The Science of Avatar

by Stephen Baxter

  Aboard Venture Star, this problem has been solved by using Pandoran unobtanium, a room-temperature superconductor, to generate the intense magnetic fields necessary for successful containment (see Chapter 15).

  More fundamental than containment, however, is the problem of where the antimatter is going to come from in the first place.

  How much antimatter would Venture Star need?

  Robert Forward came up with some numbers for his piondrive propulsion system. He figured that to get a small unmanned one-tonne probe to Alpha Centauri at a tenth lightspeed would require around a third of a tonne of antimatter. Venture Star is a lot bigger than that, and goes a lot faster, and, as you can imagine, the fuel load thereby increases; probably the antimatter required is going to be the same order of size as the mass of the ship itself—hundreds of tonnes, perhaps, or thousands. That’s why those spherical fuel pods in Venture Star’s engine stack are so large. Where is RDA going to find that much antimatter?

  The trouble is, antimatter doesn’t seem to be easy to find in nature. Here we’re getting into questions of physics and cosmology. Dirac’s equations were symmetrical—they predicted that equal amounts of antimatter and matter should have come spilling out of the Big Bang in the first place. If so, where is the antimatter? How come we don’t see matter-antimatter annihilation events all around us? As far as we can tell the observable universe is basically just matter, aside from traces of antimatter emerging from natural high-energy events like supernova explosions, which leave signatures in cosmic rays.

  The answer seems to lie in the subtleties of high-energy physics and the details of creation after the Big Bang. The laws of physics may not be quite symmetrical after all. A bit more matter than antimatter came spewing out of the Big Bang. A carnival of annihilation followed, filling the universe with a bath of radiation, and eliminating all the antimatter, and all but a trace of the matter. The excess of matter over antimatter had only been one part in ten billion, but that was enough to provide all the matter that makes up the galaxies, stars, planets, and you. This is a nice bit of physics, though the details are far from settled. But for a would-be antimatter rocket engineer all that’s important is that nature seems stingy when it comes to coughing up the juice.

  Could we manufacture it? At the moment our only antimatter “factories” are particle accelerators, like Fermilab in Chicago. The antimatter produced by slamming fundamental particles into each other at near-lightspeed inside such machines amounts to around one ten millionth of a gram per year. (And that’s at a cost of around a hundred thousand trillion dollars per kilo! This, you will note, is somewhat higher than unobtanium’s twenty million per kilo, as Selfridge quotes to Grace Augustine.) To make a few hundred tonnes at that rate would take millions of billions of years, a time which exceeds the age of the universe by a factor of… oh, let’s not even go there.

  There will clearly have to be a revolution in antimatter procurement to make all this work, and maybe that will come. Antimatter does have some practical applications today, such as in the PET (positron emission tomography) imaging system used in medicine. Maybe that will promote advances in its manufacture and storage. And Robert Forward pointed out that a factory dedicated to producing antimatter could be a lot more efficient than high-energy physics experiments producing it as a by-product.

  This is what has been achieved in the universe of Avatar, in which a tremendous particle accelerator on the far side of the moon churns out antimatter in the quantities needed to send Venture Star and its sisters to Pandora—and the reason this giant engine is on the lunar far side is to keep the Earth safe from the huge energies it handles.

  Interstellar travel is hugely challenging. For now we can say that we know Venture Star could work in principle, but we don’t yet know how to build it, and couldn’t yet manufacture the antimatter needed to run it. But we do believe it could one day exist, and could take us to Pandora.

  And, even though Jake Sully sleeps through the whole thing, the journey itself would be a tremendous adventure.

  11

  STARS TO SAIL BY

  Interstellar distances are appalling. To scale, the stars are like grains of sand separated by kilometres.

  Thomas Henderson, the first man to measure the true distance to Alpha Centauri in the nineteenth century (see Chapter 12), was so shocked by his result that he hesitated to publish it. It will be daunting even for an interplanetary civilisation; the distances between the stars are hundreds of thousands of times the distances between the planets of the solar system.

  That’s why the cruise of Venture Star, even to the nearest star system and even moving at a respectable fraction of lightspeed, will take years. And why the journey itself is a significant challenge.

  To begin with, Jake Sully’s five years, nine months and twenty-two days is a long spaceflight. The longest human spaceflight to date was by Valeri Polyakov, a Russian cosmonaut who stayed on the Mir space station from January 1994 to March 1995, during which time, endlessly circling the Earth, he travelled some three hundred million kilometres, or around seventeen light-minutes. That’s why the fifteen-strong crew of Venture Star is rotated in three waking shifts of five each, so nobody has to endure the whole journey.

  What about life support? Whether you’re on the moon or on Mars or suspended between the stars, there are common technological challenges in maintaining small habitable volumes for long periods, with closed loops of air, water and other essentials. We don’t know how to do this yet; small systems tend to be unstable, as discovered from the “Biosphere II” experiment in Arizona in the 1990s. Today we are running simulated long-duration “missions” on Earth, such as the Russian Mars500 project, in which six Russians, Chinese and Europeans were locked away in steel tanks without resupply from outside for the length of a near-future Mars mission. The “mission” had such real-life features as communications time delays, and a “landing” in which the crew were separated into “surface” and “orbit” teams. Perhaps soon, according to President Obama’s new vision (see Chapter 6), we will be running real space missions to near-Earth objects that could last hundreds of days away from the Earth.

