Radioactively hot, that is.
While the Curiosity Mars Rover was cruising for eight months between Earth and Mars, one of the instruments aboard was measuring the radiation levels encountered along the way. The Radiation Assessment Detector—RAD—was inside the spacecraft, shielded in much the same way as astronauts would be on an interplanetary mission. Now the same instrument is continuing to monitor those radiation levels on the Martian surface. The results from the interplanetary cruise phase of the mission were recently released, and while they don’t rule out interplanetary voyages by humans, they do highlight a significant risk.
Let’s do a Q & A to address some background material and some of the issues involved in order to better understand this.
Where does radiation come from?
On the surface of the Earth, it comes mostly from…the Earth. There is natural radioactivity in soil and rocks and radioactive gas emitted by them. The Earth’s atmosphere shields us from most of the harmful radiation coming from off the Earth. Extraterrestrial sources of radiation start with the Sun, but some of the radiation zipping through space likely comes from beyond our solar system or even from beyond our galaxy.
What does this radiation consist of?
We can most conveniently separate harmful radiation into two categories: electromagnetic radiation and energetic particles. As the wavelength of electromagnetic radiation shortens, its energy increases. Earth’s atmosphere is transparent to visible light, but it blocks the very high-energy radiation given off by the Sun in the form of X-rays and gamma rays.
The Sun also emits energetic particles that are (mostly) blocked by the atmosphere, which also blocks even more energetic particles (rather confusingly called cosmic rays) that can come from extrasolar or even extragalactic sources. When these particles collide with the nuclei of atmospheric gases, they can interact to create a shower of secondary particles that reach the ground even when the original particle does not. As you might intuit, highly energetic particles can do a lot of damage when they encounter frail human flesh.
Why isn’t this a problem for the astronauts aboard the International Space Station?
They are clearly well above the protection of the atmosphere. But they are in an orbit that is still inside the protective bubble provided by the Earth’s magnetic field. Particle radiation (mostly protons—hydrogen nuclei—and electrons, both from the Sun) is deflected around the Earth by this field, and some of it is trapped in the famous Van Allen radiation belts. Both of these belts (inner and outer) are well above the altitude of the International Space Station. Even so, the ISS astronauts’ lack of atmospheric protection increases their exposure. Your exposure is also increased every time you fly in an airplane (less atmosphere between you and the Sun), and people who live at higher altitudes receive more exposure.
When the Sun cuts loose with a solar flare (a sudden brightening and consequent energy release of X-rays and gamma rays), it often follows that up with a coronal mass ejection, a massive “burp” of solar plasma, a dangerous brew of charged and energetic particles. When those particles hit the Earth’s magnetosphere, we are generally well protected, although they may generate some spectacular aurorae near our polar regions.
The only humans ever to travel beyond the protection of this magnetic “bubble” are the 24 Apollo astronauts who flew to the moon and back, three of them twice. They reported seeing light flashes which scientists concluded were probably caused by cosmic rays passing through their eyes.
So how much radiation did the Curiosity instrument measure between here and Mars?
So stated, the question is easy to answer, although I will spare you the details. We know the energies and types of each incident of radiation encountered during that interplanetary cruise. The trickier part is translating that information into how much potential damage it could inflict on human beings.
Turning a measurement of absorbed radiation dose (a clear and unambiguous physical measurement) into a value of effective dose (which measures biological effects) is not straightforward and necessarily involves some assumptions and resulting uncertainty. Effective dose is measured in sieverts (Sv), where one Sv corresponds to a 5.5% risk of eventually developing cancer. A more convenient measure is a millisievert (mSv), one thousandth of a sievert. For some perspective:
• Dental X-ray: 0.01 mSv
• Two-view mammogram: 0.5 mSv
• U.S. annual average from all sources: 3.6 mSv
• Abdominal CT scan: 8 mSv
• Radiation worker annual limit: 20 mSv
• Average for 6 months on the International Space Station: 75 mSv
• National space agencies limit on astronaut career exposure: 1000 mSv
In a paper published in the May 31, 2013 issue of Science, the authors convert the RAD measurements into a dose of 662 mSv for a hypothetical 360-day round trip to Mars. This does not include whatever radiation exposure astronauts might experience on the Martian surface.
Is that significant? Yeah, it is. It’s not just the amount of radiation absorbed that matters, it’s how quickly it is absorbed: whether the exposure is spread out over a decade or more or whether it all occurs in a relatively short period such as the 18 months or so of a round-trip mission to the Martian surface. 1000 mSv over a lifetime represents a slightly increased risk of cancer. After all, an average 80-year-old has gotten 288 mSv over her lifetime. 1000 mSv received over an 18-month period would probably result in clinically detectable symptoms that could impair performance.
The problem is that we know a whole lot more about the effects of some types of radiation than we do about others. In particular, we don’t know much about the effects of massive and energetic cosmic rays, simply because we aren’t exposed to them except in travel beyond low Earth orbit. It seems reasonable to assume, however, that they would not be less damaging than their lighter and less energetic cousins.
Does this rule out human interplanetary travel?
My own take is that it probably does not, but it certainly means that this risk has to be taken into account and dealt with to a greater degree than it currently has. Curiosity flew to Mars during a quiet phase of the Sun’s activity cycle. Future astronauts could well encounter bigger events. Curiosity’s radiation shield was pretty much what is being designed into NASA’s Crew Exploration Vehicle for use beyond low Earth orbit. Increased shielding means increased weight, which means less room for everything else. If I were an astronaut angling for a ride, I would be pushing for reconsideration of this issue.
Mars has neither the Earth’s thick atmosphere nor its strong magnetic field for protection, so more than a spacesuit is needed to keep an astronaut safe from overexposure on the surface. But once you are there, you can take advantage of the natural environment. A shelter constructed from available materials (stone, bricks made from Martian soil) would still require tools for that job. In some locations, however, you might be able to go underground. Several possible openings to caves have been observed from Mars orbit, and these could provide more than adequate protection from radiation.
Space is a hostile environment, and space beyond low Earth orbit is a bit more hostile than we knew. But human beings have a remarkable ability to use their technology to adapt to harsh environments. It just may take us a little longer to surmount the hurdles.