specific direction, however, it's possible to orient a spacecraft so that it presents the most mass – the stern of a well-stocked service module, for instance – to the oncoming protons. Water, which can stop the protons, could be stored in module walls as added shielding.
Also, astronauts could don suits of radiation armor being developed. One approach uses layers of polyethylene one to two inches thick to develop a suit with a Tokugawa shogunate sort of look.
"I tried on one of those garments once," said Chris Moore, deputy director of advanced exploration systems at NASA headquarters in Washington. "It reminded me of samurai armor."
Such planning also implies a need for substantial improvements in forecasting the sun's behavior – so-called space weather, researchers say.
Looking back at forecasts in 2008 or 2009 for the current solar cycle, predictions for the state of the sun and its magnetic field during Curiosity's cruise phase anticipated "sort of a normal solar maximum" with a similarly beefed-up solar magnetic field, said Cary Zeitlin, a scientist at the Southwest Research Institute in Boulder, Colo., who is a member of the RAD science team and is the lead author on the Science paper describing the results.
"In fact, what we're having is a very weak solar maximum," which exposed RAD to more galactic cosmic rays than would have been envisioned, he said. "In my mind, that really raises the question of how well we can predict the solar cycle" in ways that help reduce the risk of exposure to astronauts operating beyond low-Earth orbit.
Technological approaches involve more powerful rocket propulsion – solar-electric propulsion or nuclear-thermal motors – to reduce the travel time, and hence exposure.
Engineers also are looking at generating magnetic fields around a spaceship. For instance, NASA is exploring the use of high-temperature superconductors, which carry electricity with no resistance when the material is cooled sufficiently. In the process, these superconductors generate enormous magnetic fields compared with typical electrical conductors.
An initial one-year concept study conducted by a team led by Shayne Westover at the Johnson Space Center looked at different exterior magnet designs and found one particularly promising. Results from that study were published last November. The team is undertaking a second, more detailed study to refine overall spaceship-and-magnet design concepts.