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
“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
.  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

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.

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

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.

Charles L. Limoli’s article
Deal Breaker” is on pp. 54-59 of the Feb. 2017 edition of Scientific American.

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