The Interstellar Vehicle Venture Star is a starbound freighter, one of a fleet of twelve sister ships regularly plying the route between Earth and Pandora. The frequency of the voyages is why Colonel Quaritch is able to offer Jake a ride home only a few weeks after his arrival. Typically the ship will carry two hundred sleepers like Jake out to Alpha Centauri, and hundreds of tonnes of unobtanium back to Earth. In addition to a fifteen-strong crew there are also ten medical personnel on board, woken at the end of the journey to supervise the waking of the cryosleepers.
If you have your Avatar DVD to hand, take a look at the early scenes featuring Venture Star, as we see it in orbit around Pandora. It certainly looks an impressive piece of engineering, and so it should. James Cameron wrote a “bible,” a ten-page briefing document, detailing how the ship works, drawing on our best understanding today of how to build a starship. (There have been more starship studies than you might think; I’m personally involved in a study called Project Icarus, about how to send an unmanned probe to a nearby star.)
At the rear of the ship is an engine stack. The ship’s main drive is a rocket powered by the annihilation of matter and antimatter—in fact hydrogen and antihydrogen, contained in those big spheres, cryogenically cooled and held safely in magnetic bottles. As we’ll see the rocket engine is only used at the Pandora end of the journey. The engine stack is at the end of a long strut that leads to the crew compartments, which include a rotating arm. These components in turn huddle behind an array of forward-facing shields.
That ship is big, no less that fifteen hundred metres long. One reason for its sheer size is the need to keep the crew separated from the hazardous radiations of the engine. We have no spacecraft of anything like that dimension. Our largest space artefact is the International Space Station (ISS), which is seventy-three metres long and a hundred metres wide, including the solar panels, and masses over three hundred tonnes—a total mass which is in fact less than Venture Star’s cargo capacity.
How does such a ship fly to the stars?
Venture Star undergoes short bursts of acceleration at the beginning and end of the voyage, and then spends most of its transit coasting, with the engines powered down. You can tell it must cruise because you can see that rotating arm turning around the ship’s spine, evidently a device to give the handful of alert crew artificial gravity. This makes engineering sense; Apollo cruised most of the way to the moon and back, with most of the fuel load of its huge Saturn V booster burned up in the first few minutes of the journey. During the boost phases, the fragile rotating arm is folded back against the ship’s spine.
When Venture Star is accelerating from the solar system, it doesn’t rely on its own engines at all. Instead it is pushed by beamed power from Earth, light from a tremendous laser that is caught by a huge sail, a bowl sixteen kilometres across held stable by rotation. The ship carries big mirror shields that in this phase of the voyage protect the habitable compartments from the laser beam’s intense glare. When the acceleration phase is over the sail is folded away. All this is done to minimise the mass of fuel Venture Star must carry.
The “lightsail” is in fact an interstellar propulsion technology whose underlying principles were, remarkably, defined and demonstrated by the end of the nineteenth century, with a theoretical prediction of the pressure exerted by light by the Scottish physicist James Robert Maxwell, followed by an experimental demonstration by a Russian scientist called Peter Lebedev in 1900. Later the American physicist and science fiction writer Robert L. Forward did much to develop the idea. In his novel Rocheworld Forward described a manned starship propelled by the collected light of a thousand laser stations in orbit around planet Mercury. But the basic idea is more elegant, even beautiful; if you didn’t mind a journey time of a few thousand years you could sail to the stars powered by sunlight alone, pushing a huge, filmy sail.
Pushed by the energy beam, Venture Star accelerates at an uncomfortable (for the awake crew) one and a half gravities for a hundred and sixty-eight days. Then it cruises. Jake Sully, having slept away the journey to Alpha Centauri in cryosleep, is told on waking that the journey took five years, nine months and twenty-two days. That’s certainly a long enough time to justify putting most of the passengers in the freezer rather than try to keep them fed, watered and occupied all that way; an active human consumes around two tonnes per year of oxygen, water and food.
But it’s still a pretty rapid crossing. According to my venerable Norton’s Star Atlas, Alpha Centauri is 4.39 light years from the sun. Despite a perhaps confusing name, a “light year” is a unit of distance, not time; it’s the distance a light beam travels in a year, around nine trillion kilometres, or about sixty thousand times the distance between Earth and sun. So it would take a beam of light four years, four months and around nineteen days to reach Alpha Centauri. Any estimate of the ship’s precise speed depends on whether Jake’s five years, nine months and twenty-two days is measured on Earth or aboard the ship—as we’ll see, there is a difference! But you can see immediately that to cross more than four light years in less than six years Venture Star must have been travelling at a respectable fraction of the speed of light. In fact, the cruise speed is seventy per cent of lightspeed, and the ship travels at this speed for five years ten months.
Such a high velocity immediately raises another hazard: interstellar debris. Space isn’t empty, not even between the stars. Out there between the sun and Alpha Centauri the average density of matter is around one hydrogen atom per cubic centimetre. That may not sound a lot, and in the four-light-year-long tunnel bored by Venture Star the ship will only encounter a gram or so of material. But a gram hitting you at seventy per cent lightspeed would be equivalent to the three hundred tonnes of the International Space Station hitting you at Earth-orbital speeds—whammo! So after launch the ship is turned head over heels, so that during the cruise those mirror shields that protected the crew from the laser beam are held ahead of the craft, to act as multi-layer protection against the debris.
As Alpha Centauri approaches, the ship is flipped over again and the great antimatter engine is at last fired up, burning to give a deceleration of one and a half gravities for another hundred and sixty-eight days. On return to Earth, the sequence is reversed, with the antimatter engine pushing Venture Star on its way, and the beamed-energy laser bank slowing it down at the solar system. (Incidentally many candidate designs for starships use hybrid designs like Venture Star, with more than one propulsion system; the huge distances involved push our technologies to the limit.)
How long does the journey take? Well, if you add up all the times I quoted above you’ll find the total one-way mission duration, including acceleration, cruise and deceleration, is about six years and nine months—
Wait. Jake Sully was told he’d been sleeping for five years, nine months and twenty-two days. That’s a discrepancy of a year. What’s gone wrong?
The reason these numbers are different is because of another aspect of that tremendous velocity, those vast distances, that no amount of ingenuity will let you engineer away: relativity.