The Science of Avatar

¹² FIRST PORT OF CALL

The very first interstellar journey we make is likely to be, just as in Avatar, to our sun’s nearest neighbour.

Alpha Centauri is a triple star system. The two principal stars, known as A and B, are bound close together by gravity. The twins don’t orbit each other, but both circle a common centre of mass, just a point in space, following looping elliptical trajectories. Each of the two central suns is similar to our sun, A in particular, but these near-twin stars are no further apart than the planets in our solar system. Alpha B comes about as close to A as the planet Saturn does to the sun, though it loops out to Pluto’s distance.

Imagine standing on a planet orbiting A, the brighter star (as Polyphemus does). From here A looks like our sun in the sky. The companion, B, is a brilliant, orange-ish star. Even at its furthest distance from A, B is about two hundred times brighter than the full moon; at its closest it is over two thousand times as bright as the moon. In fact it shows a disc to a sharp enough naked eye.

And somewhere in the complex sky around you is Proxima, the third star in the system, orbiting the main binary pair four hundred times further away from those twins than they are from each other, trundling around an orbit that takes half a million years to complete. (Proxima is so far out that there’s some controversy about whether it’s really part of the Alpha system at all.) Proxima is actually the closest star of all to the sun, which is why it’s so named: like “approximate,” the name “Proxima” comes from a Latin root meaning “near.” Proxima is an unspectacular red dwarf, a minor component of this system—but of great interest to astronomers, for it is actually more representative of the Galaxy’s stars than either Alpha A or B, or indeed the sun; seventy per cent of stars are like Proxima.

You are here! Alpha Centauri: the first port of call beyond the sun’s realm.

As the closest star system, Alpha Centauri has, not surprisingly, featured in many starship studies, and fictional depictions of interstellar travel. For example there’s Leigh Brackett’s thrilling Alpha Centauri—Or Die! (1963), Encounter with Tiber (1996) co-written by moonwalker Buzz Aldrin with John Barnes, my own Space (2000)—and Footfall by Larry Niven and Jerry Pournelle (1985), about an invasion of Earth from Alpha Centauri, rather than the other way around as in Avatar. Avatar in fact seems to be the first depiction of the system in the movies, although it was the target for the hapless star travellers of the TV series Lost in Space (1965–8).

We’ve known Alpha Centauri is the closest star system for nearly two centuries now. This was established in 1832 by a Scottish astronomer called Thomas Henderson, working at an observatory in South Africa (Alpha Centauri is invisible from the northern hemisphere). He used a method called parallax. If you hold a finger up closely before your nose, and then inspect it through first one eye and then the other, you’ll see it apparently shift against the more distant background. If you know how far apart your eyes are, and you measure the apparent shift, you can do a bit of geometry to work out how far your finger is from your nose. This is the method Henderson used, scaled up a mere hundred thousand trillion times. He knew the diameter of Earth’s orbit around the sun, and by studying the way Alpha Centauri apparently shifted across the background of more distant stars as Earth crossed from one side of its orbit to the other in the course of a year, he was able to establish Alpha’s distance. Parallax was a well-established method at the time, having been used to measure the distances between the sun’s planets. But the interstellar distance Henderson worked out was so large it made him hesitate to publish his result; suddenly the universe was bigger than everybody had thought.

Even so, a starry night seen from Alpha Centauri might seem nostalgically familiar.

Of course if you stand on a world of Alpha A, because of the glare of B, you won’t get many dark starry nights. And if your world is a Pandora, a close-in moon of a giant planet, the glare of that primary world will crowd the sky even more—although you will get a spectacular show as the giant goes through its phases, and eclipses one or other of the suns.

With time however you’ll see B track slowly around the sky, like an outer planet in our solar system. Sometimes B will be in the “night sky” of A, and will banish the darkness. But when B is in the daytime sky, and especially when the suns are close together, they will act as if they are a single point of light, like our own solitary sun, and the day–night cycle will seem normal to a terrestrial like you. You may even see a very strange solar eclipse indeed—the eclipse of one sun by another, as B passes behind A.

And there will be a few nights, when the suns are close together and both below the horizon—and when your local Polyphemus has set too—when the distant stars will at last be visible.

You’re a mere four light years from home. If you look around the sky, just as you saw from Venture Star, the constellations are little changed, because most of the stars are much further away than that. But if you look back the way Venture Star came, you will see a compact constellation familiar to any amateur astronomer. That W shape is surely Cassiopeia, one of the most easily recognisable of our star figures. But there is an extra star to the left of the pattern, turning the constellation into a crude zigzag. That star is our sun: just a point of pale yellow light, bright, but not exceptionally so. And from where you stand, the sun, the Earth and all the planets, and all of human history before the first colonists left for Alpha Centauri, could be eclipsed by a grain of sand.

Alpha Centauri, then, is a spectacular place. But the key question is: are there planets? Could Pandora actually exist?

¹³ FINDING NEW WORLDS

In the Avatar universe the geography of the Alpha Centauri system has been worked out in some detail.

All the three stars, Alpha Centauri A, B and C, have planets. Even C, the red dwarf, has a close-in gas giant and two rocky worlds. B has one gas giant and five rocky worlds, and an asteroid belt; B’s subsystem is perhaps most similar to our own solar system.

A, the largest star, has three gas giants and three rocky worlds. Polyphemus is one of the gas giants, with similar size and mass to Saturn in our system, though without the rings. It orbits at about the same radius from Alpha A as Earth does from the sun—unlike Saturn, which is about nine times further out from the sun than Earth. Interestingly, rather like the Trojan asteroids in our solar system (see Chapter 6), two rocky bodies share Polyphemus’ orbit, at points of gravitational stability sixty degrees ahead of and behind the planet: one significant rocky world and one planetoid. Polyphemus has fourteen moons (compared to Saturn’s astounding sixty-two, at the latest count, of which seven are spherical). All these (fictional) bodies have names, by the way. All of them await explorations of the imagination, in movies, books and comics…

The world we care most about is, of course, Pandora, fifth moon of Polyphemus.

The larger moons, like Pandora, probably formed from the same swirl of debris that formed Polyphemus itself; the smaller ones may be captured asteroids. There are limits on where big moons might be found in relation to the primary world. Sensible spherical moons need to be outside the primary’s “Roche limit,” within which tidal effects are so strong they pull the moon apart; inside the Roche limit you may get shapeless asteroid-like lumps of rock, but not round worlds. The precise distance depends on the mass and rotation of the primary, and on the composition of the moon, but as a rule of thumb the Roche limit is around two and a half times the primary’s radius, measured from the planet’s centre. Thus Saturn’s innermost spherical moon Mimas is three Saturn radiuses out. You can see from the onscreen size of Polyphemus in Pandora’s sky that Pandora is safely out beyond the Roche limit. Some close-in moons of gas giants are “tidally locked,” so that they keep one face permanently set towards the primary, as the moon does to the Earth. This isn’t the case with Pandora; during its twenty-six-hour day Polyphemus rises and sets.

