Extreme Medicine (23 page)

Keeping an image stable and clear in your visual field is a pretty difficult task to achieve. First you have to focus the image onto the layer of light-sensitive cells at the back of your eye called the retina. Now, your retina isn't the same all over. At the rear, near the center, is a cluster of densely packed cells, conelike in shape, that account for less than 1 percent of the area of the retina. This tiny but all-important area is called the fovea and is responsible for tasks such as reading or studying a picture. This high density of specialized cells resolves the critical detail of a scene and its colors. The rest of the retina, by comparison, is populated by rods—good in low-light conditions but rubbish at subtlety. Uninterested in nuance, they're there chiefly to pick up movement in the periphery, to identify a target on which you should focus your attention more closely.

Those receptors report to a specialized part of the brain called the visual cortex. What's interesting is that, although the fovea accounts for less than a hundredth of the surface area of the retina—one voice among a hundred—the visual cortex dedicates 50 percent of its mass to listening to the superdiscriminating fovea.

All this effort, and we're still talking about a stationary eyeball focusing on a stationary object.

Now let's start shaking things up a bit. Imagine that the thing you're looking at is no longer stationary but is instead moving. As it moves, you have to rotate your eyeballs to keep its image focused in the right spot. Once it reaches the point at which you can't track it with your eyes anymore, you start to move your head, too.

Now you have two spheres, capable of rotating independently, carrying a lens system that is trying to keep the image of a moving object sharp, on an area at the back of the eye that is only a few millimeters across.

It is the slaving together of the accelerometers in your inner ear, the muscles that rotate your eyeball, and those that turn and tilt your head that allows you to achieve this remarkable feat.

Now imagine that the system doesn't work and that the stable image of the world you take for granted is replaced by a gently oscillating, nausea-inducing scene from which there is no escape. If you've ever suffered from seasickness, imagine the worst possible episode of that, on a ship that you are never allowed to leave and under which the rolling seas will never calm down. That's what it feels like when the organs of the inner ear malfunction. And that can be caused by disease, drugs, poisons, and—as it turns out—the absence of gravity.

—

W
EIGHTLESSNESS MAY SOUND LIKE FUN,
but the majority of rookie astronauts feel sick in the first forty-eight hours of spaceflight. Antiemetic medications—those drugs that act to combat feelings of nausea—are among the most commonly prescribed during NASA spaceflights. The undesirable effects don't stop there.

Deprived of gravitational load, bones fall prey to a kind of space-flight-induced osteoporosis. The balance between the populations of cells responsible for laying down bone and those for removing bone is lost, and so bones become less dense and more prone to fracture. And because 99 percent of your body's calcium is stored in the skeleton, as it wastes away, that calcium finds its way into the bloodstream, causing yet more problems.

Hypercalcemia—a pathological state in which the levels of calcium in the blood are raised—is famous for causing a tetrad of clinical problems. Constipation is the least of these, followed by pains in the long bones. More seriously, renal stones can form, blocking the route from your kidneys to your bladder, causing excruciating pain. And finally there is the possibility of psychotic depression. Medical students remember this list as: bones, stones, abdominal groans, and psychic moans. All four are problematic when you could be two years and more than four hundred million miles from your closest family practitioner.

It's not just your bones that waste away. Muscles do too—the antigravity groups at an alarming rate. In experiments that charted the changes in the quadriceps of rats flown in space, more than a third of the total muscle bulk was lost within nine days. More interesting still, astronauts' muscle fiber switches from slow twitch—the efficient, fatigue-resistant type suited to marathon running—toward the fast-twitch variety that a sprinter might prefer.

Meanwhile, the heart and its system of vessels, deprived of the need to work against the force of gravity, become deconditioned. Spaceflight enforces a sedentary existence on otherwise well-exercised physiological systems, slowly taking athletes and turning them into couch potatoes.

For the cardiovascular system, the finely tuned reflexes that on Earth constantly cope with changes in posture sharply deteriorate during extended spaceflight. Picture yourself lying on the sofa, watching back-to-back movies. The doorbell rings, and you spring to your feet; your cardiovascular system is forced to make a sudden alteration. Having gone from lying to standing, the blood in your body now suddenly tries to pool in your lower limbs, reducing the volume that returns to the heart and as a consequence the force with which it beats. In addition, the blood that was lazily flowing between your heart and brain along your carotid arteries is now trying to travel vertically against the pull of gravity.

Combined and unopposed, these changes will leave your brain deprived of an adequate blood supply and you unconscious on the floor.

All that stands between you and that fate is a reflex that senses the drop in pressure in the carotid arteries and tells the brain to increase the rate and force of contraction of the heart, while simultaneously constricting peripheral blood vessels to restore blood pressure. This primitive reflex is all-important. Without it, you'd end up lying in a crumpled heap every time you stood up too suddenly.

This is what we see in astronauts returning from long missions aboard the space station. Asked to stand still and upright for ten minutes, a significant fraction are unable to do so without feeling faint. This we call postflight orthostatic intolerance—an inability to maintain an upright posture.

The impairments don't stop there. There are other, less well-understood alterations. Red blood cell counts fall, inducing a sort of space anemia. Immunity suffers, wound healing slows, and sleep is chronically disturbed.

In short, most astronauts return from long-duration spaceflight—missions of more than six months—in a temporarily diminished state: sleep deprived, their cardiovascular system deconditioned, their muscles and bones weakened, and their hand-eye coordination impaired. As blissful as the experience of floating around might appear, it erodes the body's ability to function when challenged again by the force of gravity.

When astronaut crews arrive back on Earth, they are met by a support team that includes nurses and physicians, and they are spirited away to recuperate from the experience. And even then, with all the care that the assembled terrestrial recovery forces can muster, there are still incidents. Returning crew members have been known to vomit at celebratory banquets, collapse in showers, or run their vehicles off the road because of transient disorientation.

