A month or two back there was a news article (or several) about how spending long-duration flights in space caused astronauts’ eyesight to deteriorate. Since “long-duration” in this case means flights of over a month in low earth orbit the doctors who published the study were rightly concerned about the effect a longer trip in space might have on the astronauts making it; any exploration of the solar system outside of the Moon’s orbit is going to involve flights lasting many months, meaning that this symptom of time spent in low-G environments – which wore off after the astronauts had spent a week or two back on Earth – might suddenly become a hugely relevant problem. However, while I completely get that this is a bad thing that must be avoided or mitigated if at all possible, when you stack it up against the other hazards involved in true long-duration space flights it starts to look positively benign. If only cataracts were the worst an extended journey in interplanetary space could do to us. Unfortunately it’s not quite that simple.
The list of things that can go horribly wrong during even a routine flight up to the ISS in LEO is a very long and varied one. Mostly they revolve around some kind of catastrophic equipment/craft failure like the Columbiadisaster, and despite being only a few hundred kilometres away from the surface of the Earth the chances of astronauts surviving such a catastrophic failure are not particularly higher than they would be if they were in orbit around Mars. However, while spacecraft are complex they are also some of the most rigorously tested machines in history with multiple failsafes; the only reason the shuttle had any fatalities was because of its reusable nature, and the US (at least) has zero other in-flight astronaut fatalities despite making numerous trips to the moon. This makes flights to LEO reasonably safe, and even the moonshots weren’t that hazardous. Modern-day spaceflight isn’t as risky as you might think, and there’s no equipment-related reason why, if the appropriate precautions were taken, a Mars mission wouldn’t be equally as safe.
So if we were to hypothetically spend some astronauts out to Mars, we could be fairly certain that their own spacecraft wouldn’t kill them. That’s good, because there’s a lot of other stuff that can either kill or cripple them in unpleasant ways. Probably the primary concern is cosmic radiation. This is exactly the same as “normal” radiation you’d find spewing out of a breached nuclear reactor – i.e. incredibly dangerous and deadly. There’s a region of space surrounding the Earth called the Van Allen belts which is highly radioactive, but the moonshots proved that astronauts can pop through the belts very quickly without suffering any long-term ill effects. Interplanetary space features a moderate level of background radiation, and Mars astronauts would spend enough time in interplanetary space (9-12 months each way) to have a higher risk of developing radiation-related diseases such as cancer. The big problem, however, is coronal mass ejections, or “Sun burps” as I once described them to a class of ten year olds.
Coronal mass ejections are exactly what it says on the tin: the Sun vomits forth a whole bunch of harmful particles and electromagnetic waves that sleet throughout the Solar System and blanket everything in high-energy radiation. If you’re on the surface of the Earth you usually won’t notice a CME since you’re protected from harmful cosmic radiation by the magnetosphere. All you’ll see are some very pretty aurorae if you happen to live at the correct latitudes. Anyone who happens to be situated higher up in the Earth’s atmosphere – high-altitude planes, astronauts in LEO – are at slightly greater risk but their exposure, if any, will be very short term and almost certainly harmless. Astronauts in interplanetary space are in for a whole world of hurt, however; toutside the magnetosphere there’s nothing to protect them from cosmic radiation, and while the skin of their spacecraft should block some of it high-frequency gamma waves will go right though the spacecraft and the astronauts inside it, shredding their DNA and probably causing horrific organ failure within 72 hours. In other words, being caught unprotected in interplanetary space when a CME hits is like staring an unshielded nuclear reactor in the face, turning anyone unlucky enough to do so into a walking corpse.
What can we do about CMEs? We can’t predict them, so while it’d probably be a good idea to launch a Mars mission in a period of low solar activity to try and minimise the risk we also have to make sure that if one does happen it doesn’t spell instant death for the astronauts on board. The only way to do this is shielding, and lots of it. Only very dense materials like lead provide adequate shielding against gamma radiation, and the drawback of this is that dense materials are also heavy. Getting enough lead into orbit to shield the entire spacecraft is completely out of the question; the best we could do is a radiation-shielded chamber that the astronauts could retreat to when a CME occurs. The good news is that CMEs travel at the speed of the solar wind and take several days to reach the Earth, so the astronauts would have plenty of warning that one was coming up, and since they only last a matter of hours it’s entirely feasible to have them shelter inside the chamber for the duration.
So the radiation problem is one which can be overcome to the extent that we can be fairly sure the astronauts won’t die en route, although they’d be at significantly higher risk of developing cancer than the average person. A trickier problem is muscle and bone deterioration in low-g environments. When you’re walking around on Earth your body is under constant tension in order to counteract the force of the Earth’s gravity. You may not notice it, but your muscles are constantly doing work just to keep you standing upright. Remove the gravity and the muscles stop doing the work, and since muscles are lazy bastards they will immediately start to waste away. Worse is the fact that since bone grows in response to taking loads, no load means the body starts to reabsorb bone minerals and the bones begin to atrophy. This is potentially far more serious than muscle atrophy, which can be reversed in a few months on the ground; bone atrophy can take years to correct, and the longer you are in space the worse the degree of atrophy. A two-year round trip could inflict permanent skeletal damage to the astronauts involved.
Happily there is also something we can do about this, and that something is an absolute crapload of exercise. The daily schedule of an astronaut usually features at least 2-3 hours of workouts in order to give muscles something to do and stop them from getting weaker, and this has had good results in counteracting muscle atrophy. No-one has (as far as I know) come up with a decent solution to bone atrophy, although the effects can be ameliorated by having astronauts wear elastic braces that compress their limbs and provide some of the mechanical stress that’s missing in a weightless environment. Importantly the space endurance record of 437 days is longer than a one-way trip to Mars would be, and the guy who did that recovered fully from his experience1 thanks to a punishing exercise regimen that he stuck to religiously. It is possible to send people to Mars and back without having their bones shatter the moment they set foot back on Earth.
Finally there is the psychological component, and this is one that’s less understood. It is possible to find people who can survive over a year spent in a small tin can without going crazy; that part is no problem. The tricky part is finding a group of people who can spend over a year in a small tin can without killing each other. There are some indirect comparisons you can draw with nuclear missile submarines, which at the height of the Cold War were submerged for months at a time, but even the cramped confines of a submarine would look positively roomy when compared to the living space available to a prospective Mars astronaut. Being stuck in such an environment with the same faces day in, day out, for two years is a significant psychological burden to carry, which is why there’s a number of studies currently going on to see what happens to people in those conditions. For example, Mars 500 was an experiment which locked six volunteers in a Mars craft mockup for 500 days in order to test both the psychosocial effects of that confinement and the ability of the volunteers to adequately deal with any complications that might arise, like a medical emergency. Mars 500 concluded successfully, but while I’m no psychological expert I suspect that no Earth-based mockup will be able to adequately reproduce the feeling of isolation that being several million miles from Earth will create. The true effects of this one aren’t going to be fully known until we actually do it.
Anyway, when weighed up against muscle wastage, bone disintegration, possible irradiation, higher cancer chances and the ever-present risk of going nuts, some slight vision impairment starts to seem a little bit trivial. Certainly I don’t think an astronaut, when presented with the chance to go to Mars, will say “Well I could do that, but on the other hand it might damage my eyesight!” Spaceflight is full of risks, and they are used to taking them. One more isn’t going to make any difference whatsoever.
1. Nearly all the spaceflight endurance records are held by Russians; this is because Russians are intrinsically crazy. Also because one of the main reasons for them putting Mir up there was so they could investigate what long-duration spaceflight would do to people, I guess. But mostly because they’re crazy.