The atmosphere cooled and thinned. The pressure dropped.

The ocean froze over.

New methane lakes formed, which converted slowly to ethane. Sunlight broke up atmospheric ammonia, to release a new atmosphere of nitrogen.

The moon settled into its long freeze.

But it was not inert. Ultraviolet photons from the sun and charged particles trapped in Saturn’s magnetic field beat down on the thick layer of air. Chemistry continued in the new atmosphere, and complex organic deposits rained down on the frozen surface.

Thus, for billions of years, Titan waited.

An object looking a little like a comet streaked across the sky of Titan, battering atmospheric gases to a plasma twice as hot as the surface of the sun itself.

Cooling, it fell towards the surface slush.

A parachute blossomed above it.

Huygens was built like a shellfish, with a tough outer cover shielding a softer kernel, with its fragile load of instrumentation. When its job was done, the outer aeroshell broke open, like the two halves of a clam shell, and the main chute unfolded.

So, after being carried across a billion miles, the aeroshell was discarded. It had absorbed nearly a third of the probe’s entire mass.

The descent module, exposed, was built around a disc-shaped platform of thick aluminum. Experiments and probe systems were bolted to the platform. The equipment was shrouded by a shell of aluminum, with a spherical cap for a nose and a truncated cone for a tail. It looked something like an inverted clam. Now cutouts in the shell opened, and booms unfolded from the main body. Instruments peered through the cutouts, or were held mounted on the booms, away from the main body.

Tentatively, the lander sought contact with the orbiter.

Fifteen minutes after its unpackaging, the main chute was cut away, and a smaller stabilizer chute opened.

The probe began to fall faster, into the deep ocean of air. Vanes around its rim made it rotate in the thickening air.

Diaphragms slid back. A series of small portals opened in the protective shell of the craft, and sensors peered out.

At the base of Titan’s stratosphere, some thirty miles above the surface, the temperature began to rise a little. Gradually, the surface became visible. Downward-pointing imagers peered, in visible and infra-red light, and as the probe slowly rotated, mosaic panoramas were built up.

At last, the probe crashed into the slush. Slowed by Titan’s low surface gravity, and the density of the lower air — half as dense again as Earth’s — the impact was slow, as gentle as an apple falling from a tree.

The probe continued its battery of experiments, pumping telemetry up to the orbiter, which sailed onwards towards Saturn.

Huygens was primarily an atmospheric probe. It had not been certain that the probe would survive the impact. And the probe had actually been designed to float if need be, for none of its mission planners had been sure whether oceans or lakes existed here, or if they did how extensive they were, or whether the chosen landing site would be covered by liquid or not.

Just six minutes after landing, the probe’s internal batteries were exhausted.

Melted slush frosted over the buried portals of the inert, cooling lander. And a thin rain of light brown organic material began to settle on the upper casing.

The chatter of telemetry to Cassini fell silent. The orbiter passed beneath the horizon, and then turned its high gain antenna away from Titan, to Earth. Patiently, Cassini began to download everything the lander had observed.

Some of the results were unexpected.

Paula Benacerraf worked through her EVA suit checklist.

She connected her Snoopy hat comms carrier to the suit’s umbilical. She set the sliding oxygen control on her chest pack to PRESS. She put on her gloves and snapped home the connecting rings.

Then she lifted her helmet over her head. The suit built up to an overpressure, and she tested it for leaks.

The ritual of checks was oddly comforting. It took her mind off what she was about to do.

Tom Lamb rapped on her backpack.

Paula Benacerraf turned, awkwardly. Foot restraints held them both in standing positions, packed in head-to-toe. In her EMU — her suit, her EVA mobility unit — she felt ludicrously bulky, awkward in the confines of Columbia’s airlock, which was just a cramped, cylindrical chamber in the orbiter’s mid deck.

“That’s it, Paula. I think we’re go.”

She said, “Already?”

“Already.” Lamb grinned out of his helmet at her, and she could see silvery stubble in the creases of his leathery cheeks. “You’re an independent spacecraft now.”

Her heart was hammering under the tough surface of her HUT, her hard upper torso unit. “Spaceship Paula. It feels good.”

Tom Lamb had once been the youngest Moonwalker. Now, at sixty-two, he was one of the oldest humans to have flown in space.

And Benacerraf, forty-five, a grandmother, was one of the oldest rookies.

Benacerraf disconnected her suit from the wall mount.

Lamb said, “Houston, we’ve got the hatch closed and we’re waiting for a go for depress on time.” His native Iowan twang was overlaid with a Texan drawl acquired over long years at Houston.

“Affirmative, EV1; you have a go for depress.”

Lamb turned to the control panel and turned the depress switch to position 5. Then, with the pressure down to five psi, Lamb turned the switch to its second position. “Depress valve to zero.”

Benacerraf heard a distant hiss. She moved the oxygen control on her chest pack to its EVA position.

“Pressure down to point two,” Lamb said now. “Let’s motor.” He kicked his feet out of their restraints. With a confident motion he twisted the handle of the outer airlock hatch. Benacerraf thought the hinges and handle looked old, like bits of a school bus, with the polish of long use.

Lamb pushed the hatch outward, and Paula Benacerraf gazed into space.

She was looking along the length of the orbiter’s payload bay.

The big bay doors were gaping open, the silvered Teflon surfaces of their radiator panels gleaming, and the bay itself was a complex trench, crammed with equipment, stretching sixty feet ahead of her. There was no direct sunlight; the bay was in the shadow of a wing, and the light in the bay was like a diffuse daylight.

Tom Lamb moved out through the airlock’s round hatchway, and drifted over to the left payload bay door hinge. There was a handrail and two slide wires that ran the length of the big hinge, and Lamb tethered himself to the wires. She could see his bright EV1 armbands.

He turned and waited for her.

“Houston, the hatch is open and EV1 is out.”

“We see you, Tom.”

“EV1 is halfway out, getting ready.”

Benacerraf, with her hands on the doorway, felt as if she was frozen in place, as if she really couldn’t step out there.

Lamb lifted up his big gold visor, so she could see his face. “Just stay with it, kid. One step at a time.”

She grunted. “Some kid,” she said.

Somehow, though, Lamb’s gravelly words punctured her tension.

She kept her eyes down on the floor of the payload bay and drifted through the hatch, just as she had done a hundred times in training, in the big swimming pool in the Sonny Carter Facility at Ellington Field. She fixed her own tether in place. Now, at least, she wouldn’t go drifting off into space…

For the first time she looked up.

Columbia was flying with her instrument-laden payload bay pointing at Earth, so that the planet was a ceiling of light above Benacerraf, a belly of ocean strewn with white, shadowed clouds.