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.

Life in Microgravity:

Benacerraf had a lot of trouble sleeping.

When her little alarm watch sounded she was already awake, her eyes crusty and sore. She wriggled out of her sleeping bag; it was a little tight at the neck and she had to squirm.

Wearing just her underwear, she emerged from her private compartment into the bulk of the hab module.

Nobody was around. That suited Benacerraf; she liked to have a little time alone, to start the day. Right now, though, according to the schedule, somebody should be using the centrifuge; but she couldn’t feel the characteristic rhythmic judder of that big, heavy arm going through its six-revs-a-minute cycle. She made a mental note; somebody was goofing off.

The hab module looked clean, intact, its systems humming and whirring. The module was cylindrical, sized to fit into a Shuttle orbiter cargo bay. But inside, the module had a straightforward square cross-section, with flat walls, ceiling and floor, and rounded edges. The color scheme was a cool Earthlike blue, and the lighting was designed to provide plenty of up-down clues. Benacerraf, prone to dizziness and vertigo, appreciated that aspect of the design.

The gaps between the flat walls and the curved hull housed racks — ORUs, orbital replacement units — which could be folded out and replaced. The design rule was that life support and emergency systems and supplies were housed in the ceiling and floor, and systems the crew would use routinely were located in the walls. And strung out along the length of the hab module were the crew quarters, a health care bay, a galley area, and wardroom and hygiene facilities.

Briskly, she used the waste management facility. This was a little booth containing a Shuttle-technology commode, with pin-down bars over her thighs, and a unisex urination cup, color-coded for her use. When she closed the switch, fans started up with a rattling whine. Her urine was drawn away by a current of air, for storage and reclamation.

Benacerraf was proud of the work that had been done on the hab module, under her supervision, at Boeing’s Station assembly facility at Huntsville. They had stripped out the equipment racks, floors and utility systems; they’d taken the thing right down to its structural subassemblies and started again. They even stripped all the paint off, until it looked like it had just come out of the horizontal boring mill. They ran structural tests to check decade-old welds, and pressure and leak tests, and fixed a thousand strain gauges to measure stresses.

Then there was a whole series of modifications. They had adapted a hab module — intended as part of a frequent-resupply low Earth orbit station — to serve as the core of a many-year deep space mission. They had reconfigured the systems to take power from a couple of reconditioned Topaz fission reactors, for instance. And they had restructured the module to put shielding material around the hull, like water tanks. It was a lot of work; the engineers had to redesign and rebuild on the fly.

But for Benacerraf it had been a kind of relief, after a decade of frustration. So much fine work had been done on the Station components, only for them to be left standing around in assembly facilities. She had been involved right back when they put together the external structure of the first lab module, back in 1995. Three thousand one hundred inches of weld, all of exceptional quality. You couldn’t buy quality like that. You had to earn it. It was good to see this fine work put to use.