In real life we’ve yet to detect any worlds of Alpha Centauri. But we have found an awful lot of worlds orbiting other stars.

One of the true scientific miracles of my lifetime has been the discovery of “exoplanets,” indeed in some cases whole other solar systems. When I was a boy not a single planet beyond the sun’s family was known. Some scientists maintained there were no other worlds—that the solar system was a freak, a matter of chance. Now, at the time of writing, we know of more than four hundred other worlds. We’re starting to learn a good deal about the distribution of planets and planetary systems, and are coming up with new theories of planetary formation. And we have new ideas of how planets may be habitable, suitable for life, even if in some cases they are dramatically different from our own Earth. It’s certainly timely for Avatar, a movie of travel to alien worlds, to appear just now. Suddenly we see a sky full of Polyphemuses—and, maybe, Pandoras.

The challenge of detecting worlds beyond our own is formidable, because planets are small and faint compared to their parent suns.

Suppose we were studying the solar system from a planet of the star Altair, in the constellation of the eagle (Aquila), about seventeen light years away. Even mighty Jupiter, the largest of the sun’s planets, would be lost in the sun’s glare. Jupiter’s apparent distance from the sun, from the point of view of an Altairean, would be only one-thousandth the width of a full moon seen from Earth, and its light, which is just reflected sunlight, only a billionth of the sun’s. It was once believed that you would need truly ginormous telescopes flying in space to resolve worlds like Jupiter out of the glare, let alone Earths, smaller, closer to the sun, even fainter. Not so.

While there had been tentative observations of planets orbiting pulsars (small supernova remnants) since the 1980s, in 1995 the scientific world was startled by the first observation of a planet orbiting a star called 51 Pegasi, a “main sequence” star (that is, a star in the middle of its normal lifetime, like our sun). The discovery was made not with giant telescopes but with improved instruments, careful observation and a dash of ingenuity.

An exoplanet is generally detected indirectly: not by observations of the planet itself, but by studying its effects on its parent star. The most productive technique to date has been the “radial velocity” method. As the planet orbits its star, the star itself is pulled out of position, just a little, and if some of this motion is towards or away from Earth you can detect it with a subtle shifting of the lines of the star’s light spectrum. This is the Doppler effect, the same phenomenon that causes the blue shift and red shift so familiar to hardened interstellar travellers like us. Alternatively there is the “transit” method. If the planet happens to pass across the face of its sun as seen from Earth—just like transits of Venus and Mercury, planets inside Earth’s orbit crossing the face of our sun—the dip in the star’s apparent brightness can be detected. Other techniques include using stars in the line of sight as gravitational “lenses.”

As you can imagine, these effects, though detectable, are small and subtle. The more massive the planet, and the closer it is to its parent star, the larger the effect and the more likely it is that the planet will be detected. Thus the first exoplanets found tended to be more massive than Jupiter, yet orbiting (to everybody’s surprise) very close to their parent stars. The very first discovered, at 51 Pegasi, was a “Jovian,” in the jargon, a gas-giant planet like Jupiter, orbiting its sun in just four days (our closest-in world Mercury takes eighty-eight days). Polyphemus is another example, a gas giant not much further from Alpha Centauri A than the Earth is from the sun.

There is an inevitable “observational bias” in our exoplanet detection. For a long time yet we are going to find more large, close-in worlds than small, further-out worlds, and the statistics of the planets we’ve found so far must reflect that. Nevertheless we have enough data now to start to classify the exoplanets and make some tentative predictions.

For example, eighty per cent of the exoplanets discovered have been in multiple-planet “solar systems” (which can be detected by observing the multiple tweaks the planets apply to their parent star’s motion). It’s thought that about a third of all sunlike stars will host planets the size of Neptune (around seventeen Earth masses), or “super-Earths,” worlds somewhere between Earth and Neptune in size. A super-Earth, by the way, would be a spectacular place, despite the higher gravity; the larger the world is the more geologically active it is likely to be, as the Earth is much more active than Mars or the moon. Expect fiery worlds, tremendous volcanoes.

The observational techniques are improving, but we’re still some way from being able to detect an “Earth,” orbiting at an Earthlike distance from a sunlike star. This would produce only a thousandth the deflection of the parent star of a close-in Jupiter (Jupiter has over three hundred times the mass of Earth).

So suddenly we’re seeing all these planets. But what about life?

It used to be thought that if it is to be liveable for creatures like us or the Na’vi, a world would have to be more or less Earth-sized, and would have to occur in the “habitable zone” of its parent’s star—orbiting at a distance from the star that would allow liquid water to occur on its surface, not too hot and not too cold, so at something like Earth’s distance from a star like the sun.

But in recent years we have discovered life surviving in quite extreme environments on Earth: in the deep sea where no sunlight ever penetrates, in conditions of cold and heat, even subject to radiation. Maybe life is more robust and flexible than we used to think.

And we have discovered new kinds of worlds, like Jupiter’s moon Europa, which under a crust of ice has a water ocean, kept liquid by tidal effects. Europa’s ocean seems a prime arena for life, even though it is far outside the traditional habitable zone.

In Avatar’s fictional universe Pandora too is an example. Alpha Centauri A is about fifty per cent brighter than Sol, and its habitable zone is about twenty-two per cent wider than the radius of Earth’s orbit around the sun. Polyphemus with its moons follows an orbit about forty per cent wider than Earth’s, so is just outside the traditional habitable zone of Alpha A—but oxygen, a signature of life, was detected in Pandora’s air anyway. It turns out that Pandora is kept warm by complex effects include tidal heating, and by a greenhouse effect from an atmosphere thick with carbon dioxide, and by other aspects of its complex environment as a moon of a gas giant in a double star system. No doubt we will turn up many other exceptions to the habitable-zone rule in the future.

These days, in fact, we no longer even think the parent star has to be like the sun to support a habitable world. Even red-dwarf stars, like Proxima Centauri, could conceivably have life-bearing planets. Such stars are small and dim, and the planet would have to huddle close to the central fire, probably so close that it would be “tidally locked” like our moon orbiting the Earth, with a single face perpetually presented to the star. You would think that the dark side, a place of eternal night, would be so cold that all the water, and even the air, would freeze out. But it’s believed that even a thin layer of atmosphere would transport enough heat around the planet to keep this ultimate chill-out at bay. From such a planet’s surface the sun would be huge—pink-white rather than red to the vision—and forever fixed in the sky, no sunrises or sunsets. The lack of tides, and the comparatively low-energy sunlight, would surely shape the origin and evolution of life. Perhaps plants would be characteristically black, to soak up all the energy available from the sunlight. It could be a dangerous environment, for stars like Proxima are prone to violent flares.

