Extreme Medicine (24 page)

“You see,” he shouted above the din of the bar, “these people who go to Mars, they will literally 'av to eat their own shit.”

—

I
F THAT HASN'T PUT YOU OFF
the trip already, then consider the radiation hazards. As far as anyone can tell, the background radiation you would be exposed to while traveling between Earth and Mars should be within safe limits—unless there's a solar flare.

These giant eruptions of plasma from the surface of the Sun are accompanied by an intense shower of high-energy particles that rain through space. For the astronauts and cosmonauts operating in low Earth orbit, within the cage of protection provided by the Earth's magnetic field, this presents little problem. The charged particles are caught and trapped by the lines of Earth's magnetic flux, depositing their energy more or less harmlessly, well away from the human crews.

But for a vehicle venturing outside the Earth's immediate neighborhood, there is no such protection. A solar flare is like a neutron bomb going off next to you. Energetic particles—charged helium nuclei, neutrons, protons, and the like—would pass through your body, wreaking havoc and irreversibly damaging cells. Such an exposure would be like taking the DNA blueprints of each cell, shooting cannon balls through them, and then trying to build something based on the information that remains. The resulting structures would be dangerously unstable and prone to malfunction.

The fastest-proliferating cell populations would be worst affected: hair follicles, skin, and the lining of the gut. The rapidly dividing cells of the bone marrow, too, would fall victim. With blood cells decimated, the sufferer would be left anemic, short of platelets to help clot blood and bolster the immune system. This explains the familiar depiction of acute radiation sickness: hair falling out in clumps, diarrhea, bruised skin, and bleeding gums. Without a shield, it would be impossible to survive such an exposure.

To make matters worse, solar flares arise sporadically, and we're about as good at predicting them as we are at forecasting the British weather. And there's no straightforward way of combating their effects. Building a ship coated with lead wouldn't help—even if you could find a way to lift that mass into orbit. Lead and other heavy metals are great at shielding against X-ray radiation and lighter particles, but when it comes to highly energetic heavy particles, they are worse than useless. Massive particles, arriving at close to the speed of light, would smash into the atoms of a metal shield and scatter them like a cue ball hitting a billiard pack. These scattered atoms would then give rise to secondary radiation, as deadly as the particles they were supposed to shield against.

One possibility lies in building a sort of bomb shelter in the spacecraft, an area more resistant to the radiation storms brought by a solar flare. This you could shield, not with layers of metal, but with a jacket of water. It turns out that water is very good at attenuating solar particle radiation. But this is pretty speculative. When it comes to the radiation hazards of a human mission to Mars, if you ask the experts, they tell you that we simply don't yet know enough.

—

E
VEN IF WE FIGURE OUT A
way to negotiate the radiation and build a life-support system that is at least partly regenerative, we keep getting back to the most elemental problem: having to contend with the absence of gravity. The longest mission in human spaceflight history was 437 days, 17 hours, 58 minutes, and 16 seconds; it was completed by cosmonaut Valeri Polyakov aboard the Russian space station
Mir
between 1994 and 1995. By all accounts, he arrived back on Earth in reasonably good health, but it is far from clear that this would be true of all space explorers.

Polyakov is in an exclusive club. Around five hundred people have flown into space. Of these, only ten have flown for more than two hundred days and only two for more than a year.

Most of our experience in the field of astronautics involves missions of less than two weeks' duration. The impairments seen in crew members who have flown for between three and six months are significant and tend to vary from individual to individual.

A range of countermeasures to combat the effects of longer missions is available to astronaut crews. These include medications, special diets, and regimens of resistive exercise. And while they have gone some way to mitigating the consequences of human spaceflight, none appears uniformly effective.

It is because of this that the idea of generating artificial gravity has surfaced time and time again. The concept is not new. The earliest rocket scientists realized that their crews would experience weightlessness and that this might be problematic, even if they could not predict all of its effects.