Others, forgetting that they have returned to a world ruled by gravity, drop expensive equipment or fragile gifts, having got used to the idea that released objects float rather than sink to the floor. Back at home, one astronaut reportedly got out of bed to change his infant son's diaper and stood for a while wondering how he might Velcro the baby to the cot while he searched for some wipes.

The problems of spaceflight are principally those of readaptation to a world in which gravity is the shaping force. Reacclimatizing to that, both physically and psychologically, is a challenge. On return to Earth, astronauts are carefully monitored while their bodies readapt. But on a mission to Mars, they'd arrive and be entirely on their own.

The crews that arrive at Mars would do so after six to nine months of flight and would experience many if not all of these problems. There they would have to perform the most challenging landing in the history of human spaceflight. The communication delay between Earth and Mars might be up to twenty minutes. In that moment of touchdown, they would be truly alone. Assuming they land safely—and remember that around 50 percent of everything we've thrown at Mars has crashed or disappeared—they'd then have to leave their vehicle to walk to the pre-prepared habitat. That habitat might be up to half a kilometer away.

And that's assuming they even make it that far.

—

I
T'S WORTH BRIEFLY CONSIDERING
what it takes to get to Mars. The term
spaceflight
is something of a misnomer. Human-rated spacecraft don't really fly through space. Their rocket motors fire for only a few brief minutes at the start of the journey, throwing the vehicle and its occupants toward their intended target, like a medieval ballista hurling a missile at the walls of a castle. The spacecraft have their own rocket motors and thrusters, but these are far less powerful than the launcher that set them on their way. Once they're traveling, only subtle course corrections can be made. So astronauts on their way to their destination are engaged in an activity that might more accurately be described as spacefall.

While the vehicle and its crew are busy falling across space, Mars is out there somewhere in the darkness, tearing around its elliptical orbit at a little over fifty thousand miles per hour. Mars's journey around the Sun takes 687 days. Earth completes its orbit in 365.25 days, moving at around seventy thousand miles per hour, which leaves the two planets constantly changing their relative positions in the sky.

This has consequences. It means that you can't decide to go to Mars any time you want. You have to wait for precisely the right opportunity, launching from low Earth orbit at exactly the right time, so that Mars is there when you arrive. And the same is true upon your return.

Despite these restrictions, there are as many different recipes for getting to Mars as there are for the perfect chicken noodle soup. Mission architects have to juggle propulsion systems, trajectories, vehicle velocities, and atmospheric entry strategies and trade these against payload mass and crew size in an attempt to design something realistic in terms of risk and cost. They have to decide, for example, between exotic deep-space maneuvers—which might use the orbital energy of Venus as a slingshot to propel vehicles on their way to and from Mars—and more prosaic but potentially safer journeys.

But in the end, all of the mission designs boil down to two broad scenarios: those that see you arrive and stay on Mars for a few weeks and those that leave you on the surface of the Red Planet for more than a year. These are the so-called short-stay and long-stay mission architectures for Mars.

For the short-stay missions, crews would travel for close to nine months to get to Mars. But once there they could then take advantage of an early opportunity to return to Earth, which would arise between thirty and ninety days after their arrival. This, after having spent close to nine months in flight, would be like flying from London to New York, milling around in the gift shop at JFK for an hour, and then flying straight home. But it has the advantage of shortening the total mission duration to less than twenty-four months.

For the long-stay missions, you can get to Mars a little faster, closer to six months than nine, but in this case, the elliptical movements of the planets mean you don't get a chance to come home again for something like eighteen months.

That means you'd spend at least a year traveling and a year and a half or more on Mars. That mission would approach three years in duration—all of which would be spent weightless or working in the reduced gravity of Mars.

There are a number of formidable problems that accompany missions of such duration. The first is life support. How do you invent a system that can keep a crew of four alive for nearly three years? For space stations, breathable oxygen is generated by electrolyzing water: using a current to decompose it into hydrogen and oxygen. This requires a steady supply of water, which is conveniently resupplied from Earth via the Russian Progress vehicles: automatically piloted, space-age delivery trucks. The carbon dioxide that would otherwise accumulate is scrubbed out using chemical sieves—canisters of lithium hydroxide that react with the CO
₂
and remove it from the atmosphere. These too need to be resupplied aboard the Progress vehicles, along with food for the crew.

But there is no easy way to resupply a team traveling to Mars, and so a number of ingenious solutions to this problem have been proposed. One involves a grow-your-own approach to life support and nutrition.

One of the experiments under way when I first visited Johnson Space Center in 1997 was exactly this. Plants respire photosynthetically, by taking in carbon dioxide and generating oxygen and water. It turns out that if you grow ten thousand wheat plants, you can generate more than enough oxygen to breathe while removing the human waste gas of carbon dioxide. Better still, you have a partial source of nutrition. For a while, the Space Center had a team of four volunteers locked up in a hermetically sealed tube, subsisting pretty independently on this self-regenerating, hydroponically grown life-support system. And that's all great—until you factor in the possibility of crop failure.

Another solution, discussed at a European Space Agency human space-exploration symposium, would be to grow vats of algae, which might be easier to sustain than wheat and would also provide a source of protein. Between that and the wheat plants, you could get halfway to a diet of pizzalike food—bread coated with flavored algae—and massively reduce the weight and volume of the food and life-support apparatus required for a Mars mission.

After that conference, I remember listening wide-eyed in the bar while an excitable Frenchman who specialized in the field of regenerative life support told me how it might work, going so far as to explain the recycling of urine and the use of feces as a source of fertilization.