"Titan" - читать интересную книгу автора (Baxter Stephen)

BOOK THREE Cruise A.D. 2008 — A.D. 2014

Cassini was travelling at three miles per second: more than four Titan diameters every hour. And as Cassini climbed out of the heart of the Saturn system, Titan itself lay dead ahead, a featureless, orange-brown ball, dimly lit by the remote sun.

As Titan approached, a human passenger on Cassini might have been exhilarated, or terrified, by the probe’s plummeting towards the moon.

Cassini was a survivor. It had endured a two-billion-mile cruise through some of the most hazardous sites in the Solar System to get here. It had even survived Earthbound attempts to cut its funding, to abandon it to its fate, here among the moons of Saturn.

Cassini had already completed sixty orbits of Saturn. The orbits, pumped and shaped in three dimensions by Titan flybys, had periods ranging from a hundred days to ten, Saturn closest approaches ranging from three Saturn radii to seven, orbital inclinations ranging up to sixty degrees above Saturn’s equator.

There were more than thirty close Titan flybys during the tour. Cassini had even grazed Titan’s atmosphere, scooping particles of the thin, high layers of air into its mass spectrometer. The flybys had brought Cassini as close as six hundred miles from Titan’s cloud tops, passing at a speed of twelve thousand miles per hour.

Mission planners on the ground had eked out the spacecraft’s remaining propellant supplies and power in order to keep Cassini functioning effectively as long as possible. Perhaps, the planners dreamed, Cassini could survive through a single complete Saturn year, while Earth travelled around the sun thirty times. It could even slingshot off Titan to head for another planet, or an asteroid.

But now Cassini had a new mission: an assignment which, ultimately, it could not survive.

Humans were coming to Saturn. And Cassini would have to serve them.

It was January 18, 2008.

Communications from the surface of Titan would not be easy, for human colonists there.

As Titan kept the same face to Saturn at all times, a colony would be out of line of sight of Earth for half of each sixteen-day orbital period. That compounded the problems of Saturn’s billion-mile remoteness from Earth, and the difficulties of superior conjunctions: those periods, occurring once a year, when the geometry of the orbits of Saturn and Earth was such that the sun got between the Earth and Titan.

What the colony would need was a relay satellite in orbit around Titan. The human mission could have brought along its own relay satellite, and left it in orbit after its crew descended to the surface.

It would prove cheaper to use Cassini.

Cassini was to be placed in Clarke orbit around Titan, a synchronous sixteen-day orbit, so that it hovered above the ground station continuously. That way the satellite would be in line-of-sight with Earth almost all the time, save for those brief periods when it was eclipsed by Titan, or Titan passed behind Saturn, or when a superior conjunction made communication impossible in any case.

Cassini had on board an electronics package called the Probe Support Equipment, which had been designed to enable it to pick up data from the Huygens probe during its descent to Titan’s surface, and later downlink the data to Earth. And now this old piece of hardware and computer software, used only once, could be used to communicate with a surface human colony.

But to be captured by Titan, Cassini was going to have to shed most of its twelve-thousand-mile-per-hour approach velocity.

There was no way Cassini’s rocket propellant — tanks of hydrazine and nitrogen tet — could deliver such a velocity change. Even at the start of its tour, the total the propellants could have delivered had been about five thousand miles per hour; now, they were much depleted.

So Cassini — ageing, space-soaked and battered, short on energy and fuel — was going to be dipped into Titan’s atmosphere, and aerobraked.

For Cassini, which had never been designed for such a mission, there were some drawbacks.

Principally, it had no heatshield. And it had no aerodynamic surfaces for control. Cassini had the typical angular, non-streamlined look of a craft designed for the vacuum of space. Now, it would have to function as a mixture of entry capsule and aircraft.

As Titan neared, it began to open outward, turning from a socked-in ball to a wall of cloud, its scale overwhelming the hardy probe. Cassini was not heading for the heart of Titan’s face, but was passing the moon tangentially: aimed, roughly, at the edge of the atmosphere.

Cassini’s temperature rose rapidly as it encountered the first wisps of Titan air, nitrogen and methane and hydrogen.

Cassini plunged into the atmosphere of Titan with its lower equipment module, and its cluster of engine nozzles, leading; the umbrella-shaped high-gain antenna followed behind, acting as a kind of keel to keep the spacecraft stable.

