Slowly, slowly, the sail billowed out, driven by the energy-thin drizzle of photons from the fat, faded sun. The probe, still orbiting Saturn, began to spiral outward, fine lines hauling at the sail so that it tacked in the unwavering breeze of light.

It took a thousand years to achieve solar escape velocity.

The journey took twenty thousand years.

The cruise was uneventful. The minuscule acceleration reduced as the light pressure from Sol dwindled with distance, and the interstellar medium — hydrogen atoms and ions — exerted a tiny but constant drag at the sail.

Each capsule contained diverse species. Many were extremophiles, able to adapt to extremes of temperature, pressure, acidity. Those landing on the Earth-like worlds contained organisms similar to blue-green algae. Most of the species were single-celled, but some were multicellular eukaryotes. Eukaryotes were more fragile. But there was evidence that on both Titan and Earth the progression to multicellular forms had formed an evolutionary bottleneck, of such low probability that on many worlds it might never happen. If eukaryotes could be protected and prosper, billions of years of evolution could be shortcut.

But the micro-organisms travelled through a deeply hostile environment.

They had to be shielded against ionizing particles and ultraviolet radiation. And the organisms were engineered to withstand heavy radiation fluxes; what was carried amounted to spores: biologically inert, free of water or ammonia.

At the mid-point of the twenty-seven light year journey there was a shift in polarization, so that the sail’s silvered surface was now directed towards the new star, the darkened side towards the diminished red blur that was Sol.

With the mirrored sail reversed, a long deceleration began.

There was a variety of designs, of strategies.

Some of the probes from Titan headed for clouds where new stars were being formed. Some of them were designed to colonize comets; at closest approach to a parent star the comet would spew spores into interplanetary space, for later capture by planets.

And so on.

This was panspermia: the delivery of life forms to other worlds, other star systems.

There were some on Titan who hypothesized that the worlds of Sol had themselves been seeded, in the remote past, by an early starfaring race. If that were true, the resulting life forms were morally obliged to continue, to spread life further, as far as possible.

On the other hand, if it were not true, if Sol life was the first, then the moral imperative to spread, to propagate, was all the greater.

So a cloud of solar sails drifted outwards from Titan, like thistledown on the light of dying Sol, fleeing the doomed world. A wind of life, blowing between the stars.

The star was the heart of a young, vigorous system. A disc of protoplanetary debris still encircled it, through which its planets swam.

On arrival, the sail was ejected.

The probe entered a neat elliptical orbit around the brilliant central star. The outer edge of the ellipse touched the orbits of the Titan-like planets, the inner edge the orbits of the Earth-like worlds. At the inner and outer points of its orbit, the probe ejected a multitude of tiny parcels, hundreds of thousands of them, shielded against ultraviolet radiation, each containing thousands of organisms.

Over twenty years, the parcels distributed themselves into sparse rings around the central star. The parcels were coated with a substance whose reflectivity varied with the intensity of the light falling on it. Thus, each parcel oscillated between the limits of its habitable zone, maximizing the probability of capture.

The target planets moved through the rings, sweeping up capsules.

Many of the capsules, entering at unfavorable geometries, were burned up in the thick atmospheres of their target worlds. But some survived, and drifted down through cloud layers — of water vapor or ammonia — to settle like silvered snowflakes on land, or oceans.

The thin metal coating of the capsules corroded. The parcels in which the micro-organisms arrived were egg-like, containing a small amount of prepackaged nutrients. This helped the organisms survive as they adapted to local conditions.

The awakening micro-organisms, released, began to disperse and evolve. They were adapted to rapid and efficient mutation.

Biological processes began.

The surfaces of these worlds bore the scars of recent, and continuing, planetesimal bombardment. This would not be an easy place in which to survive. But on each planet, a handful of organisms survived. And began to breed.

Together, the children of Sol began to remake the worlds of a new star.

AFTERWORD

The untimely death of Carl Sagan (1934-1996), who has a cameo role in Book Two of this novel, was a sad footnote to a year full of scientific wonders.

Sagan was an astronomer and planetary scientist, and author of accessible and uplifting non-fiction and science fiction. As a scientist, Sagan played an active role on spaceprobes such as Mariner 9 to Mars — Sagan ensured the probe was positioned to photograph Mars’s moons — and Pioneer 10 to Jupiter and beyond, on which Sagan was responsible for placing a message to alien life. Sagan’s speculations on terraforming Venus — the first serious scientific speculation on the subject — on the possibility of permafrost on Mars, and on conditions on Titan, helped influence the thinking of subsequent workers and writers — including myself.

Like H.G. Wells, Sagan seems to have believed that the future of mankind would be a race between education and catastrophe. In 1984 he co-authored the concept of nuclear winter which may, perhaps, have helped avert that very catastrophe from befalling us. As we near the end of a millennium still largely gripped by the madnesses which dominated its opening, we cannot afford to lose Sagan’s brand of clear-thinking, cheerful, communicative rationality.

Carl Sagan’s death was announced after I had drafted his appearance in Titan. So, sadly, this book is already alternate history. But I decided Sagan should stay in.

Stephen Baxter

Great Missenden

January 1997

An earlier version of one chapter in this novel appeared in a very different form in Interzone magazine, No. 105, 1996.