  By the time we launch Venture Star we’ll surely have solved these problems. Even so, to survive more than five years, even with their passengers stored in cryosleep, the waking crew of Venture Star will have to manage their resources with almost one hundred per cent efficiency. It is a supreme irony that to reach their interstellar goal the crew, citizens of an evidently supremely wasteful civilisation, will have to become experts at recycling.

  They will also have their own health to think of.

  The design of Venture Star and its mission must be constrained by human factors. The higher the acceleration during the boost phase, and the longer it can be sustained, the better, as the overall mission time is reduced. But how much acceleration can a human body stand?

  Since the arrival of high-performance aircraft and the space age our tolerance of G-forces has been studied by organisations like NASA and the military. Most of us can withstand a couple of G (Earth standard gravities) for short periods. That’s what you would experience on a mild roller-coaster, though some can pull you through as much as five G, briefly. We are most vulnerable to accelerations when we’re standing, because that drains the blood away from the brain; ten seconds at five G leads to tunnel vision and then blackouts. Fighter jets can impose up to nine G vertically, and pilots trying to stay conscious wear stretchy “G-suits” to force the blood up to the brain. Pilots with the highest tolerance are known in the trade as “G-monsters.” You can improve your G-tolerance with training in centrifuges, like the spinning rotor arm on Venture Star, though turning a lot faster. The secret is to tense your leg and abdominal muscles to force the blood to the upper body; you strain, as if you were suffering a particularly difficult bowel movement.

  It seems unlikely however that without major re-engineering the human body is ever going to be able to function effectively in gravit
y fields of more than a few G. You could move around in an exoskeleton like Colonel Quaritch’s AMP suit if you had to, but your cognitive functions would likely be impaired. The RDA designers probably pushed the G load in the boost phases as high as they could. But even to withstand months at Venture Star’s one and a half gravities, the crew must have been hardened by some serious time in the centrifuge.

  Meanwhile the long cruise phase holds its own hazards as well: not too much gravity, but too little.

  On Pandora we see Colonel Quaritch ferociously exercising, because, he says, low gravity makes you “soft.” He’s probably right. Without gravity pulling on your body, you would suffer what’s become known as “space adaptation syndrome.” You’d suffer immediate effects such as a redistribution of the fluids in your body, and in the longer term a wasting of your relatively unused muscles, in your legs, for example. There are other effects which appear to be permanent, such as a decrease of bone density.

  To compensate, astronauts on the space stations have always tried to exercise, to put their bones and muscles under regular stress. One good recreational way to do this, incidentally, might be through contact sports like wrestling or sumo, where you stress your body against somebody else’s—I can see Quaritch putting his rookies through that, en route to Pandora.

  On board Venture Star there is a higher-tech solution. The awake crew have been given artificial gravity during the cruise by that rotating “arm” turning around the ship’s spine.

  It certainly would feel like gravity if you stood inside one of the compartments at either end of the arm. On Earth, the planet’s gravity is constantly pulling you down towards the centre of the world; you’re stopped from falling by the reaction of the ground beneath your feet, pushing back at you. Inside the ship’s rotating compartment the floor is similarly pushing at your feet, so it feels like a reaction against gravity. But in fact the floor is pushing to keep you moving in a circle. If the compartment suddenly dissolved and you were released, you’d go flying off in a straight line at a tangent to the circular motion—just like a bolas whirled and released by a Na’vi hunter. The artificial gravity you feel is what the engineers call a “fictitious force”; it is a “centripetal force,” which means “centre-seeking.”

  As you can imagine, the faster you are whirled around by the ship’s arm the greater the apparent gravity. And also the longer the arm is, the more “gravity” you would experience—but the greater the engineering challenge, for all that spinning mass would have to be compensated for, if the ship itself wasn’t to start spinning the other way in reaction.

  How much gravity is “enough” for the human body—a sixth of Earth’s like the moon, a third like Mars? We know something of the effect of extended periods of zero gravity on human physiology, but we know nothing at all about extended periods of partial gravity, as you might experience in Venture Star’s spin module, or on low-gravity worlds like the moon, Mars and Pandora. We’ll have to find this out before we can design a ship like Venture Star.

  And there’s another “fictitious” force to contend with in a spinning environment, called the Coriolis force. This acts on a moving body to curve its motion in the opposite sense to the spin. This has real consequences for us here on the turning Earth, such as the deflection of moving masses of air into weather systems. In a spinning habitat Coriolis effects will interfere with the inner ear, causing dizziness, nausea and disorientation. Experiments have indicated that at two rpm (revolutions per minute) or below, most people will suffer no adverse effects from Coriolis forces; at seven rpm or above, most people will suffer. Venture Star’s arm turns at around three rpm—you can see this in the movie and time it—which looks a sensible compromise.

