The Science of Avatar

⁴ DREAM WORLDS

The Resources Development Administration (RDA) is a mighty impressive organisation, judging by what we see of it in Avatar.

Consider its competences. It mounts interstellar missions on a huge scale, transporting and building vast industrial and military infrastructures. It mines an alien world. It wages war against the natives. It brings resources back to Earth—and it turns a profit in doing so.

RDA came about because of the discovery of unobtanium on Pandora. After telescopes in the solar system discovered planets of the nearest star Alpha Centauri (see Chapter 13), with Pandora particularly showing tantalising hints of life and strange magnetic effects, two unmanned spacecraft were sent to the system, using prototype versions of the technologies that would one day power Venture Star (see Part Three). From Pandora, landers sent back images of the floating rock masses that would become known as the Hallelujah Mountains, and the landers sampled an “unidentifiable mineral” (later called unobtanium) that seemed to be involved in the exotic physics that was keeping those mountains afloat (see Chapters 15 and 16).

The potential for industrial development, and huge profits, was immediately obvious. Corporations and governments quickly formed the Resources Development Administration, an international quasi-governmental consortium, to manage the development of resources from Pandora. RDA was to have complete control over operations on Pandora, but is accountable to shareholders (as administrator Parker Selfridge is all too aware), is limited by treaty in its military operations—no weapons of mass destruction—and is obliged to work on Pandora “for the good of all mankind.”

And then a blue face was seen peering intently into one of the landers’ cameras, and things got complicated…

RDA’s skills wouldn’t have come out of nowhere. Before mankind could launch an interstellar mining operation, governments and corporations would have developed off-Earth operations on a smaller scale, starting with the worlds of our own solar system, feeding a resource-hungry Earth—and all the while making a fat profit in the process. What’s interesting about that is, while we’re not likely to see humans reach the worlds of other stars in our lifetimes, we could well get to see proto-RDAs exploiting the worlds of our solar system.

And maybe that will start with another small step on mankind’s nearest neighbour, the moon. A small step, followed by the chewing of a drill-bit in the lunar dirt.

I once met a man who, like Jake Sully, journeyed through space to another world. He travelled only about a light-second, not four light years. The trip took him three days, not six years. He didn’t need suspended animation, though on the way back, exhausted, he did sleep a lot. But, like Jake, he too walked on a low-gravity world. His name is Charles Duke, and he flew to the moon in 1972 aboard Apollo 16.

I interviewed Duke over lunch in a hotel in Bond Street, London. He told me how the handling of the Apollo lunar lander reminded him of the fighter planes he flew earlier in his career: “It was like being a rough acrobatic pilot. Oh, great ride…” Duke’s low-gravity moonwalks were actually typical of our near-future experience in space. Aside from the four gas giants, Earth is the largest world in the solar system; anywhere humans can land in the sun’s family we’ll find the gravity lower, just as on Pandora.

Then, during his journey home, with the spacecraft suspended between Earth and moon, Charlie Duke assisted in a space walk to retrieve instrument records. “As I floated out, the Earth was off to the right, probably about a two o’clock low, real low. I could see it beyond the hatch, beyond the Service Module. And it was just a little thin sliver of blue and white. And then I spun around this way and directly behind me there was this enormous full moon, and it was, I mean it was overwhelming, that kind of feeling. And you could see Descartes, you could see Tranquillity, all the major features, and it just felt you could reach out and touch ’em. No sensation of motion at all. The sun was up above my eye line but it’s so bright you don’t look at it. And everything else was just black…” He mimed for me his spacecraft, suspended between Earth and moon and sun. What an experience! Even a moonwalk would have some familiar features—ground below your feet, a sky above, a horizon. Duke’s walk between the worlds was something no human being had enjoyed before Apollo, in all our evolutionary experience—which is one reason why, in my opinion, we should continue to send humans into space.

But even as Duke was having his astonishing adventure, President Nixon’s administration was making the decision to can the later moon flights. For the foreseeable future American human spaceflight would be restricted to just the low-orbit hops of the space shuttle. It might have been very different: building on the successes of Apollo, Americans might already have reached Mars. But they didn’t.

