sharp shadows on the cloud tops.
Titan – the largest of the moons, orbiting twenty Saturn radii from its parent – was a reddish-orange pinprick, well outside the ring system.
Titan appeared to lie directly ahead of the spacecraft.
It was time.
Pyrotechnic bolts fired, silently, releasing puffs of vapour that immediately crystallized and dispersed. Three springs pushed Huygens away from Cassini, and a curved track and roller made the released probe spin, at seven revolutions per minute.
The path of Huygens and its parent probe diverged, at half a mile per hour.
Two days after the release, with the two craft about thirty miles apart – each clearly visible from the other, as a bright, complex star – Cassini fired its main engine once more, to deflect its orbit. Now Cassini and Huygens parted more rapidly.
Cassini’s nominal mission was a four-year orbital tour of the Saturn system. Its objectives were to study Saturn’s atmosphere, the atmosphere and surface of Titan, the smaller icy satellites, the rings, and the structure and physical dynamics of the magnetosphere.
And while Cassini flew on, Huygens – dormant, unpowered, a mere ten feet across, spinning slowly for stability – fell directly towards the burnt-orange face of Titan.
It was November 6, 2004.
… It would be a second-generation star.
It formed from a spinning cloud, of primordial hydrogen and helium, polluted by silicon, carbon and oxygen: rock and snow, manufactured by the first stars, the oldest in the Universe.
The cloud was a hundred times the width of the Solar System, to which it would give birth.
The cloud collapsed, and spun faster. It heated up. At last, the cloud became unstable, and broke up into successively smaller fragments.
It shrank. The cloud became opaque, and the heat it generated as it collapsed could no longer escape.
The core imploded suddenly.
The collapse made the core, a protostar, shine brilliantly, ten thousand times as bright as the sun that would shine on mankind.
Eventually the core was so hot that hydrogen nuclei began to fuse to helium. The thermonuclear energy generated balanced the inward gravitational force. The protostar stopped contracting.
It was a star. The sun.
The remaining nebula cloud condensed into dust particles and snowflakes. The orbiting particles collided with each other, and – because of the stickiness of the ice, and the organic tars coating the dust – they formed a flat disc of swarming planetesimals, objects ranging in size from a few yards across to several miles.
The planetesimals collided; some grew in size, forming planets, and others fragmented.
Most of the nebula’s mass was lost in the process.
Earth formed in a million years. Earth was dominated by rock, its snow boiled away by the young sun’s heat, its surface pounded by planetesimals.
Further out, it was different.
Further out, everything was moving more slowly, and the nebula was less dense. It was cold enough for water, carbon dioxide, ammonia and methane to condense into ice. So while the inner planets were dominated by rock, the accreting planetesimals at Jupiter’s orbit and beyond swept up dirty snow.
Hundreds of millions of years after Earth, Saturn formed, gigantic, gaseous. It radiated heat as it collapsed, warming the orbiting fragments of nebula gas and dust.
Around Saturn, an accretion disc formed. Moons coalesced, from a mixture of water ice, silicates, ammonia, methane and other trace elements.
One of them was massive.
It was half rock, half ice. It heated as it collapsed, because of its huge mass; the primordial ices melted and vaporized. The rock settled to the center, because of its greater density. At last, at the core of the moon, a ball of silicate formed, overlaid by a shell of ice, six hundred miles thick.
An ocean gathered. It was a mixture of ammonia and methane. A dense atmosphere was raised over it. The new world was a cauldron, with pressures hundreds of times that of Earth’s sea level in human times, and temperatures measured in hundreds of degrees.
The high pressure and temperature were sustained, for millions of years. And in the organic soup of the ammonia-water ocean, complex chemistry seethed.
But the new ocean and atmosphere were not stable. Ultra-violet flux from the young sun beat down; planetesimals continued to fall, blasting away swathes of the air; the atmospheric gases dissolved in the ocean.
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