Cassini was travelling at many multiples of the speed of sound. The temperature of the spacecraft’s structure rose rapidly, and a thin bow shock of plasma, glowing grey-white, formed ahead of the craft’s squat, angular prow.

The onboard processors monitored the spacecraft’s status. Internal thermometers noted the temperature rise within the body of the craft, and accelerometers recorded the reduction in velocity. If the velocity drop was too great, or the spacecraft began to overheat, the processors would fire the main engines. That would boost the spacecraft rapidly back out of the atmosphere, to the relative safety of space.

But Cassini would leave the atmosphere with most of its velocity intact. On this first entry Cassini would not even shed enough speed to be captured as a satellite of Titan; for now it remained in orbit around Saturn, although on a lower-energy trajectory, and must return to Titan for more aerobraking. Eventually, after several passes, the craft would shed enough energy to enter an elliptical orbit around Titan. And at last, using a combination of aerobraking and engine burns, Cassini would circularize its orbit and take up its position over Titan’s equator.

Later, the craft could be moved to station-keep over the eventual human colony’s position.

Many engineers on Earth gave low odds for Cassini to survive so many Titan passes. But in any event that was for the future, many months away.

For now, Cassini blazed in the thin, high air of Titan, a man-made meteorite, dragging a straight yellow line across the orange face of Titan’s cloudscape.

Even during this first atmospheric pass Cassini suffered some damage. Many of the covers of its sensors were corroded; its ability to function as a science platform was already degraded.

But much of its yellowed paint and pitted, blackened insulation blankets had been stripped away, the underlying metal surface exposed, gleaming. That was going to give the mission controllers, in the future, some heating problems. But the spacecraft itself looked young again, its scoured-clean surfaces shining.

Cassini,in fact, looked as if it had just come out of the clean room at JPL.

The human spacecraft Discovery, laden with fuel tanks and habitation modules and antennae, sailed away from Earth, towards the sun.

Discovery was an airliner shape suspended in black infinity, the radiators of its payload bay doors gleaming in the harsh, flat sunlight. The orbiter looked much as it had done in Earth orbit, save for the wings — reshaped for Titan’s atmosphere — and the removal of the tailplane.

Beyond the leading edges of the wings supplementary tanks protruded, massive, blunt-nosed cylinders swathed with reflective insulation blankets. The tanks carried the fuel for the final big OMS burn that would place Discovery into orbit around Titan.

In the payload bay was lodged a lumpy Space Station habitation module, with its front end docked to the orbiter’s big crew compartment, its rear fixed to a docking node. Designed for low Earth orbit, where it would have been protected by the Earth’s magneto-sphere, the hab module had been crudely toughened up with layers of aluminum to provide radiation shelter for the crew. The water tanks were clustered around the walls, too, making the interior of the hab module the nearest thing the crew had to a storm shelter in case of a violent radiation event, like a solar flare.

The docking node, too, was scavenged from the Space Station program; it was a squat, compact cylinder, every face sprouting docking nodes and airlocks.

Two Apollo Command Modules were stuck on the side of the docking node like suckling aluminum piglets.

Behind the node was the CELSS farm: it was an adapted Spacelab module, filled with the racks and lamps of the crew’s little hydroponic homestead.

And behind the farm, heavily shielded, was the cluster of fission generators. They were heavy, reconditioned Soviet-built antiques, of a design called Topaz. Each Topaz was a clutter of pipes and tubing and control rods set atop a big radiator cooling cone of corrugated aluminum, that looked like a hollowed-out Mercury spacecraft. The whole thing was perhaps five yards tall. The Topaz, intended to power ion rocket deep-space probes, was the only fission reactor design that had flown in space.

The launch of the reactors, aboard Endeavour, had been one of the most controversial aspects of the mission.

All the modules in the payload bay were swathed with gold-colored sun-shielding insulation blankets; they looked like presents wrapped up for Christmas. And the orbiter’s big, filmy high-gain antenna had been oriented to provide some shade from the approaching sun: the double-hide maneuver, the mission planners called it. But after three months’ exposure to the strengthening sunlight, parts of the blankets had already baked and turned black.

In the sunlight, the payload bay was brilliantly bright, and the sky beyond was black and empty of everything except the fiery disc of the sun itself.

Discovery coasted, unpowered, on its long trajectory towards the sun. Discovery had left Earth behind, and entered a realm governed only by the simplest of laws, gravity and Newton’s laws, utterly predictable.

Shadows shifted steadily across the cluttered payload bay as the orbiter went through its slow thermal roll.