Forty years later it’s easy to forget that human beings walked on the moon at all. And it’s easy to forget that the Apollo astronauts didn’t just go there “in peace for all mankind,” as the plaque on Apollo 11’s lunar lander said, or just for the science, or even just for national prestige. Just like RDA on Pandora, they went there in search of resources.

And today, would-be prospectors of the sky are again looking out at the solar system with calculating glints in their eyes.

Before 1969 the exploration and colonisation of the solar system, beginning with the moon and working outwards to Mars and beyond, was pretty much a given. In a favourite novel of my boyhood, Leigh Brackett’s Alpha Centauri—Or Die! (1963), this is nicely summed up in a few lines (Chapter IV): “There are men in space again… [The message] was heard and repeated. Inward from Mars it travelled, across Earth and Venus and into the sun-bitten, frost-wracked valleys of Mercury. Outward from Mars it travelled, to the lunar colonies of Jupiter and Saturn, to the nighted mining camps of the worlds beyond…”

Our view of the solar system then, going back centuries to the pioneering telescopic observations of Galileo, was that it was a family of worlds, most if not all of which would host life. Why shouldn’t there be life? Earth is just another planet; if life is here it ought to be everywhere.

And, following old theories of the formation of the planets, it was thought that the further out your world was from the sun, the older it would be. So “young” Venus, blanketed in cloud, was thought to be a world of ocean and swamp, the seas fizzing like soda pop from excess carbon dioxide, the land probably dominated by dinosaur-like monsters. And Mars, further out from the Earth, must be older than Earth, and host to an advanced, ageing civilisation—and, being older, Mars must be drying out. Around 1900, astronomer Percival Lowell put these ideas together with tentative, blurred telescopic observations of Mars to construct one of the most beautiful (if most wrong) theories in the history of science. Lowell believed the Martians were working together on a planetary scale to fix their own climate change crisis, their own ecocide; they had built a global network of canals to use polar cap meltwater to irrigate the drying fields. Lowell believed he saw these canals through his telescope.

This was the Mars that inspired some of the greatest works of early science fiction, including H. G. Wells’ The War of the Worlds (1897), in which the Martians reverse the Avatar story and come to our world for its resources—including human blood!—and Edgar Rice Burroughs’ “Barsoom” novels, beginning with A Princess of Mars, serialised from 1912. In Burroughs’ books “John Carter, gentleman of Virginia” is transported to a Mars of warring tribes and exotic multilegged beasts, and finds a beautiful humanoid girl to fall in love with, “Dejah Thoris, Princess of Helium.” James Cameron says that his absorption in science fiction of all kinds over thirty years fed into the creative process behind Avatar, and he was specifically inspired by Barsoom, and the adventures of John Carter, a soldier on Mars.

Burroughs allowed for Mars’ low gravity, by the way. Like the Na’vi, some of his Martians are taller than humans—“fifteen feet tall.” And on Barsoom there are immense life-sustaining machines of the kind I speculated in Chapter 2 must support a post-ecocide Earth: “Every red Martian is taught during earliest childhood the principles of the manufacture of atmosphere…”

This, anyhow, was the solar system, bursting with life and ripe for colonisation, that shaped the expectations of the early space explorers. So it was quite a shock when the first unmanned spacecraft sailed past Mars in 1964, over an area where “canals” were expected to be seen (even though it was no longer thought they would be artificial)—only to find craters, like the desolate moon.

And then there was the moon itself. It might be lifeless but, before the Apollo missions, space visionaries believed that the apparently barren moon would harbour hidden riches for future human colonists—especially water. As late as 1968, Arthur C. Clarke, in The Promise of Space, wrote, “The most valuable substance of all—as it is on Earth, when in short supply—would be water… [Water] certainly exists on the moon; the question is where, and in what form.” But Apollo brought a grave disappointment. Analysis of the moon rocks seemed to show not the slightest trace of water, either now or in the past. The dark lunar “seas” proved to be made of basaltic dust, not organic sea-bottom scum. To many, even inside the space programme, Apollo, intended as a first step into the cosmos, in the end served only to prove that we cannot colonise space.

So the space planners turned away from the old dreams. Moonwalkers like Charles Duke were suddenly left stranded. And if you wanted to write science fiction about Barsooms and other inhabited worlds, you had better set them among the stars, like Avatar.