In 1923 Hermann Oberth proposed a solution: a vehicle tethered to a counterweight that would spin end over end like a twirling baton, subjecting the occupants to an artificial gravitational load as it went. It's the same load we feel on spinning fairground rides, the force that pins us against the side of the car.

So far, so good. But the problem with artificial gravity lies not in the underlying physics of the idea but with engineering a rotating vehicle capable of the feat. Here design is narrowly constrained by the biological frailties of the astronaut crew.

The force of artificial gravity generated by a rotating vehicle depends upon the radius of the vehicle and its rotation rate. To generate enough force it must either be small and spin extremely quickly or be large and spin more slowly.

Everybody differs in their tolerance to fairground rides; some people can be spun at head-snapping rates without apparent ill effect while others feel sick just watching the thing go around. This, again, is down to the apparatus of the inner ear: detecting rotational accelerations, trying to make sense of what is happening, and expressing displeasure through the vomiting center if it cannot. But if the rate of rotation is kept slow enough, to four revolutions a minute or less, everybody in time can adapt to the motion.

With that requirement fixed, the radius of rotation necessary to produce a force of 1 G—equivalent to the load you would feel at the surface of the Earth—can be calculated. It leaves you with a vehicle around 125 meters across—coincidentally about the same size as the London Eye. If the thought of something of that size whacking around four times every minute seems daunting, imagine building a vehicle of that scale and then launching it into space.

NASA did more than imagine. In the 1990s, Kent Joosten and a team of engineers at Johnson Space Center came up with a broad-brush design for an artificial-gravity vehicle that might actually work. This returned to Hermann Oberth's original idea of a tether between a crew habitat and a counterweight. In Joosten's design, the module and its counterweight were separated by an ingenious, ultralight, liquid-crystal pylon structure. This could be compressed and stored during launch from Earth and then deployed after the vehicle had arrived in orbit. The whole thing would then tumble end over end all the way to Mars, with the crew living in a module about the size of a four-bedroom house under conditions that approximate terrestrial gravity.

Joosten's artificial-gravity study represents the most mature technical approach to the subject so far seen. There are, however, a number of significant problems to be overcome before such a vehicle design can be realized. It presents an entirely new paradigm in our concept of what human spaceflight is, and this has in part contributed to a reluctance to embrace or further investigate the idea.

Among the hundreds of studies that have considered how best to get to Mars, nearly all of them have involved smaller, simpler vehicles of the type that took us to the Moon. But there is a way to deliver artificial gravity inside such spacecraft, even if the vehicle itself can't be spun.

In our daily lives, our bodies do not experience constant gravitational load. When we stomp up and down stairs, our joints become shock-loaded, with regions of our skeleton transiently experiencing up to three or four times the gravity they would at rest. When we lie down to sleep, the long axis of our body is more or less perpendicular to the force of gravity, and our skeleton, cardiovascular system, and antigravity muscles are left unloaded. This quasi-weightless state quite closely resembles the weightlessness of spaceflight. Indeed, when researchers want to mimic the effects of microgravity here on Earth, they simply send a bunch of people to bed.

So on Earth our physiology is maintained by only intermittent exposure to gravitational load—the standing up and stomping around we do during the day. And even that isn't constant. From this realization grew the idea that we might prescribe gravity like a drug, giving it in short but large doses. Cue the short-arm centrifuge as a countermeasure to the effects of weightlessness. Instead of building a spacecraft as big as the London Eye and rotating it slowly, you could build a much smaller spinning device, rotate it very quickly, and pack that inside a conventional spacecraft module.

If you do the math on this, a centrifuge with a radius of three meters would have to spin around forty times a minute to generate a load of about 3 G at its edges. This bizarre regimen of loading might nevertheless be enough to protect the body from weightlessness. Better still, it can be administered in short doses; as little as an hour a day might be sufficient. And with this knowledge in hand, NASA went out and built one.