This may not sound like much fun. But remember that not so long ago people thought that to have life you had to have a sunlike star, with planets at an Earthlike distance. Since, as noted in Chapter 12, seventy per cent of the Galaxy’s stars are red dwarfs, with this model we have multiplied the potential number of habitable worlds in the Galaxy many times over. Not only that, the dwarfs have very long lives as stable stars, perhaps a hundred times as long as the sun’s. Suddenly the universe looks a lot more hospitable for life.

As it happens, the best candidate found so far of another Earth, the fourth planet of a star called Gliese 581, orbits a red dwarf. And as our nearest neighbour, Proxima, is a red dwarf, maybe it’s there we will find a “Pandora,” in reality, not orbiting the more glamorous Alpha A or B.

We may detect signs of life even before we manage to image habitable worlds directly. Spectroscopy, the analysis of the light reflected by a planet, or of starlight passing through a planet’s atmosphere during a transit across the face of its parent, can show evidence of the gases making up the planet’s atmosphere. Some gas giants have already been shown to have methane in their atmospheres. Direct spectroscopy may be possible in the next decade or so, through such missions as ESA’s infrared telescope Spica (to be launched possibly in 2017). Detecting such gases as oxygen in a world’s atmosphere would be a good indicator that life was present, even before we could see the green. This, in fact, in the Avatar universe, was how Pandora’s life was first detected.

The holy grail is to image an Earthlike world—to see its seas and polar caps and continents—as well as to detect the makeup of its atmosphere. This is the goal of future space missions including NASA’s proposed Terrestrial Planet Finder. And if such a world were discovered there would surely be pressure to develop and send a space probe. In the Avatar universe the first discovery of the Alpha Centauri planets prompted a rapid development of technology, leading ultimately to the sending of the first interstellar probes.

But could Polyphemus and Pandora exist? And if they do, given Alpha Centauri is the nearest star system, why haven’t we seen them yet?

Much of what we used to think we knew about Alpha Centauri has turned out to be wrong.

We used to think that in a multiple-star system like Alpha Centauri you might get close-in rocky worlds, but the formation of Jovian gas giants could be inhibited because of the closeness of the suns. After all, Alpha B is sitting at an orbit where Alpha A’s Jovians should have formed, and vice versa. But in October 2002 astronomers in Texas announced the discovery of a Jovian planet orbiting a star of the Gamma Cephei binary system, about forty-five light years from Earth, a system with twin stars with the same kind of spacing as the two suns of Alpha. The Jovian they found is about twice as massive as Jupiter, orbiting happily about twice as far as Earth is from the sun.

Then we used to think that even if multiple star systems like Alpha Centauri grew planets the stars’ gravitational perturbations would destabilise their orbits and throw them out of the system altogether. But recent studies have shown that for planets as close to Alpha A as Earth is to the sun, B’s gravity would have no significant effect on their orbital stability. So Alpha Centauri may not just have twin stars. It may host twin solar systems: two planetary systems just a few light-hours apart, so close that if humans had evolved there we might already have made interstellar journeys.

And we used to think that we would never find a giant planet like Polyphemus so close to its star, as close as Earth is to the sun. When we only had the example of our solar system to study, we believed that gas giants would only be found far from the parent star, beyond the “snow line,” where, out in the stillness and cold and dark, the worlds grow immense, misty, stuffed with light elements like hydrogen and helium that were boiled out of worlds like Earth that formed close to their sun’s heat. Thus in our solar system the closest-in Jovian, Jupiter itself, is five times as far as Earth is from the sun. But as we’ve studied the new exoplanets we’ve found endless examples of gas giants orbiting much closer to their suns than was thought possible. Indeed, as I noted earlier, it’s the very closeness of these huge worlds to their suns that allow us to detect them in the first place.

It seems a Jovian may well be born out beyond the snow line, but then it can suffer a kind of friction with the sun-surrounding disc of dust and gas from which it formed, causing it to lose orbital energy and spiral inwards. Several such planets may be eaten by their sun until at last the growing sun’s radiation and solar wind, or perhaps a blast from a nearby supernova, clears away the last of the debris, leaving the survivors to settle where they are. In our system, perhaps Jupiter and the other three giants are the last survivors of a flock of gassy worlds, most of which were consumed by the young sun.

In other systems we’ve seen “hot Jupiters,” left stranded in stable orbits much closer to their suns than Jupiter is to the sun. The most extreme example found so far, reported in 2010, is a planet of a star called WASP-12, nearly nine hundred light years from Earth. While Jupiter takes around twelve years to orbit the sun, this wretched world orbits in a mere day. The star’s gravity will have pulled it into an egg-shape, its surface temperature must be thousands of degrees, and the star’s heat, boiling away its atmosphere, will some day ensure its break-up altogether.

Even without being a hot Jupiter, being close in would make a difference to a gas giant’s formation, to its weather, and ultimate fate. And indeed Polyphemus has a different composition to Saturn—it is smaller and denser—and it is lot more stormy, with a “great red spot” storm larger than the red spot on Jupiter.

So it’s entirely possible that a Jovian like Polyphemus could indeed be found at an Earthlike distance from Alpha Centauri A, with a nice spherical moon like Pandora. But even if we found Polyphemus using exoplanet-tracking techniques, would we be able to see Pandora? Maybe. One recent computer simulation, of an Earth-sized “exomoon” orbiting a Neptune-sized giant, showed that the moon’s orbit would affect the giant’s path sufficiently for it to be detected by a “transit” observation by a future space telescope.

In reality we haven’t yet detected a Polyphemus orbiting Alpha Centauri, or indeed any worlds in that system, despite its closeness. In the Avatar universe the explanation is simple. The plane of the planets is tipped at sixty degrees to our own; our current detection methods, the transits and Doppler tracking, work best when the planets’ orbits are in our line of sight. There are other factors too, such as the comparative instability of planetary orbits within the system. This could well be the case. Planet-hunting is still a tentative game. But we are planning more subtle exoplanet searches, with powerful spaceborne instruments. I think we can be confident that if Poly-phemus and Pandora, or anything like them, do exist, some day we will see them.

And, someday, maybe, visit them.

¹⁴ THE CASE OF THE CYLINDRICAL BIOLOGIST

Polyphemus and Pandora: what evocative names!

In giving these new worlds names from classical mythology, their discoverers followed a tradition that dates back to 1781, when the British astronomer Sir William Herschel discovered the solar system’s seventh planet, the first found beyond the wandering bodies visible to the naked eye that had been known since before humans were humans. Eventually the new planet was named Uranus, in Greek mythology the personification of heaven and the son and husband of Gaia, the Earth goddess—though Herschel had hoped to name it Georgium Sidus, in honour of King George III: a planet called George!

In myth, Polyphemus, with a name meaning “very famous,” was a Cyclops, a cannibalistic one-eyed giant encountered by Odysseus in Homer’s Odyssey. It seems an apt name for a giant world dominated by a single glaring-eye storm. And to the Greeks, Pandora, whose name means “giver of all,” was the first woman. Out of curiosity she opened up the famous “Pandora’s Box” (actually a jar), thus releasing all the evils of mankind, leaving only Hope inside the box as consolation. Certainly it seems appropriate that a world as fecund as Pandora should be given the name of the Greek Eve.

