Radiation: The Unsolved Problem of Space Travel

These appear to be great times for future space travelers.  Commercial rocket outfits such as SpaceX are thriving, the Dutch organization Mars One is still refining their pool of potential Martian astronauts for a flight presently scheduled for 2032, and National Geographic's "Mars" docu-drama TV series has intermingled interviews with space experts and a dramatic portrayal of what an actual flight to Mars might be like, complete with death-defying crises. 

Then along comes Charles L. Limoli, a radiation biologist at UC Irvine, with his mouse brains shot full of holes, as he describes in an article published in the February 2017 issue of Scientific American.  They're not holes you can see with the naked eye—just tracks of ionization damage caused by nuclear particles intended to simulate the damage caused by cosmic rays and solar particle radiation that a typical years-long round trip to Mars would involve.  The bad news Limoli has is that when you take smart mice and shoot their brains with that much radiation, it damages certain fragile parts of the nerve cells:  the dendritic spines.  And they don't grow back.  So even high-IQ mice who get zapped in a NASA particle accelerator designed to simulate the type of radiation that astronauts will be exposed to, end up with what amounts to a mouse form of Alzheimer's.  The performance of these mice in certain mouse intelligence tests (don't ask me how they figured out how to do that) fell to only 10% of what it was before the zapping.  And the damage seems to be permanent, at least in mice.

So what, you say?  We'll just shield the space capsule.  Think again.

The more energetic a particle is—the faster it's going and the heavier it is—the harder it is to shield against.  Turns out that galactic cosmic rays, which are one of the two main kinds of radiation astronauts on deep-space missions will encounter, are the highest-energy particles known, with many of them having more energy than the most powerful earth-bound particle accelerators can produce.  The only practical radiation shielding presently used involves putting a lot of heavy stuff—concrete, steel, lead—between you and the radiation.  On earth, this isn't such a problem if you happen to have a disused salt mine handy—you just go underground.  But weight is the enemy of space travel, and a rocket that was shielded well enough to reduce cosmic-ray fluxes to something comparable to what we encounter on earth (shielded as it is by its magnetic field and atmosphere) would be prohibitively heavy.  We are talking shield thicknesses of many feet, all around the living areas of the vehicle. 

So at present, all these nice dreams of people spending years in deep space are still only that—dreams.  Based on the work of Limoli and others, sending astronauts on an unshielded rocket to spend a year or more in deep space would likely turn their brains into Swiss cheese, radiation-wise, with dire consequences that would affect everyone on board.  I don't know about you, but I can't think of a more depressing end to a space flight than to have everybody turn into candidates for assisted living in a matter of months.  And that's just if we try to get to Mars.  If the astronauts somehow manage to get there without losing their minds, the first thing they'd have to do would be to dig a deep burrow to shield themselves, or apply whatever unknown shielding technology they used on the trip to the newly established colony as well.

Space optimists look at this problem as just another speed bump on the road to Mars.  There may be medical ways of alleviating some of the damage that radiation causes to neurons.  But any such treatment is far in the future, and prospective space travelers need it now.  There is even less likelihood of finding a lightweight way of shielding against high-energy cosmic rays.  The physics of the problem has been well understood for decades.  In principle, you could use magnetic fields to create a shield, but the field intensity to deflect such particles is absurdly high—even the superconducting magnets in  today's MRI machines, which have to be carefully isolated from any ferrous object, probably wouldn't do the job.  And if you don't do something, you're condemning your space travelers to virtually certain mental decline.  Of course, some may think that this is just a risk we have to take.

Having watched the entire National Geographic "Mars" series over the Christmas holidays, I noted a disturbing tendency on the part of some enthusiasts for what I would term secular millennialism.  The religious millennialists called the Millerites were followers of William Miller, who convinced both himself and many others that he had figured out when the second coming of Christ would occur:  Oct. 22, 1844, about ten years ahead of when he was writing.  In the years leading up to 1844 he accumulated a large following who sold their farms, businesses, and houses and gathered in small groups, waiting for the big day.  In what became known as the Great Disappointment, nothing unusual happened, either then or later.

Secular millennial movements such as classical Marxism exact arduous and even painful sacrifices today for a promised millennial paradise tomorrow—and somehow, tomorrow never comes.  Some present-day promoters of deep-space travel have convinced themselves that the future of the human race lies not on Earth, but on Mars or other places where we'll be able to start over again and do it right.  If you really believe this, it's going to affect your attitude toward life on Earth.  After all, if we're just going to move soon, why paint the walls? 

It's not clear at this point whether radiation will pose a "deal-breaker" problem to deep-space travel.  I suppose if we get clever enough about orbital assembly stations, we could eventually manage to build a rocket that could carry enough conventional shielding to protect astronauts on their way to Mars.  But that does not seem to be in the current plans of many space enterprises. 

If somebody started calling for volunteers, or even offered lots of money, for people to jump off a thousand-foot cliff without parachutes "just for the experience," I hope we would find a way to shut them down.  That hasn't happened so far with Mars One, the Dutch outfit that is currently selecting people to go on a one-way trip to Mars.  But if by the time astronauts gets to Mars, their brains are so fried that they don't know where they are, it would be a shame for everybody—especially the astronauts and those who put them knowingly into a situation that was going to end badly. 

Maybe we'll figure this one out, but in the meantime, any time I see someone promoting manned deep-space flight, I'll be wondering what they plan to do about radiation.  And so far, I don't see anyone taking it seriously enough.

Sources:  Charles L. Limoli's article "Deep-Space Deal Breaker" is on pp. 54-59 of the Feb. 2017 edition of Scientific American.