But maybe we jumped to conclusions. Since Apollo we have come to suspect that the sky is after all full of riches, even the much-maligned moon. But to reach them, we’ll first have to get off the Earth.

⁵ A RIDE ON A VALKYRIE

It has always been difficult to make the first step off Earth and into space.

It’s easy on the moon, with its one-sixth gravity. Charlie Duke and his colleague aboard their tiny Apollo lunar module were able to return to lunar orbit with an engine and fuel tanks you could have fitted in a camper truck. By comparison, to climb out of Earth’s gravity well, the space shuttle stack was over fifty metres tall and weighed around two thousand tonnes, most of which was fuel, and oxidiser to burn that fuel. If Earth’s gravity was just a little stronger, in fact, no chemical-fuel rocket system like the shuttle would be able to escape from Earth. And if not for the pressure of military requirements which drove the development of rocketry, we might never have reached space at all; the first astronauts and cosmonauts rode into orbit on converted ballistic missiles.

The space shuttle worked, flying for three decades, despite the design flaws that led to two terrible accidents. But now the programme has been cancelled. And in February 2010 President Obama also dropped funding for NASA’s follow-up “Constellation” programme, which would have replaced the shuttle with a new range of human-rated rockets and spacecraft. The hope is that private industry will step up to the plate with a replacement launch system. Obama’s intention is evidently that the money freed up by not having NASA develop its own vehicles will help prime the pump for a new age of access to space. But for now it looks as if U.S. astronauts will have to hitch rides to orbit on Russian rockets.

Space is an expensive business, however, especially as a start-up. There’s a saying in the business that you need to spend billions to make millions out of space. But there are individuals with such means, and a drive, it seems, to make childhood ambitions come true. Companies like SpaceX and Blue Origin are rushing to develop their own launch systems capable of taking humans safely to orbit. NASA would be a customer, as would companies like Virgin Galactic, with its plans to take passengers on hops into space. SpaceX was established by South African dotcom entrepreneur Elon Musk, one of the creators of Paypal, and Blue Origin was founded by Jeff Bezos, the president of Amazon. This is new money being leveraged to achieve old dreams.

However, while the money might be new many of the designs are relatively conservative: capsules launched aboard chemical-rocket firecrackers, just like Apollo. Even the space shuttle had bits that were either discarded, like the external fuel tank, or had to be fished out of the ocean and rebuilt, like the solid rocket boosters.

What we need is not another throwaway rocket system. What we need is a true spaceplane. What we need is Avatar’s Transatmospheric Vehicle Valkyrie.

The space shuttle was boosted by rockets to orbit, but then could only glide back to Earth, unpowered. A true spaceplane would be capable of taking off unaided from a runway like a conventional aircraft, reaching orbit, and then returning to land. (In the industry jargon this is SSTO—single stage to orbit.)

This is an old dream. Before the development of Project Apollo the U.S. Air Force dreamt of spacecraft with wings. It flew the famous X-15 rocket plane, and it funded extensive research into “lifting bodies,” capable of very high-speed flight. Some of this research fed into the space shuttle programme, and today the USAF is experimenting with a scaled-down spaceplane known as the X-37B.

There are technologies on the horizon that could be developed to achieve a true SSTO craft. One promising technology is the “scramjet”: a supersonic combustion ramjet, which would enable aircraft to reach extremely high speeds within the atmosphere. A conventional “ramjet” draws in air to collect oxygen with which to burn its fuel, but the airflow within the engine is subsonic (below the speed of sound), so if the craft itself is travelling faster than sound, the intake of air has to be slowed down, creating drag. But in a scramjet the air passing through the engine can be supersonic—faster than sound, inside the engine itself. This enables the aircraft itself to reach much faster speeds.

The fastest air-breathing aircraft to date is NASA’s X-43A which has reached Mach 9.8 (that is, 9.8 times the speed of sound) using scramjet technology. In theory it is believed that scramjets could reach almost orbital velocity (which is Mach 25). The great advantage is in weight savings; unlike a rocket such as the space shuttle, a scramjet would need to carry virtually no oxidiser to burn its fuel, extracting it all from the air.