(There are in fact already two astronomical Pandoras in our solar system. One is a main belt asteroid discovered in 1858, a rock about sixty kilometres across. The other is the seventeenth moon of Saturn, an even more battered lump of rock around a hundred kilometres long by eighty wide, which shepherds the outermost of Saturn’s rings, following a very complex and chaotic orbit.)

But if you were to follow Jake Sully down the ramp off the Valkyrie, it’s probably not the moon’s name you’d be thinking of in your first moments on Pandora, but its low gravity.

Colonel Quaritch is suspicious of Pandora’s low gravity. He obsessively pumps iron to avoid being made “soft” as a result.

Pandora’s gravity is about eighty per cent of Earth’s. Its diameter is three-quarters of Earth’s, and its mass about half; in size it’s a world somewhere between Earth and Mars, which has around one-third Earth’s gravity.

But Pandora’s air is thicker, about twenty per cent denser than Earth’s. You might wonder how a low-gravity world can hold on to a thick atmosphere, as Pandora evidently does. On any world air molecules can be heated to “escape velocity” and just fly off into space, like tiny spacecraft. A battering by the solar wind, charged particles from the sun, adds to that leakage as well. The lower the gravity, the more air will escape to space. Thus our moon with around a sixth Earth’s gravity is all but airless.

But gravity isn’t the only factor when it comes to a world keeping its air. Titan has around the same gravity as the moon, but, as we saw in Chapter 7, its atmosphere is more massive than Earth’s. This is because it is so cold out there at the orbit of Saturn; Titan’s air molecules move much more slowly, on average, and fewer escape. On the other hand Venus, only a tad smaller than Earth, has a much more massive atmosphere than the Earth because it’s too hot; all the heavy carbon dioxide that’s locked up in the rocks on Earth is baked out into the air on Venus.

Leakage of an atmosphere can be surprisingly slow too. It’s thought that an Earthlike atmosphere somehow delivered to one-third-gravity Mars (perhaps as part of a “terraforming” project, making Mars into a second Earth) would take around ten million years to leak away. That’s a slow enough process for a civilisation to manage an artificial atmosphere if it had to (recall the air machines on Burroughs’ Barsoom). There are natural inputs to a planet’s air too, from outgassing via volcanoes, and impacts from comets. And there are other special factors. Pandora orbits between radiation belts surrounding its primary Polyphemus, which deflect the charged-particle wind from the sun. This is evidently a complex question; a world’s lower gravity does not imply it must have thinner air.

But what effect would a different gravity have on living things?

Galileo was able to figure out the basic physics of gravity and bodies back in 1638: “It would be impossible to fashion skeletons for men, horses or other animals which could exist and carry out their functions [proportionally] when such animals were increased to immense weight…”

This work was the origin of the famous “square-cube law.” If you double the size of an animal, its cross-section goes up as the square of the size—four times—but its volume, and therefore its mass, goes up as the cube—eight times. This basic rule is central to “biomechanics,” the discipline of how living things are put together mechanically. This means that you couldn’t just double the size of an elephant in some genetic-engineering brainstorm and expect it to function; its four-times-thicker muscles wouldn’t be able to raise its eight-times-greater weight.

Ah, but what if you transported said elephant to a lower-gravity world, like Pandora?

There’s a difference between mass and weight. Mass is a resistance to motion. You would have the same mass even in zero gravity, in space. You get weight in a gravity field. Weight is mass multiplied by the acceleration due to gravity, which is approximately ten metres per second per second on Earth. Weight is the load you have to carry around. In space you would still have mass, but no weight.

We all have a maximum weight we can bear, given the strength of our bones and muscles. But in a lower gravity field, you could carry around more mass: higher mass times lower gravity comes out to the same weight. How much more mass depends on the weakness of the gravity.

Humans have complicated geometries, so let’s simplify things. There’s an old joke about the farmer who’s having trouble with his milk production, and he calls in theoretical physicists from the local university to help. After weeks of intensive study, back comes the report which begins: “Consider a spherical cow…” (Well, it made me laugh.) The point is, to figure out basic principles, scientists will often make simplified models of the real world to make the calculations easier, even if the models are somewhat unlike the real thing.

And in that spirit, consider a cylindrical biologist.

Here’s Dr. Grace Augustine, standing tall on Earth, probably giving some RDA desk jockey a hard time. She could be represented by a pillar a bit less than two metres tall, say twenty centimetres diameter. The pressure she’s exerting on the bones holding her up is her weight divided by her cross-sectional area.

Now let’s suppose we stretch her up by twenty-five per cent, without making her any wider. She’ll still be shorter than the average Na’vi, at around three metres. Her mass has gone up twenty-five per cent, and so has her weight, but her cross-section hasn’t changed. So the pressure on her bones is up twenty-five per cent too.

That could be a problem, if we kept stretching her. Grace’s bones can support only a certain maximum weight, because beyond that the pressure would overcome the binding energy of her bones’ molecules; the bones would splinter and Grace would fall. So, in a given gravity field, and with bones of a given strength, there is a limit on Grace’s height—and indeed her mass—unless you thicken up her bones like an elephant’s.

But now let’s whisk Tall-Grace to Pandora. The gravity here is eighty per cent of Earth’s. And so, though her mass is unchanged, her weight (twenty-five per cent more mass times eighty per cent gravity) is the same as Short-Grace’s back on Earth, because the lower gravity has cancelled out the extra height. And thus the pressure on Tall-Grace’s bones is as low as it was for Small-Grace back on Earth, and she feels no discomfort.

There are plenty of subtleties beyond this simple argument. Even given their height the Na’vi look remarkably slender—narrower bones mean higher pressure—but, as we’ll see in Chapter 25, their bones are strengthened by a naturally occurring carbon fibre.

And we should remember that the Na’vi didn’t have to be as tall as they are. No animal has to grow as large as the laws of physics allow it to. The Na’vi’s apparent close relative, the prolemuris, is no more than a metre and a half tall, just as on Earth our hominid ancestors were all chimp-sized until the emergence of Homo erectus, about as tall as us, a couple of million years ago. The Na’vi are as tall as they are because something in their evolutionary history made it right for them to be so. However their height does illustrate that a human body form that would be impossibly tall and slender on Earth can work on Pandora.

What are the limits? How big could a land animal grow on Pandora?

Pandoran beasts are big. Even the direhorse is larger than any horse on Earth. The heaviest living land animal on Earth is the African elephant; a bull can stand some four metres tall at the shoulder. The heaviest animal of all was the brachiosaurus which died out some hundred and thirty million years ago, and stood around seven metres tall at the shoulder. The largest land animal we see on Pandora in Avatar is probably the hammer-head titanothere at maybe six metres tall—like an elephant scaled up in Pandora’s gravity field. Perhaps greater beasts roam in parts of Pandora yet unexplored.

