Pale Blue Dot - Carl Sagan [106]
On March 25, 1993, a group of asteroid and comet hunters, looking at the photographic harvest from an intermittently cloudy night at Mount Palomar in California, discovered a faint elongated smudge on their films. It was near a very bright object in the sky, the planet Jupiter. Carolyn and Eugene Shoemaker and David Levy then asked other observers to take a look. The smudge turned out to be something astonishing: some twenty small, bright objects orbiting Jupiter, one behind the other, like pearls on a string. Collectively they are called Comet Shoemaker-Levy 9 (this is the ninth time that these collaborators have together discovered a periodic comet).
But calling these objects a comet is confusing. There was a horde of them, probably the fragmented remains of a single, hitherto undiscovered, comet. It silently orbited the Sun for 4 billion years before passing too close to Jupiter and being captured, a few decades ago, by the gravity of the Solar System’s largest planet. On July 7, 1992, it was torn apart by Jupiter’s gravitational tides.
You can recognize that the inner part of such a comet would be pulled toward Jupiter a little more strongly than the outer part, because the inner part is closer to Jupiter than the outer part. The difference in pull is certainly small. Our feet are a little closer to the center of the Earth than our heads, but we are not in consequence torn to pieces by the Earth’s gravity. For such tidal disruption to have occurred, the original comet must have been held together very weakly. Before fragmentation, it was, we think, a loosely consolidated mass of ice, rock, and organic matter, maybe 10 kilometers (about 6 miles) across.
The orbit of this disrupted comet was then determined to high precision. Between July 16 and 22, 1994, all the cometary fragments, one after another, collided with Jupiter. The biggest pieces seem to have been a few kilometers across. Their impacts with Jupiter were spectacular.
No one knew beforehand what these multiple impacts into the atmosphere and clouds of Jupiter would do. Perhaps the cometary fragments, surrounded by halos of dust, were much smaller than they seemed. Or perhaps they were not coherent bodies at all, but loosely consolidated—something like a heap of gravel with all the particles traveling through space together, in nearly identical orbits. If either of these possibilities were true, Jupiter might swallow the comets without a trace. Other astronomers thought there would at least be bright fireballs and giant plumes as the cometary fragments plunged into the atmosphere. Still others suggested that the dense cloud of fine particles accompanying the fragments of Comet Shoemaker-Levy 9 into Jupiter would disrupt the magnetosphere of Jupiter or form a new ring.
A comet this size should impact Jupiter, it is calculated, only once every thousand years. It’s the astronomical event not of one lifetime, but of a dozen. Nothing on this scale has occurred since the invention of the telescope. So in mid-July 1994, in a beautifully coordinated international scientific effort, telescopes all over the Earth and in space turned towards Jupiter.
Astronomers had over a year to prepare. The trajectories of the fragments in their orbits around Jupiter were estimated. It was discovered that they would all hit Jupiter. Predictions of the timing were refined. Disappointingly, the calculations revealed that all impacts would occur on the night side of Jupiter, the side invisible from the Earth (although accessible to the Galileo and Voyager spacecraft in the outer Solar System). But, happily, all impacts would occur only a few minutes before the Jovian dawn, before the impact