This is how the Valkyrie flies. Four times the size of the space shuttle, with its black heat-resistant tiles and white insulation reminiscent of the shuttle’s bodywork, the Valkyrie returns from orbit using friction with the atmosphere to brake, like the shuttle, and glides most of the way home. But to return to orbit it uses air-breathing turbojet engines to get off the ground, and switches to a scramjet mode at three times the speed of sound. It has rocket engines for the final burn to orbit. All this is powered by a fusion engine.

A compromise design with some potential is Skylon, being developed by a company called Reaction Engines Ltd based in Bristol, UK. Skylon’s engine works like a conventional jet up to five times the speed of sound at twenty-six kilometres altitude, at which point the air inlets close and the engine switches to an internal liquid oxygen supply, working as a rocket to complete the climb to orbit. As of February 2009, ESA, the European Space Agency, announced that it was funding a million-euro development of the engines.

Perhaps when the new private spacecraft start flying we will find ourselves on the verge of a new transport boom, like the spread of the railways in the nineteenth century. Aside from access to orbit, spaceplanes could be used for suborbital hops, such as a two-hour flight from New York to Sydney.

And, I suppose, other applications could be military, as we see in Avatar. Valkyrie-class vehicles acting like sub-orbital C-130s could deliver troops and materiel to combat zones anywhere in the world within hours.

But even if we do start getting into space at some reasonable price—so what? If you’re a budding space prospector, a proto-RDA, what are you supposed to be mining, the vacuum? As we’ve seen, in the wake of the Apollo missions scientists believed that even the closest destination, the moon, entirely lacked the most basic resource, water.

It turns out, though, that they might have been too hasty. And beyond the moon, the sky seems to be full of riches.

⁶ FOLLOW THE WATER

After Apollo, the space scientists examining the returned moon rocks thought that they contained no trace of water. They concluded that the moon must be dryer than old bones (literally).

But this conclusion has long been questioned. It could be that any evidence of water those early researchers did see was dismissed because of fears of contamination from Earth’s atmosphere; none of the boxes in which the lunar samples were returned kept their vacuum.

The picture began to change significantly in 1994 when a joint NASA–military satellite called Clementine was thought to have detected traces of water frozen in the shadows of a lunar polar crater. This raised great hopes, though doubt was cast on the result later. In 1999 another NASA probe called Lunar Prospector was deliberately crashed into a south pole crater, in the hope of raising a plume of dust laced with sparkling water—but again the results were inconclusive.

Today, however, thanks to discoveries in 2009 from India’s Chandrayaan-1 spacecraft, and NASA’s Lunar Reconnaissance Orbiter and Lunar Crater Observation and Sensing Satellite, we believe there might be three sources of water on the moon. The shadows of polar craters, forever dark, could act as cold traps. There could be trace amounts of water in volcanic glasses. And finally there might be water scattered over the moon’s surface—just traces, the slightest dew in the regolith (the lunar soil), delivered by comet impacts after the moon’s formation.

Water in space would be hugely valuable, far more so than gold, given the cost of hauling water up from Earth. On the moon, water would support life, and using electrolysis (passing an electric current through it) water can be broken down into hydrogen and oxygen to make rocket fuel. The moon could become a filling station outside Earth’s deep gravity field that could be used to support a general expansion into the solar system, just as was dreamed of before Apollo.

Another key resource to be found on the moon is helium-3, the isotope of this light element that is most useful in fusion reactors. Unfortunately, like the water, the helium-3 is implanted thinly in the regolith, having been deposited there by the solar wind over aeons. (In the Avatar universe RDA does in fact maintain a lunar helium extraction facility.)

The moon, however, is only the beginning of our search for water and other resources beyond the Earth. And it may not even be the first place we’ll look. In April 2010 the Obama administration set out a startling and terrifically exciting new vision for the future of American manned spaceflight. The next small step an American astronaut makes on another world might not be the moon, or even Mars, that traditional destination, but an asteroid.

On the very first day of the nineteenth century, a new world was discovered. Smaller than any planet, it was an asteroid, now called Ceres, the first discovered, and the largest of them all, as it turned out, circling in the great waste between Mars and Jupiter. Other asteroids soon followed: more than four hundred lumps of rock and ice were discovered by the end of the nineteenth century. The asteroids are thought to be relics of the solar system’s formation, fossil remnants never gathered up into planets.