Pandora’s low gravity would help you fly, especially with the aid of that thick air. On Titan, the air is so thick and the gravity so low that a human could fly by the power of her own muscles, flapping artificial wings. So we could have predicted big flying animals on Pandora.

Earth’s largest flying creature was the winged reptile Pteranodon ingens, which flew over Kansas some eighty million years ago, with a wingspan of around nine metres. On Pandora a mountain banshee exceeds that at around twelve metres wingspan, and a leonopteryx would dwarf it, with a wingspan of thirty metres. The size a flying creature could reach depends on other factors than gravity, such as the density of the air and the oxygen content—the more oxygen, the more energy you have available to keep you aloft.

On Earth you have to look to the sea for the real monsters in size. The blue whale is thought to be the heaviest animal ever to have existed, weighing in at some hundred and ninety tonnes (compared to around five tonnes for an African elephant). If we visit Pandora’s oceans in the future, there will be monsters, I have no doubt.

And what of the tremendous trees of Pandora?

On Earth, the basic physical constraint on tree height is the need for the tree to be able to lift water to its uppermost leaves. The tallest known tree on Earth is a sequoia in northern California, at a hundred and sixteen metres tall. The theory says that a tree could possibly reach as much as a hundred and thirty metres—and there have been historical accounts of trees a hundred and twenty metres tall. By comparison Hometree on Pandora is some three hundred metres tall, nearly three times the size of that big old sequoia. This is more than the simple gravity scaling might suggest, but Hometree evidently has a different architecture from a sequoia, with pillar-like multiple trunks, themselves as sturdy as sequoias, enclosing a large internal hollow.

Pandora’s low gravity would enable some wistful architectural designs: impossibly long arches, impossibly slender columns. We don’t see any native architecture on Pandora; with the hometrees available for habitation I suppose building is unnecessary. And the humans at Hell’s Gate show no imagination in their own functional building schemes. Maybe the Stone Arches are a glimpse of what would be possible.

But in fact the Stone Arches seem to be a product of the single most remarkable physical phenomenon on Pandora: its unobtanium, and the magnetic fields with which it is associated. And if you followed Jake Sully to Pandora you would very quickly learn that unobtanium is the reason you, and RDA, are here.

¹⁵ OBTAINING THE UNOBTAINABLE

What is it about unobtanium that makes it so valuable?

Unobtanium is a room temperature superconductor—we’ll find out later what that means. On Parker Selfridge’s desk we see demonstrated one of its apparently magical properties, that a chunk of it can float in the air, defying gravity, over what looks like a magnet. Unobtanium has shaped Pandora’s geology. It is unobtanium’s gravity-defying properties that hold up the floating Hallelujah Mountains. When Jake climbs the “stairway to heaven” on his way to Iknimaya, his mountain-banshee challenge, you can see what look like lumps of rock embedded in the roots and tendrils, straining to rise like trapped balloons, boulders presumably laced with unobtanium.

But the real value of unobtanium lies in its superconducting properties, which have led to a new industrial revolution on Earth, including the building of Venture Star-class starships—and generating vast profits in the process.

Is all this fanciful?

The very name “unobtanium” suggests that we’re dealing with impossible physics. According to science-fiction archivist David Langford, the word is an engineer’s in-joke dating from the middle of the twentieth century, applied to any ideal substance you need to achieve the impossible—frictionless bearings, for example. The word “unobtanium” was actually formally defined in the U.S. Air Force University’s Interim Glossary of 1958 as “a substance having the exact high test properties required for a piece of hardware or other item of use, but not obtainable whether because it theoretically cannot exist or because technology is insufficiently advanced to produce it.” The word has been used in science fiction before, for instance in David Brin’s 1983 novel Startide Rising. Cameron has suggested that maybe the discoverers of unobtanium on Pandora adapted the old tongue-in-cheek name as a joke for this magical stuff, and it stuck.

But in fact there may be nothing unobtainable about unobtanium. Superconductivity is a real property. And a superconductor really can defy gravity, at least in the presence of a magnetic field.

As the name suggests, a superconductor is a material that is a “super” conductor of electricity—so super, in fact, that unlike common conductors like copper wire, it conducts with virtually no resistance at all. This means that no electrical energy is wasted in heating up the conductor, and the current could apparently run for ever, without losses.

This seemingly impossible property was first discovered by accident, as a consequence of research into low temperature physics.

In 1908 the Dutch scientist Kamerlingh Onnes was the first experimenter to turn the gas helium into a liquid. Whereas water liquefies from steam at a hundred degrees centigrade, to liquefy helium you need to reach the astoundingly low temperature of just four degrees above absolute zero—around two hundred and seventy degrees below zero centigrade. Having achieved his liquid helium Onnes tried dunking familiar materials in it, just to see what happened. (Well, you would, wouldn’t you?) And he discovered that in certain pure metals, as they cooled down, electrical resistivity suddenly switched off—or at least, it dropped to values too low to measure.

The industrial applications of such a substance are startling. You could run extremely high currents, for instance to power the very strong electromagnets needed by fusion reactors and starship antimatter traps, without the fear of heat damaging your apparatus. Low-loss power transmission lines are another possibility. Heat produced by electrical resistance is a problem in computers, forcing a limit to how much connectivity you can jam into a finite space—the smaller your computer is physically, the faster it can operate. With superconductivity there would be no heat limitations, in principle.

And superconductors can be used to generate lift: to defy gravity.

A superconductor in a magnetic field has a remarkable property called “perfect diamagnetism”; it expels the magnetic field from its interior by creating an electrical current running on its surface. The magnetic field reacts by pushing back at the superconductor. This is called the Meissner effect, and was first discovered in 1933—and it is presumably the effect we see holding up the lump on Selfridge’s desk, as the magnetic pressure balances gravity.

This effect, “magnetic levitation”—“maglev”—can be harnessed as a friction-free load-bearing mechanism. You could imagine using it for frictionless bearings and flywheels. Larger-scale industrial applications could include lifting heavy weights, and running trains on frictionless tracks. Maglev trains are mentioned in a deleted scene in the 2007 script for Avatar. In fact maglev trains have already been trialled, though using only conventional electrical conductors. In Japan in 2003 such a train reached a speed of nearly six hundred kilometres per hour, faster than the record set by conventional trains. With no friction from the track, the main resistance to the train’s motion comes from the air; if it were run in an evacuated tunnel it’s thought that such a train could reach speeds of thousands of kilometres an hour. This might be very useful on the airless moon, where you could build a “mass driver,” an idea of Arthur C. Clarke’s, basically a train so fast it could take off into orbit…

So superconductivity is a real phenomenon, and superconductors do indeed have enormous industrial potential. The trouble with the first superconductors, however, was that it took extreme cold to trigger the superconductivity in the first place. You couldn’t realistically run a maglev train track through a hundred-kilometre-long tunnel filled with liquid helium.

