Her suit made her more dense than the water, and so she sank into darkness.

She fell slowly. She let her arms and legs relax, and she felt them drift away from her torso, separated by the flow of water.

She turned slowly onto her back.

Above her she could see the surface of the water, the dim orange glow of the sky above, huge oily ripples creasing the meniscus. But the surface receded, its detail lost, and soon the sky was invisible, save for the faintest of orange glows.

The water felt comfortable as she fell deeper into it, as if she was returning to a kind of home.

Now, at last, it was all gone. The Universe had collapsed down to the layer of water that pressed against the surface of her suit, the bubble of air in her helmet. There were no more choices, no decisions, no plans.

Maybe this was mankind’s last moment, she thought, here on this remote beach, the furthest projection of human exploration. Maybe, in fact, the sole purpose of the human story, fifty thousand years of crying and living and loving and dying and building, had been to deliver her here, now, to this alien beach, the furthest extension of mankind, with her little canister of seeds.

The cold dug deeper. For a while she found herself shivering, and she wrapped her arms around her torso. But that seemed to pass, and she felt comfortable again.

She knew what was happening. This was hypothermia, her core body temperature falling, as her body heat leaked out through the suit’s unresisting layers into the giant welcoming mass of fluid beyond.

It didn’t really matter.

She thought she was unconscious for a time.

It was hard to be sure.

Then she thought she could see Columbia, far below, rising towards her.

She smiled.

The orbiter’s leading edges glowed, a faint orange. The floodlights in the payload bay glowed like a captive constellation. And beyond Columbia there were stars: thousands of them, easily visible to her dark-adapted eyes, like the blackest desert night on Earth. She could even see the great sweep of the Galaxy, the ragged edge of the dust-clouds at the core.

The EVA was over. She reached up her hands, and started to take off her helmet.

BOOK SIX

Titan Summer

Voyager One reached the boundary of the Solar System.

This was the heliopause, the sheet in space where the wind of ionized particles from the sun grew so feeble it was overwhelmed by the broader stream of interstellar ions. Already Voyager was a hundred times Earth’s distance to the sun, ten times Saturn’s distance.

When gushes of solar plasma hit the heliopause, immense radio blasts — a hundred trillion watts — were generated. Voyager’s instruments, almost overwhelmed, recorded this, and faithfully attempted to download the data back to Earth.

Still there was no reply, no reassuring command stream.

Even beyond the heliopause, the sun’s gravity held sway; there were clouds of objects out here — ice moons, a trillion comets, never observed by humans — circling the central star. Voyager soared through this new realm, its radioisotope power slowly fading.

Voyager tried to contact Earth until its reaction gas failed, and it could no longer point its antenna. And by 2020 there was no longer sufficient power to drive the radio transmitter. Still the software cycled through its reacquisition algorithm, sending commands to inert attitude thrusters and radio transmitters, until the last trickle of power died.

It took twenty thousand years for Voyager to cross the Oort Cloud, the sun’s immense swarm of comets. At last it was free of the Solar System, its final gravitational bonds broken.

Its power and radio transmitter long dead, Voyager embarked on a new journey through the silent calm of interstellar space: an endless circling of the heart of the Milky Way galaxy.

There was almost nothing here to damage the derelict craft. The stars were so sparsely scattered that Voyager would never encounter another stellar system…

As time eroded, the logic of physics unfolded implacably. The sun was no longer young. Its core became denser and hotter, as it clogged with the accumulated helium ash of billions of years’ hydrogen fusion. The sun got brighter, at the rate of eight percent per billion years.

But for a long time Earth’s surface temperature remained the same. Earth was protected by matter and energy feedback cycles maintained by living and geological processes. And as the temperature rose silicate rocks weathered more easily, absorbing carbon dioxide from the atmosphere.

But it couldn’t last forever.

Eventually the carbon dioxide concentration in the atmosphere fell so low that the plants and trees could no longer photosynthesize. That put an end to the biosphere’s carbon supply. The rocks continued to weather, and the carbon dioxide concentration fell still more rapidly, and Earth heated quickly.

Maybe humans could have prevented this, with some huge feat of planetary engineering. There were no humans around to try.

On the parched planet, one species after another faced what human biologists had called thermal barriers to their survival. The more complex plant and animal species diminished first, as Earth shed the biological complexity painfully gained over billions of years.

After a billion and a half years the surface temperature averaged fifty degrees Centigrade, above which no animal, fish, crustacean or insect could survive. Most vascular plants and mosses succumbed as well, leaving the land and oceans empty save for micro-organisms: multi-celled animals like algae and fungi.

But above sixty or seventy degrees the structural characteristics of even the simplest multi-celled creatures — like membrane systems — could not be sustained. The survivors now were one-celled creatures, like cyanobacteria and some photosynthetic bacteria.

Above seventy degrees photosynthesis ceased at last.

The last survivor of Earth’s once-rich biosphere was a hardy bacterium, swimming through the sulphur-rich waters surrounding a black smoker ocean-floor vent. The story of life on Earth had come full circle, for the heat-loving archaebacteria were among the oldest life forms: they had arisen on a younger, hotter Earth, and become the progenitors of all subsequent life.

The end came at two hundred and fifty degrees Centigrade, above which the very stuff of life — the giant molecules, nucleic acids and ammo acids — was broken down.

After another hundred million years the oceans began to boil.

Huge clouds of vapor were suspended in the atmosphere. A new greenhouse factor came into play, driving temperatures higher still, ever faster.

The water clouds did not last long. The water vapor was broken up by energetic sunlight and its hydrogen was driven off into space, leaving a planet baked permanently free of water.

And the loss of all water stopped the weathering of silicate rock, the process which drew carbon dioxide out of the atmosphere. Volcanic carbon dioxide began to accumulate in the atmosphere. New clouds rose, and the planet began to bake…

At that remote time, Venus and Earth became at last what humans had dreamed in ancient times: twin planets, alike in every significant detail — scorched dry, their surfaces cracked and flattened under a dense, sluggish atmosphere, utterly lifeless.

It was different, for Titan.

The heating of the sun ruined the old surface of Titan, the gumbo-streaked, icy landscape humans had explored. The ethane of Clear Lake boiled, evaporated. The gases dissolved there — nitrogen, methane, hydrogen — thickened the atmosphere still further, adding to a greenhouse effect that accelerated the warming of the atmosphere.