  Maybe the human body is going to prove more adaptable to long-term spaceflight than we think. I once met Sergei Krikalev, the cosmonaut who holds the record for the most time in space accumulated on separate missions, an astounding eight hundred and three days. And I have to say he looked pretty healthy to me.

  There would be plenty of work for the crew to do through the long cruise. There would be basic systems maintenance; in a system as complex as a starship, over such a long journey, you can bet that a lot of glitches, and even multiple failure modes where one fault compounds another, are going to crop up. This is one reason an awake human crew will be required, for their flexible problem-solving capability—evidently beating out the capabilities of even the super-advanced artificial intelligences of the twenty-second century.

  And, outbound, the most essential work the alert crew will have to undertake is to care for their precious live cargo: the avatar bodies being grown in their tanks, and Jake Sully and the other (human) passengers undergoing “cryosleep,” suspended animation.

  The idea of using cold to induce suspended animation—to halt, temporarily, all the body’s functions—has a long history. There have always been cases of humans being saved for example from near-drowning accidents by hypothermia, the deep chilling of the body, which induces a kind of natural cryosleep. In antiquity the pioneering doctor Hippocrates advocated packing wounded soldiers with snow to keep them alive. There is good science behind this. For every six degrees’ drop in your core body temperature your metabolic rate drops by fifty per cent.

  Deep cold is already used routinely in medicine. Some tricky heart operations require that the body’s blood flow be cut off entirely, while the surgeons get on with their repair work. But at normal body temperature, brain cells can survive only five minutes or so without oxygen from the blood. After that you get brain damage, and, ultimately, death. This survival interval can be greatly extended if the patient is cooled down, to give the surgeons a chance to do their work. The technique is known as Deep Hypothermic Circulatory Arrest. Suspended animation has other potential applications, for instance for patients waiting for organ donation—or, to go back to Hippocrates, to stabilise soldiers critically wounded on the battlefield.

  But there are complications. Cells can be damaged by the cold itself; Jake wouldn’t have been helped to wake up with frostbite. In the Avatar universe RDA scientists have found a way to use microwaves to “jostle” water molecules in cells, and so prevent the formation of damaging ice crystals. But even without actual damage the effects of cooling on the body are complex; humans after all are not animals that naturally hibernate. For example, immunity reactions are slowed.

  For now, NASA and ESA are not funding any research into suspended animation, though both appear to be keeping an eye on developments elsewhere.

  One last job for Venture Star caretaker crew, and perhaps the most glamorous, is interstellar navigation.

  Navigation is the science of figuring out precisely where you are and where you’re heading. And, given the vast distances involved and the relative smallness of the target, you might imagine that navigation and some kind of mid-course corrections will be necessary during Venture Star’s cruise.

  To some extent interstellar navigation will be based on principles developed over millennia on Earth, principles we’ve already adapted as we’ve sent probes out beyond the planets, and have landed humans on the moon at target destinations with an accuracy of metres. We’ve all become used routinely to locating our positions with enormous precision thanks to the GPS system of satellites, a system consulted by smart phones and satnav systems. Conceivably, by the time Venture Star carries Jake Sully to Pandora, some chain of interstellar location beacons could be established to help a passing starship figure out its position. The receipt of pulses from beacons on Earth and at Alpha Centauri could also be useful.

  Alternatively, many vehicles (and indeed modern mobile phones) carry accelerometers which can sense movement; keeping track of this allows “inertial navigation,” with which a ship computes where it must be in space simply from its internal sensing of motion. But inertial navigation systems tend to accumulate errors.

  Venture Star’s principle system of navigation is in fact the very oldest: by the stars. Many unmanned space
craft have carried star sensors for just this reason; out in space, surrounded by a shell of brilliant stars, it’s easy to pick out target stars by their characteristic light, and so to fix your position in three dimensions.

  Before any star mission becomes practical, a vast exercise will be needed in nailing down interstellar distances, star positions and velocities precisely. Work has already begun on such a catalogue of stars, with the first space probes dedicated to “astrometry.” ESA’s Hipparcos space mission (High Precision Parallax Collecting Satellite), which ran from 1989 to 1993, produced a high-precision mapping of a hundred thousand stars. The upcoming Gaia, ESA’s successor to Hipparcos to be launched in 2012, is set to catalogue a billion stars.

  For Venture Star it won’t be sufficient to treat the stars as fixed markers, as navigators can on Earth. From the ship, moving among those very stars, the crew will see the stars themselves shift across the sky—though not by much; the distance to Alpha Centauri, four light years, is still relatively short compared to the distances to the furthest visible stars. The stars of the Orion constellation, for example, are scattered through a volume of space a thousand light years deep, and the nearest of them is no closer than five hundred light years from the sun. Perhaps the interstellar navigators will actually measure the shifting of nearby stars against the background to fix their position (this is effectively how Thomas Henderson calculated the distance to Alpha Centauri in the first place (see Chapter 12)).