There were military police vehicles on the fringe of the recovery convoy, holding back while the technicians moved in on him.

The first tech opened up the canopy, and began to secure the ejection seat. Fresh, cold air pushed into the cockpit; Deeke breathed of it deeply.

To the north, that tower of vapor still dominated the horizon, misshapen, slowly dispersing.

Endeavour,still inverted, was crossing the equator.

Benacerraf looked up. The glowing skin of Earth scattered rich, cool light over the consoles and equipment of the cluttered cabin. Thunderclouds, ten miles high, were piled up along the equator; visibly three-dimensional, they seemed to reach down from a solid sky, clutching at the wounded orbiter.

My God, she thought. I’m still alive. I survived it. Again.

“We’re working on a revised OMS-two for you, Bill. Hang in with it. In the meantime, you want to proceed with your checklist?”

“Rog.”

“APU auto shutdown enabled. Boiler controller off. APU control off.”

“OPS 105 PRO. Gotcha.”

“ET umbilical door mode to manual. Left and right door, left and right door latch switches…”

“Houston, Endeavour. You want to tell us where the hell we are?”

“You’re in orbit, guys. Ah, seventy by seventy-one miles. Congratulations.”

“Jesus. We’re hardly out of the atmosphere,” Angel said.

Benacerraf knew he was right; at this altitude atmospheric drag would soon haul the orbiter back to Earth, whether the crew chose to come or not. And they had a way to climb to get to the rest of the cluster, built around Discovery.

But they would overcome all that. She felt a huge relief.

Angel whooped. “I guess we had a horseshoe up our ass the whole time, huh.”

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.