But unobtanium is self-evidently at room temperature, as we see when Parker Selfridge casually picks up the trophy lump from his desk without having his hand freeze solid. Is this possible?

After Onnes’ accidental discovery, the mechanism of super-conductivity took decades to unravel. In fact it had to wait for a whole new branch of physics to emerge. Once again we must approach the eerie science of the quantum.

Electrical current in a conductor is a flow of electrons. It turns out that at sufficiently low temperatures the electrons in a conductor bond into pairs, called “Cooper pairs.” (Leon Cooper was one of a team that won the 1972 Nobel Prize for figuring this out.) Like entanglement (Chapter 11) these couplings are a typically spooky quantum-physics effect; the electrons don’t have to be physically close to each other, but they are still attached. Physicist and science-fiction author Charles Sheffield compared them to a husband and wife at a crowded party, separated yet always joined.

Crucially, each pair stops behaving like the electrons from which it is composed, and more like another class of particle entirely—called “bosons,” which includes photons, the particles that make up light. And bosons have very different properties from “fermions,” the class that includes electrons. The electron pairs become “correlated,” lined up, as if the whole of the interior of the conductor is a single quantum object. All the photons in a laser beam are correlated in the same way. The way I think of it is that in a conventional conductor the electrons, all loners, are like a jostling crowd, cramming their way through a corridor. Cooper pairs are like a Soviet march-past, synchronised, smooth and slick, and getting by with far fewer collisions with the furniture.

The trouble is, the coupling of electrons into Cooper pairs is a fragile effect that is easily destroyed by heat. For decades it was believed that no such thing as unobtanium, a room-temperature superconductor, could ever be found because of this.

So everybody was surprised when, in the 1980s, certain ceramics were discovered which can remain superconducting at the balmy temperature of ninety degrees above absolute zero—above the temperature at which nitrogen boils, let alone helium. Later, copper-oxide-based superconductors pushed the limit up to over a hundred and thirty degrees above absolute zero. The latest developments include the discovery in 2008 of iron-oxide-based superconductors working at around the same temperatures. The scientific jury is out on how this works, presumably through a high-temperature analogue of the electron-pair correlation effect seen at low temperatures. For now, the grail of a true room-temperature superconductor is still out of reach—but it’s coming closer.

For the sake of the Avatar storyline, unobtanium has some other key properties. It can exclude magnetic fields much stronger than other superconductors can cope with—in a strong enough field most superconductors eventually break down. And it doesn’t just exclude magnetic fields, it also has the ability to anchor strong magnetic fields in parts of its structure, perhaps using non-superconducting components embedded in a superconducting matrix. This is what enables Pandora itself to support very strong magnetic fields, as we’ll see in the next chapter. None of this is entirely implausible, and unobtanium’s superconducting properties at least don’t look unobtainable, in principle, and it certainly would be highly valuable in industry.

Where did Pandora’s unobtanium come from? The answer comes from the peculiar (fictional) history of Alpha Centauri’s formation. As the system’s young stars coalesced they were perturbed by an intruder, a runaway neutron star, the surviving core of a supernova explosion, a lump composed purely of jammed-together neutrons with the mass of a star but the diameter of a city block. The neutron star, itself a source of powerful magnetic fields, ripped into the young Centauri stars, and some bizarre nuclear reactions followed. The result was a system laced with unobtanium. And that’s why unobtanium is not present in our solar system, whose origin was unperturbed by neutron stars.

But even if we could find it, could a superconducting mineral like unobtanium really lift a mountain?

¹⁶ MOUNTAINS IN THE SKY

The Hallelujah Mountains, ranging in size from boulders to many kilometres across, float thousands of metres above the ground. The Hallelujahs are a lovely visual concept, inspired in part by the Huang Shan Mountains of China, spectacular karst limestone formations that themselves look too delicately vertical to exist.

The Hallelujahs are lifted by the push of Pandora’s magnetic field on the superconducting unobtanium in the mountains’ rocks. The magnetic field itself is a complex product of the presence of the unobtanium in the ground. Indeed it was an early sighting of the Hallelujahs that led human scientists to suspect the presence of superconducting unobtanium in the first place.

In fiction, flying islands go back at least as far as the eccentric aerial kingdom of Laputa, in Jonathan Swift’s Gulliver’s Travels (1726). And as it happens Laputa is held up by magnetism too. It contains a magnetic rock, “a Lodestone of a prodigious Size… The stone is endued at one of its Sides with an attractive Power, and at the other with a repulsive… When the repelling Extremity points downwards, the Island mounts directly upwards” (Part Three, Chapter 3).

But just how strong would a magnetic field have to be to lift a mountain?

Consider Selfridge’s trophy unobtanium lump on his desk.

If this is equivalent to a ten-centimetre cube, say, and if the density is about that of rock on Earth (a couple of tonnes per cubic metre), then the mass is a couple of kilograms. It is held in the air by a push from a magnet in the base unit. The “push” comes from “magnetic pressure,” which is an energy density associated with the magnetic field. It really is a pressure, a force per unit area, measured in pascals (newtons per square metre) just like air pressure (which on Earth is about a hundred thousand pascals at sea level).

So with a cross-section of ten centimetres squared, and if Pandora’s gravity is eighty per cent of Earth’s, the pressure required to hold up the lump is (weight divided by area) about sixteen hundred pascals.

The standard formula for magnetic pressure (easy to find in any physics text) tells us that the pressure exerted by a magnetic field is proportional to the square of the field strength. And the standard unit of magnetic field strength, or “flux density,” is the tesla (T)—named after Nikola Tesla, a Serbian-American inventor once played by David Bowie, in the 2006 movie The Prestige. (A tesla is equivalent to ten thousand gauss, in other units.)

It turns out that to get a pressure of thirteen hundred pascals you need a magnetic field strength of around sixty mT (milli-teslas—each a thousandth of a tesla). How strong is this? Well, it’s several hundred times the strength of Earth’s magnetic field at ground level (which is only about a ten-thousandth of a tesla; a tesla is actually a pretty large amount). It’s stronger than a toy fridge magnet, at a few milli-teslas, but weaker than the coil gap in a loudspeaker, which might be about a tesla. So it’s certainly plausible that a lump like Selfridge’s desk ornament could be lifted by a magnet of everyday household use.

It seems remarkable that even a toy magnet is so much stronger than Earth’s magnetic field—and, if you use one to pick up a pin, you will witness its magnetism overcoming the gravity pull of an entire planet. But you have to think of magnetic field strength as a kind of density; there’s an enormous amount of energy stored in Earth’s field, which works globally—even if locally, on a very small scale, it is much weaker than the fridge magnet. And on larger scales, whereas electrical and magnetic forces can attract or repel (think of positive and negative charges, north and south poles) gravity only ever attracts. So electromagnetic forces can be strong on short scales but cancel out on larger scales, whereas the attractive force of gravity just piles up and up. That’s why the structure of your body is dominated by electromagnetic forces, but the structure of the universe, such as the orbits of planets and the spiral forms of galaxies, is determined by gravity, not electromagnetism.