Then in 1898 a new type of asteroid was discovered. Christened Eros, this flying mountain can wander within the orbit of Mars, and even comes distressingly close to Earth. Today we know of many asteroids whose paths take them near our planet. Known as near-Earth objects (NEOs), most of them are only a few kilometres across or less. There may be as many as two thousand NEOs more than a kilometre across, and maybe two hundred thousand more than a hundred metres across. About a fifth of them will eventually hit Earth—“eventually,” in this context, meaning over billions of years. The famous impact sixty-five million years ago which appears to have caused the extinction of the dinosaurs was in fact caused by a NEO. Today we are tracking NEOs with programmes run by NASA and other agencies; one day we may be able to push away any threats.

However it is not the threat of the NEOs that interests us here, but their promise.

Obama’s new vision would send astronauts to a NEO. We know we can reach them; already asteroid Eros has been orbited by an unmanned spacecraft. And surprisingly, perhaps, some of the NEOs come so close to Earth that it would take less fuel to reach a NEO and return than it takes to get to the surface of the moon and back. The catch is that it takes much longer to get to a NEO than the moon. In a way that’s a benefit; an asteroid mission could be a rehearsal for the even longer missions to Mars to come. The operation would be tricky; an asteroid’s gravity is so low that “landing” would be more like docking with an immense natural space station. Once there the astronauts could trial technologies for pushing rogue NEOs away from an Earth impact.

And NEOs themselves could prove to be very valuable prizes indeed.

Some NEOs are flying mountains of natural steel and precious metals, such as gold and platinum. The prospect of reaching what is known as a C-type asteroid, full of organic compounds, is even more exciting, because the C-types contain water. Not only that, with suitable engineering, you can also extract from the asteroid dirt carbon dioxide, nitrogen, sulphur, ammonia, phosphates—all the requirements of a life-support system, or a rocket fuel factory. You can also use the asteroid dirt to make glass, fibreglass, ceramics, concrete.

A logical early project using asteroid resources would be the construction of a solar power plant in Earth orbit. The high-technology components of the plant, such as guidance, control, communications, power conversion and microwave transmission systems, would be assembled on Earth. The massive low-tech components, cables, girders, bolts, fixtures, station-keeping propellants and solar cells, would all be manufactured in space from asteroid materials. The plant would produce energy, safe, clean, pollution-free, to be sold back to Earth.

This isn’t fantasy. Schemes to exploit the NEOs are approaching the feasibility of business plans; hard-headed entrepreneurs are considering ways to reach these mines in the sky. And once we get there, resources and power are going to start flowing down from the sky to the Earth.

Perhaps this is how we will save the world from an Avatar-style ecocide. In Part One we looked at the bottleneck we face on Earth: a bottleneck caused by diminishing resources, and the diminishing capacity of Earth’s environment to withstand the disturbances we are causing to extract those resources. If population continues to grow—and, just as significantly, if we continue to aspire to a better standard of living for all of us—we’re going to need economic growth, which means a growth in the usage of resources. And maybe space resources, extracted without further impact to the Earth, could be a way through the bottleneck.

Maybe it doesn’t have to be the way Jake Sully bleakly summarised it to Eywa. Maybe there is a way for us to keep the Earth green, without giving up our civilisation and all the benefits it brings: by using the resources of space.

What if we keep expanding? Beyond the moon, beyond the NEOs, what riches lie waiting further out in the solar system?

Let’s follow the water. We need a lot of ingredients to live, but water is by far the most fundamental.

It turns out that the whole of the inner solar system out to Mars—planets, near-Earth asteroids and all—could supply only enough water for maybe fifty billion people. That’s a lot, but only six or seven times the number of people alive today—or, put another way, seven billion people consuming seven times as much resource each.

Happily there is a lot more water in the outer solar system. There are a vast number of asteroids in the main belt, orbiting between Mars and Jupiter, perhaps ten billion larger than a hundred metres in diameter, and a hundred billion between ten and a hundred metres across. They are rich in water, metals, phosphates, carbon, nitrogen, sulphur. The main-belt asteroids could contribute about half the water available on Earth, vastly expanding mankind’s opportunities for growth.

