Pale Blue Dot: A Vision of the Human Future in Space

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 site would be carried by Jupiter’s rotation into the line of sight from Earth.

The appointed moment for the impact of the first piece, Fragment A, came and went. There were no reports from ground-based telescopes. Planetary scientists stared with increasing gloom at a television monitor displaying the data transmitted to the Space Telescope Science Institute in Baltimore from the Hubble Space Telescope. There was nothing anomalous Shuttle astronauts took time off from the reproduction of fruit flies, fish, and newts to look at Jupiter through binoculars. They reported seeing nothing. The impact of the millennium was beginning to look very much like a fizzle.

Then there was a report from a ground-based optical telescope in La Palma in the Canary Islands, followed by announcements from a radiotelescope in Japan; from the European Southern Observatory in Chile; and from a University of Chicago instrument in the frigid wastelands of the South Pole. In Baltimore the young scientists crowding around the TV monitor—themselves monitored by the cameras of CNN—began to see something, and in exactly the right place on Jupiter. You could witness consternation turn into puzzlement, and then exultation. They cheered; they screamed; they jumped up and down. Smiles filled the room. They broke out the champagne. Here was a group of young American scientists—about a third of them, including the team leader, Heidi Hammel, women—and you could imagine youngsters all over the world thinking that it might be fun to be a scientist, that this might be a good daytime job, or even a means to spiritual fulfillment.

For many of the fragments, observers somewhere on Earth noticed the fireball rise so quickly and so high that it could be seen even though the impact site below it was still in Jovian darkness. Plumes ascended and then flattened into pancake-like forms. Spreading out from the point of impact we could see sound and gravity waves, and a patch of discoloration that for the largest fragments became as big as the Earth.

Slamming into Jupiter at 60 kilometers a second (130,000 miles an hour), the large fragments converted their kinetic energy partly into shock waves, partly into heat. The temperature in the fireball was estimated at thousands of degrees. Some of the fireballs and plumes were far brighter than all the rest of Jupiter put together.

What is the cause of the dark stains left after the impact? It might be stuff from the deep clouds of Jupiter- from the region to which ground-based observers cannot ordinarily see-that welled up and spread out. However, the fragments do not seem to have penetrated to such depths. Or the molecules responsible for the stains might have been in the cometary fragments in the first place. We know from the Vega 1 and 2 Soviet missions and the Giotto mission of the European Space Agency—both to Halley’s Comet—that comets may be as much as a quarter composed of complex organic molecules. They are the reason that the nucleus of Halley’s Comet is pitch black. If some of the cometary organics survived the impact events, they may have been responsible for the stain. Or, finally, the stain may be due to organic matter not delivered by the impacting cometary fragments, but synthesized by their shock waves from the atmosphere of Jupiter.

Impact of the fragments of Comet Shoemaker-Levy 9 with Jupiter was witnessed on seven continents. Even amateur astronomers with small telescopes could see the plumes and the subsequent discoloration of the Jovian clouds. Just as sporting events are covered at all angles by television cameras on the field and from a dirigible high overhead, six NASA spacecraft deployed throughout the Solar System, with different observational specialties, recorded this new wonder—the Hubble Space Telescope, the International Ultraviolet Explorer, and the Extreme Ultraviolet Explorer all in Earth orbit; Ulysses, taking time out from it investigation of the South Pole of the Sun; Galileo, on the way to its own rendezvous with Jupiter; and Voyager 2, far beyond Neptune on its way to the stars. As the data are accumulated and analyzed, our knowledge of comets, of Jupiter, and of the violent collisions of worlds should all be substantially improved.

For many scientists—but especially for Carolyn and Eugene Shoemaker and David Levy—there was something poignant about the cometary fragments, one after the other, making their death plunges into Jupiter. They had lived with this comet, in a manner of speaking, for 16 months, watched it split, the pieces, enshrouded by clouds of dust, playing hide-and-seek and spreading out in their orbits. In a limited way, each fragment had its own personality. Now they’re all gone, ablated into molecules and atoms in the upper atmosphere of the Solar System’s largest planet. In a way, we almost mourn them. But we’re learning from their fiery deaths. It is perhaps some reassurance to know that there are a hundred trillion more of them in the Sun’s vast treasure-house of worlds.

There are about 200 known asteroids whose paths take them near the Earth. They are called, appropriately enough, “near-Earth” asteroids. Their detailed appearance (like that of their main-belt cousins) immediately implies that they are the products of a violent collisional history. Many of them may be the shards and remnants of once- larger worldlets.

With a few exceptions, the near-Earth asteroids are only a few kilometers across or smaller, and take one to a few years to make a circuit around the Sun. About 20 percent of them, sooner or later, are bound to hit the Earth—with devastating consequences. (But in astronomy, “sooner or later” can encompass billions of years.) Cicero’s assurance that “nothing of chance or hazard” is to be found in an absolutely ordered and regular heaven is a profound misperception. Even today, as Comet Shoemaker-Levy 9’s encounter with Jupiter reminds us, there is routine interplanetary violence, although not on the scale that marked the early history of the Solar System.

Like main-belt asteroids, many near-Earth asteroids are rocky. A few are mainly metal, and it has been suggested that enormous rewards might attend moving such an asteroid into orbit around the Earth, and then systematically mining it—a mountain of high-grade ore a few hundred miles overhead. The value of platinum-group metals alone in a single such world has been estimated as many trillions of dollars—although the unit price would plummet spectacularly if such materials became widely available. Methods of extracting metals and minerals from appropriate asteroids are being studied, for example by John Lewis, a planetary scientist at the University of Arizona.

Some near-Earth asteroids are rich in organic matter, apparently preserved from the very earliest Solar System. Some have been found, by Steven Ostro of the Jet Propulsion Laboratory, to be double, two bodies in contact. Perhaps a larger world has broken in two as it passed through the strong gravitational tides of a planet like Jupiter; more interesting is the possibility that two worlds on similar orbits made a gentle overtaking collision and stuck. This process may have been key to the building of planets and the Earth. At least one asteroid (Ida, as viewed by Galileo) has its own small moon. We might guess that two asteroids in contact and two asteroids orbiting one another have related origins.

Sometimes, we hear about an asteroid making a “near miss.” (Why do we call it a “near miss”? A “near hit” is what we really mean.) But then we read a little more carefully, and it turns out that its closest approach to the Earth was several hundreds of thousands or millions of kilometers. That doesn’t count—that’s too far away, farther even than the Moon. If we had an inventory of all the near-Earth asteroids, including those considerably smaller than a kilometer across, we could project their orbits into the future and predict which ones are potentially dangerous. There are an estimated 2,000 of them bigger than a kilometer across, of which we have actually observed only a few percent. There are maybe 200,000 bigger than 100 meters in diameter.