Selfridge’s toy is one thing. What about the Hallelujah Mountains?

Just as I imagined a biologist as a cylinder (Chapter 14), now imagine a mountain as a cube, a hundred metres on a side (many of the mountains are a lot larger), with the density of rock. This is a lot more mass than the desk ornament—around two million tonnes—and the pressure needed to keep it up is much greater too, at around one million, six hundred thousand pascals. And the magnetic field strength we need is greater too, around a couple of teslas.

A couple of teslas might not sound much. It’s well within what modern human technology can produce—the big magnetic resonance imaging systems in hospitals can run up to fields of several teslas, on a small scale.

But this is several thousand times Earth’s field strength. It’s stronger than the magnetic field around Jupiter. It’s stronger even than the sun’s field at the location of a solar flare, an event powerful enough to batter the Earth across more than a hundred million kilometres with enough charged particles to crash power grids. But there are stronger magnetic fields in nature; the field at the surface of a neutron star, a compressed supernova remnant of the kind that created unobtanium in the first place, can run to hundreds of millions of teslas. (Robert L. Forward’s novel Dragon’s Egg (1980) and my own Flux (1993) showed life forms shaped by this bizarre environment.)

In the movie Avatar we see visual evidence of strong magnetic fields of a poetic sort. The Stone Arches that congregate over areas of strong flux, such as the Tree of Souls, are reminiscent of “solar prominences,” areas of intense magnetic activity on the surface of the sun where glowing plasma is lifted along flux lines to form tremendous arches—some big enough to straddle the Earth. The Arches are in fact a relic of Pandora’s magnetic fields. During the region’s formation flux loops shaped the rock when it was still molten, and held it there until it cooled and hardened. As a result arch formations can be used to locate unobtanium deposits, and act as warnings for pilots of aircraft of the presence of hazardous magnetic fields.

Regarding the flying mountains, even if you had the field strength, there are also questions of stability. If you experiment with fridge magnets you’ll find that supporting an object by repulsion isn’t so easy, as the object will slide off to one side or another, or flip over so that unlike poles are drawn together. With a superconducting body the effect is different, as the floating body is cushioned by the magnetic field excluded from its interior. Maglev experiments have shown that for stability you need the supporting field to be stronger at its periphery than at its centre, to keep the floating object in place. On Pandora, how could such a shaped field come about in nature? Perhaps there was some kind of feedback effect between the magnetic fields in the floating rocks and the still-molten ground, when the mountains were formed. Or, some researchers in the Avatar universe have speculated, the Hallelujahs could represent a balance achieved by a kind of consciousness, just as Eywa is integral to the balance of the ecology… Even so the Hallelujahs aren’t entirely stable, however. They have been known to collide, hence the Na’vi name for them of “Thundering Rocks”: tktktk.

Certainly Pandora’s intense magnetic fields will add to the hazards of a very hazardous world.

¹⁷ A DANGEROUS MOON

Colonel Quaritch likes to welcome newcomers to Pandora with a scary depiction of its dangerous life forms, the plants, the animals, the natives, all of which, according to him, want nothing more than to kill humans.

But Pandora would be a ferociously hazardous place even without any life forms at all.

Pandora is a volcanic world. And it’s that way because of where it orbits.

Consider the moons of Jupiter. Of the four largest moons, discovered by Galileo—Io, Europa, Ganymede and Callisto—closest-in Io is some six Jupiter radiuses from the giant planet’s centre, while furthest-out Callisto is about twenty-six radiuses out. Io has had its orbit tweaked into an ellipse by its neighbours, Europa and Ganymede. As a result Jupiter and the neighbouring moons together raise ferocious tides on Io—and not of water, as our moon raises tides in Earth’s oceans, but of rock. The whole moon is flexed and squeezed, an effect that heats Io from within, just as a rubber ball gets hot if you knead it in your grip. It’s just the same for Pandora, which too orbits a gas giant and has sister moons, so we must expect it to suffer similar tidal flexing.

Because of the heat injected by gravitational kneading, Io is the most volcanic world known. Its calderas spew out a hundred times as much lava as from all of Earth’s volcanoes—and that from a surface area just one-twelfth the size. The whole surface is riddled with sulphurous pits, lava pools and magma-spewing fissures. In NASA spacecraft images Io looks like nothing so much as a vast plate of pizza. This is unusual for a small world. Smaller planets lose their inner heat more quickly, and tectonic activity generally seizes up; that’s the case on the moon and even on Mars. Not on Io—and not on Pandora.

Clearly Pandora is not such an active world as this. But it is an arena of much more intense tectonic activity than Earth: a world of fractured continents, of volcanoes and earthquakes, of hot springs and geysers, and with its air polluted by carbon dioxide, hydrogen sulphide and other volcanic products.

All the volcanism is bad for machinery, because of the ash and gases volcanoes inject into the air. On Earth we had an example of this in April 2010, when air travel across north Europe was closed for days because of an ash cloud emitted by a volcano in Iceland. The eruption wasn’t that big by historic standards, and away from Iceland itself you couldn’t even see the cloud. But an airliner flying through it would ingest sixty billion particles of abrasive ash every second. The worst danger was that particles of silica in the ash would melt and clog up the engines’ cooling systems, which was likely to shut down all the aircraft’s engines at once, rather than one or two dropouts which airliners are designed to handle. Pandora is evidently a tough environment for industry, as we’ll discuss in Chapter 18.

As for humans, the Pandoran air is lethal. “Exopacks on!” barks the crew chief as the Valkyrie passengers prepare to walk on Pandora for the first time. “Remember people, you lose your mask you’re unconscious in twenty seconds and you’re dead in four minutes. Let’s nobody be dead today, it looks bad on my report…”

Thanks to the volcanism, compared to Earth’s atmosphere, Pandoran thick air is stuffed with carbon dioxide, xenon and hydrogen sulphide. It’s the carbon dioxide that keeps the moon warm enough for life. But it’s the carbon dioxide that would kill you—or the hydrogen sulphide, if you gave it a chance. (The xenon is harmless.)

Carbon dioxide is an essential component of our biosphere, but it is toxic in greater concentrations: a silent, odourless assassin. In 1988 in Cameroon, carbon dioxide was expelled from lakes by volcanic events; animals in the area were overwhelmed and killed, as were seventeen hundred people. Coal miners are wary of “blackdamp” in their mine shafts, toxic air in which raised carbon dioxide levels are matched by reduced oxygen. It was to warn of the dangers of blackdamp that canaries were used as a warning system; the birds, more sensitive to bad air than humans, would succumb first.