The main belt may not be the most interesting territory to prospect asteroids, however. The asteroids tend to occur in groups, shepherded by orbital resonances with the planets. Some of the most significant groups are known as the Trojan asteroids. These are not in the main belt but in Jupiter’s orbit, at the so-called Lagrange points, points of gravitational stability. As a result the Trojans are comparatively close together; by comparison, the main belt asteroids are spread over an orbit wider than that of Mars.

And the Trojan asteroids are rich. It is believed that the asteroid mass available in the Trojans is several times greater than that in the main belt itself. Not only that, they seem to be even more volatile-rich than the C-type asteroids and comet nuclei. Some analysts think the Trojans might prove to be the richest single resource pool in the solar system.

Beyond the asteroids, ambitious prospectors could settle on the moons of the outer planets, some of which are little more than giant balls of water-ice. A single ice moon has around forty times as much water as all Earth’s oceans. The last planet to be discovered, Pluto (though it’s no longer regarded as a planet at all) is believed to be but one of a whole cloud of similar objects, icy worldlets and massive comet nuclei, circling silently in the dark. The cloud may extend some hundred thousand times as far as Earth is from the sun—that’s halfway to Alpha Centauri. The cloud may have a mass as much as ten times all the planets in the solar system combined…

What a vision this is! Water is only one of the resources waiting for us out there. Imagine an interplanetary civilisation, the solar system transformed by baby RDAs into a savage competitive arena of giant mining vessels, plying the space lanes and dismantling moons—a sky full of Pandoras.

But you might hope that amid all this industry we will find it in us to preserve the natural wonders of the solar system. Including our very own Pandora.

⁷ THE WONDERS OF THE WORLDS

We’ve sent unmanned spacecraft to inspect all the planets of the solar system save distant Pluto, and have landed on several of them, including moons. And what we’ve found everywhere we’ve looked is wonderful—even if it’s not always what we expected (though that in itself is great news for a scientist).

Where might we go, not in search of resources, but for the sheer wonder of exploration?

Even the humble moon has its wonders. For example, at the moon’s north pole, at a crater called Peary, there are mountains where the sun never sets. These “Peaks of Eternal Light” are believed to be the only site in the solar system where this is true. It comes about because the moon’s axis isn’t tilted relative to the plane of its (and Earth’s) orbit around the sun, unlike Earth’s tilt, which is the cause of our seasons.

Mars may have no egg-laying princesses as in Burroughs’ books. But it is a small, strange world, very unlike the Earth, with volcanic mountains so tall they stick out of the atmosphere, and a canyon system that stretches around half the planet, and valleys that look as if they were carved by flowing water. Right now there are robots working up there, machines built by human hands rolling across the desiccated seabeds. And, we’re increasingly suspecting, maybe there’s life there after all. (In the Avatar universe there are human colonies on Mars.)

Venus is a world only a little smaller than Earth, but swathed in a monstrous ocean of atmosphere, almost all of it carbon dioxide, a bright layer that utterly blankets the ground from our view. It’s so hot down there that at night the ground glows in the dark. But, astonishingly, despite the (literally) infernal conditions, human craft have made it here too. The Soviets achieved a landing with Venera 7 in 1970, a spacecraft as tough as a miniature AMP suit. A very Russian achievement!

One of the general wonders of the age of planetary exploration is that the solar system is turning out to be full, if not of Earths, at least of abodes where some form of life is conceivable. Consider Jupiter’s second moon out. Europa is close enough to its parent for tides to have melted a deep layer of the moon’s water-ice mantle. Its cracked icy crust looks like nothing so much as ice floes on Earth’s frozen-over Arctic Ocean, beneath which is a sea, tremendously deep, perhaps reaching all the way to the moon’s rocky core. And hydrothermal vents on that black-as-night seabed could provide nutrients for some form of life. A world with a roof.

And, a little further out, is a mysterious world that may be the solar system’s greatest wonder of all.

The furthest any craft from Earth has landed, so far, is on Titan, sixth moon of the sixth planet Saturn, nearly ten times as far from the Earth as the sun. It was an astounding achievement.

And the world the Huygens probe found is the solar system’s own Pandora.