—

S
OMEWHERE IN A
NASA
LABORATORY
in Galveston, the ceiling spins around above my head, revolving forty times a minute. I keep my head straight, eyes fixed on the screen mounted above, about three feet from my face.

Deep within my inner ear are tiny cells with hairlike protrusions that waft in a gel like blades of grass standing vertically, set in a plate of jelly. Part of my vestibular system, these exist to detect acceleration in the world around me. The more the jelly leans over, the more the blades of grass bend, and this triggers the firing of the hair cells. Right now they're struggling to make sense of what I'm being put through.

The set of hair cells in my semicircular canals, the organs that detect rotation, are screaming, firing constantly with the whirling of my body. My brain got bored of listening to that quite some time ago and has decided to ignore their messages, leaving me feeling almost comfortable. But it's a precarious state. There is profound conflict between what I'm seeing and what I'm feeling. My vomiting center—which is wired in to the same box of tricks that senses acceleration—is at this instant just about managing to stay quiet. I have to keep my head dead center to maintain that status quo. If I start jerking it around, I'll be vomiting in seconds.

I'm wearing a headset with a microphone. A researcher in the control room, watching the camera feed, asks me if I'm still OK. I tell him that I am. Another voice from the control room bombards me with a few more questions and then asks me if I wouldn't mind turning my head to take a look at a piece of equipment he's worried about on my right-hand side. I tell him that I'm not falling for that one. Somewhere off-mike, there's an evil chuckle.

I've been here now for half an hour; there are still another thirty minutes left. I am lying on my back on this experimental device: a centrifuge small enough to be accommodated in the module of a spacecraft on its way to Mars.

It looks, at first glance, like an instrument of torture. A pair of arms, each one just about long and wide enough to accommodate an adult lying supine, sprout from a central column. There are harnesses and straps to stop you from flailing around and probes and monitors designed to extract information from you. The whole thing can rotate at a stomach-wrenching rate. If Tomás de Torquemada invented a fairground ride, it would look something like this.

The apparatus is there to interrogate human physiology, to determine how it will respond to this insult. ECG electrodes are glued to my chest, an automated blood-pressure cuff inflates periodically, and a probe monitors the oxygen in my bloodstream.

This is a device for generating artificial gravity—or at least an artificial gravitational load. The forces generated when the machine rotates force me out, trying to fling me toward the walls of the room. I'm stopped from doing so by a plate at my feet. As the centrifuge spins up, I get heavier against that plate. At full tilt, the force on my body below my waist is between two and three times that of normal gravity. In my upper body, where the speed of travel is slower, the load is less. It means there's a gradient of force along my body that builds steadily from head to toe. This gives the illusion that I'm lying with my back arched, making me feel as though I'm engaged in some sort of limbo dance maneuver.

As I settle down into it, I begin to feel more comfortable, comfortable enough to begin to get bored. If I don't move my head around, the whole experience is quite doable, almost relaxing. A voice crackles into my headset.

“How're you doing?” asks the researcher. I tell him I'm fine. “We can stick something on the screen if you're getting bored.” He fumbles around in the control room and slides a DVD into the player. A Harry Potter film springs into view on the screen above me, and all of a sudden, whirling in the darkness in front of a small glowing screen feels no more abnormal than watching an in-flight movie on a long-haul flight. And I begin to think that this could be an OK way to get to Mars after all.

—

A
RTIFICIAL GRAVITY IS ONE OF THOSE
things that people tend to dismiss with a snort if they don't know much about it. It remains unclear how long humans can be deployed in space without suffering serious medical consequence, but it is unlikely that we can endure weightlessness indefinitely and maintain acceptable health. If we are to continue to push out into space, then at some point, artificial-gravity machines—compact torture chambers or giant twirling batons—will have to play a role. This is a natural progression. We take everything else with us into space: our light, our heat, our food and water; we even take our atmosphere. At some point, it seems certain that we'll take gravity with us too.