The carbon dioxide content of Earth’s air is around a fraction of one per cent. From one per cent upwards it can cause drowsiness. At higher concentrations you get dizziness, shortness of breath, difficulty breathing and panic attacks. At eight per cent you lose consciousness after a few minutes. On Pandora, the concentration is nineteen per cent… After Quaritch’s climactic attack with his AMP suit on the link shack, Jake is left exposed to Pandoran air without an exopack, and his rapid near-suffocation is convincing.

Pandora’s high concentration of hydrogen sulphide is a hazard too. This gas is deadly at concentrations of more than a few tenths of a per cent, but capable of causing coughing and skin irritation at much lower levels.

Of course Pandoran life forms are adapted to their air. There is even one sort, the “puffball tree” (Obesus rotundus) which absorbs toxic gases from the atmosphere, for the benefit of the rest of the ecology. Humans, however, will always need protection from systems like their exopacks, which remove the excess carbon dioxide and hydrogen sulphide from a user’s air.

If Pandora’s air doesn’t get you, meanwhile, there’s the magnetism.

Pandora’s own magnetic field is hazardous enough. Locally, as we see onscreen, it is strong enough to affect human technology—which is why regions of intense flux, like the Tree of Souls, are good places for Grace, Jake and the rebels to hide out. As we will see in Chapter 18, one reason why much of RDA’s technology has a heavy, retro look is simply that it has to be robust enough to keep working in Pandora’s intense magnetic environment, amid other hazards.

The magnetic field would also have an effect on living things. Conceivably you would feel the presence of a strong local field if you walked through it. You’ll recall that unobtanium is pushed away by magnetic fields because as a superconductor it has “perfect diamagnetism”—a chunk of it expels magnetic fields from its interior. But to some extent any conductor is diamagnetic, such as your own water-filled body, and can be pushed by a strong enough field. The bodies of frogs and mice can be made to float in magnetic fields, as has been proven by certain researchers with too much time on their hands.

Magnetism has more subtle influences. Life on Earth routinely exploits the planet’s magnetic field. Creatures with internal magnetic “compasses,” which get directional information from the way the field is pointing, include birds, sea turtles, bats, lobsters and newts. Some, including turtles and newts, are thought to have internal magnetic “maps” based on three-dimensional variations of the field. Such animals may “see” aspects of the field superimposed over a more normal visual view of the world, like a pilot’s head-up display. Obviously such senses are useful for migratory species of birds, but “magnetoreception” is widespread beyond that, in non-migratory species such as flies and chickens. Even cows in a field can sometimes be seen to line up with magnetic field directions.

It’s been difficult to identify the receptors for these senses because magnetic fields pass through flesh and blood; an animal’s magnetic sensors could be located anywhere in the body, not necessarily on the surface, the way eyes are. It’s not even clear how magnetic senses work: perhaps through magnetic fields causing a voltage within the body, or through their tugging at a magnetic mineral called magnetite within the body, or perhaps through the fields causing some unusual biochemical reaction within the body. A recent review of the subject in Nature (22 April 2010) summed this up as “a fascinating interplay of biology, chemistry and physics.”

Magnetic-sensitive life forms used to Earth’s gentler fields would probably suffer on Pandora. This has been demonstrated by researchers placing standard bar magnets, much stronger than Earth’s field, on homing pigeons and sea turtles, whose ability to navigate is disrupted. However, native life forms exploit Pandora’s strong magnetic fields for other purposes than direction-finding (see Chapter 21). Life is endlessly ingenious in exploiting the resources offered by its environment.

For humans, the medical effects of long-term exposure to powerful magnetic fields are not well understood. But human workers are now routinely exposed to Pandora-sized fields of several teslas, for instance from working with magnetic resonance imaging scanners in hospitals. In 2007 the European Union’s Health and Consumer Protection department published a study of the “possible effects of electromagnetic fields on human health.” The report pointed out that strong magnetic fields affect biological molecules with magnetic properties such as haemoglobin, and there has been some evidence that the electrical activity of neurons and brain areas can be affected by intense fields.

Pity RDA’s miners. Unobtanium mines tend to be located in the most intense regions of magnetic flux. In fact humans aren’t allowed anywhere near unobtanium deposits; symptoms such as vision distortions and strange tactile sensations are reported hundreds of metres away, along with irregular heartbeats, muscle tremors, nausea and other symptoms. RDA’s mining operations are perforce run by remote control.

Those flying mountains are another hazard for the miners. If you dig out unobtanium in the wrong place you could destabilise the magnetic fields holding up a Hallelujah…

So Pandora’s magnetic field is hazardous enough. Its interaction with Polyphemus’ field only makes things worse.

Jupiter’s magnetic field is ten times the strength of Earth’s. As a result the giant planet is surrounded by a powerful “magnetosphere,” a region of space filled with high-energy charged particles. This magnetosphere extends between fifty and a hundred planetary radiuses, well beyond the orbit of Callisto. This is a big structure; if it was a visible object, from Earth it would look the size of the sun. Inside the magnetosphere there are Van Allen radiation belts, bands of trapped charged particles of the kind known to be a hazard for astronauts orbiting Earth—but Jupiter’s belts are ten thousand times as intense as those around Earth. The magnetosphere has visible effects on Jupiter itself, such as tremendous auroras, caused by charged particles battering the planet’s upper atmosphere: fantastic light displays some sixty times brighter than the northern and southern lights on Earth. And the magnetosphere causes Jupiter to emit huge blares at radio frequencies, more intense than any radio source in the solar system save the sun. Io and the other big moons are all well within Jupiter’s magnetosphere.

The situation is similar at Polyphemus, whose magnetosphere envelops six of its moons, including Pandora. The interaction of Polyphemus’ magnetosphere with Pandora’s is complicated and interesting. The localised magnetic “hot spots” on Pandora’s surface funnel charged particles from Polyphemus’ magnetosphere or from the sun down to the surface. The result is storms from space similar to those on Earth caused by solar flares, violent releases of energetic particles from magnetically active regions on the sun’s surface. On Earth, extreme events can crash power lines, interfere with communications between planes and ground controllers, and affect mobile phone services.

But our own magnetosphere is basically a shield. It generally deflects the worst of the solar storms, pushing aside the charged particle flows. Pandora’s complex magnetosphere actually delivers the storms to the surface. An intense enough storm could be lethal for life forms; in the very worst case death could come instantly as the brain’s tissue is ionised, and you just “short out.”

Another remarkable feature Pandora shares with Io is a flux tube. Io is connected to its parent Jupiter by a tremendous trail of plasma, a natural conductor that carries a current of five million amps across a potential difference of hundreds of thousands of volts, with a power seventy times more than all of Earth’s generating capacity. This astounding structure pours additional heat energy down onto Io’s roiling surface. Pandora’s flux tube is more intermittent, but when it works it creates massive electrical storms, auroras, and other phenomena.

Quaritch is right. Pandora is a very hazardous world.

But RDA is on Pandora despite the hazards. The wealth to be found under Pandora’s surface makes it worth braving the hazards. And RDA is very efficient at extracting that wealth.