Titan, Saturn’s largest moon, was discovered by the Dutch astronomer Christianus Huygens in 1655. To him it was just a dot of light, glowing dull orange. But in 1944 Gerard Kuiper, another Dutch astronomer, discovered methane gas there. This was a moon with air! Titan turned out to have the most massive atmosphere of any rocky world after Venus. Bigger than our moon but only half the diameter of Earth, Titan is able to retain a fat layer of air because of its extreme cold.

Our first close-up views of Titan came in 1980 and 1981, when Voyagers 1 and 2 flew past Saturn. But Titan was just a ball of smog; we could see nothing of the surface. Then, in 2004, the Cassini spaceprobe arrived, with the Huygens lander, named for the pioneering astronomer, clinging to its side.

Titan really is like Pandora in many ways. Like Pandora it is a low-gravity moon of a giant planet, and, superficially, remarkably Earthlike. Huygens came down on what looked like a relic of a flash flood, a plain littered by worn pebbles. On Titan there are mists and clouds, and slow-falling rain; there are branching river valleys that lead to oceans crossed by waves hundreds of metres tall. One ocean, called the Kraken Mare, is as big as the Caspian Sea.

But Titan is an Earth reimagined in different materials. On Titan water-ice plays the role silicate rock does on Earth, and methane plays the part of liquid water. Those pebbles Huygens saw were ice, not rock. The methane cycle isn’t quite like Earth’s water cycle, so the weather isn’t the same; evaporation is slow, but the air can hold a lot of vapour. The result is long periods of drought punctuated by intense rainstorms. There could even be “cryovolcanoes,” volcanoes spewing liquid water; there is evidence of lava flows in the past. If you stood on Titan you would be a monster of molten lava!

And, like Pandora, Titan is full of opportunities for life.

Out of those layers of clouds, complex organic molecules—the stuff of life itself—continually drift down to the surface below. They are created by electrical storms in the atmosphere, and the reaction of sunlight and Saturn’s magnetism with the upper air. These organic molecules could be the basis of an Earthlike life: carbon-water life, maybe anaerobic (that is, oxygen-hating) methane-eating bugs.

But there could be other kinds of life. Maybe a more exotic sort of carbon-based life form, using ammonia as its solvent rather than water and a metabolism based on carbon-nitrogen bonds, could be found in the stuff bubbling out of the cryovolcanoes. This is the sort of life that might live in the oceans of “roof worlds” like Europa. Most exotic of all could be a community of slime-like organisms that use silicon compounds as their basic building blocks, not carbon as we use; they might live in the surface ethane lakes, so cold they favour the long but fragile silicon-silicon molecular chains on which this form of life depends. Such forms might also find a home on Triton, the even colder moon of Neptune, where there are lakes of liquid nitrogen.

Nowadays we envisage many kinds of life, and many diverse habitats in the solar system. But Titan is extraordinary, for it may be a junction for life forms related to types from deep within the solar system’s warm heart, and from its chill edge. Huygens only glimpsed this; we must go again.

But Titan, like the other bodies of the solar system, might have a value beyond science—and that’s what might put it at risk. Titan is a natural organic-synthesis machine, way off in the outer system. It could become a factory for future colonists, churning out fibres, plastics, even synthetic food, manufactured from carbon, hydrogen, oxygen, nitrogen. Further out in time, it may be possible to export Titan’s volatiles to inner planets lacking them; Titan nitrogen, taken away on a massive scale, could be used to terraform Mars, to make it like the Earth. Just as some once hoped the moon could be a stepping stone to the planets, so Titan, a vital resource pool on the fringe of interstellar space, may some day be a key refuelling dump for ships like Venture Star, on their way to the stars.

But surely the worlds of the solar system are more than just mines in the sky. There are already proposals to preserve the unique value of other worlds. Radio astronomer Claudio Maccone of Turin advocates a “protected antipodal circle” of radio silence covering the moon’s far side. This is the only place in the solar system permanently shadowed from Earth’s clamorous broadcasts and so ideal for radio astronomy, and worth preserving as a park of silence.

Certainly, I personally hope that by the time we get to Titan we will treat it with more respect than RDA treats Pandora.

So we have reached the effective edge of the solar system, and there have been wonders aplenty—but no true Pandora, nothing like Earth. To find life like ours we will have to go on beyond the sun’s family.

But how are we going to get there? Could we ever build a ship capable of reaching the stars? Will a Venture Star ever fly?