A fat pie-dish shape, ten feet across, clung to the side of the Cassini stack, so that the craft looked like a robot warrior going to battle, clutching a shield. In fact, the shield was a combined aeroshell and heat shield for a separate spacecraft, called Huygens, which was designed to land on Saturn’s largest moon Titan. The results Huygens gathered would serve as “ground truth,” confirmation and calibration for the more extensive orbital surveys Cassini would perform of the moon.

Now Cassini reached a point in space almost four million miles from Saturn’s cloud tops.

From here, the planet looked the size of a quarter-inch ball bearing held at arm’s length. Spinning in just ten hours, the planet was visibly flattened. A telescope might have shown its yellowish cloud tops, with their streaky shading and complex, anti-cyclonally rotating cloud systems. The sun was off to the right, with its close cluster of inner planets, so Saturn, seen from the probe, was half in shadow. The ring system, tight around the planet, was almost edge-on to the spacecraft, all but invisible, and it cast 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 vapor 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.