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

Surrounding the rings, Earthbound observers understood were the concentric orbits of the five moons then known: Miranda, Ariel, Umbriel, Titania, and Oberon. They’re named after characters in Shakespeare’s A Midsummer Night’s Dream and The Tempest, and in Alexander Pope’s The Rape of the Lock. Two of them were found by Herschel himself. The innermost of the five, Miranda, was discovered as recently as 1948, by my teacher G. P. Kuiper.[13] I remember how great an achievement the discovery of a new moon of Uranus was considered back then. The near-infrared light reflected by all five moons subsequently revealed the spectral signature of ordinary water ice on their Surfaces. And no wonder—Uranus is so far from the Sun that it is no brighter there at noontime than it is after sunset on Earth. The temperatures are frigid. Any water must be frozen.

A revolution in our understanding of the Uranus system—the planet, its rings, and its moons—began on January 24, 1986. On that day, after a journey of 8? years, the Voyager 2 spacecraft sailed very near to Miranda, and hit the bull’s-eye in the sky. Uranus’ gravity then flung it on to Neptune. The spacecraft returned 4,300 close-up pictures of the Uranus system and a wealth of other data.

Uranus was found to be surrounded by an intense radiation belt, electrons and protons trapped by the planet’s magnetic field. Voyager flew through this radiation belt, measuring the magnetic field and the trapped charged particles as it went. It also detected—in changing timbres, harmonies, and nuance, but mainly in fortissimo–a cacophony of radio waves generated by the speeding, trapped particles. Something similar was discovered on Jupiter and Saturn and would be later found at Neptune—but always with a theme and counterpoint characteristic of each world.

On Earth the magnetic and geographical poles are quite close together. On Uranus the magnetic axis and the axis of rotation are tilted away from each other by some 60 degrees. No one yet understands why: Some have suggested that we are catching Uranus in a reversal of its north and south magnetic poles, as periodically happens on Earth. Others propose that this too is the consequence of that mighty, ancient collision that knocked the planet over. But we do not know.

Uranus is emitting much more ultraviolet light than it’s receiving from the Sun, probably generated by charged particles leaking out of the magnetosphere and striking its upper atmosphere. From a vantage point in the Uranus system, the spacecraft examined a bright star winking on and off as the rings of Uranus passed by. New faint dust bands were found. From the perspective of Earth, the spacecraft passed behind Uranus; so the radio signals it was transmitting back home passed tangentially through the Uranian atmosphere, probing it—to below its methane clouds. A vast and deep ocean, perhaps 8,000 kilometers thick, of super-heated liquid water floating in the air is inferred by some.

Among the principal glories of the Uranus encounter were the pictures. With Voyager’s two television cameras, we discovered ten new moons, determined the length of the day in the clouds of Uranus (about 17 hours), and studied about a dozen rings. The most spectacular pictures were those returned from the five larger, previously known moons of Uranus, especially the smallest of them, Kuiper’s Miranda. Its surface is a tumult of fault valleys, parallel ridges, sheer cliffs, low mountains, impact craters, and frozen floods of once-molten surface material. This turmoiled landscape is unexpected for a small, cold, icy world so distant from the Sun. Perhaps the surface was melted and reworked in some long-gone epoch when a gravitational resonance between Uranus, Miranda, and Ariel pumped energy from the nearby planet into Miranda’s interior. Or perhaps we are seeing the results of the primordial collision that is thought to have knocked Uranus over. Or, just conceivably, maybe Miranda was once utterly destroyed, dismembered, blasted into smithereens by a wild careening world, with many collision fragments still left in Miranda’s orbit. The shards and remnants, slowly colliding, gravitationally attracting one another, may have re-aggregated into just such a jumbled, patchy, unfinished world as Miranda is today.

For me, there’s something eerie about the pictures of dusky Miranda, because I can remember so well when it `vas only a faint point of light almost lost in the glare of Uranus, discovered through great difficulty by dint of the astronomer’s skills and patience. In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed.

Chapter 9. An American Ship at the Frontiers of the Solar System

…by the shore

Of Triton’s Lake…

I will clear my breast of secrets.

—Euripides, Ion (CA. 413 B.C.)

Neptune was the final port of call in Voyager 2’s grand tour of the Solar System. Usually, it is thought of as the penultimate planet, with Pluto the outermost. But because of Pluto’s stretched-out, elliptical orbit, Neptune has lately been the outermost planet, and will remain so until 1999. Typical temperatures in its upper clouds are about -240°C, because it is so far from the warming rays of the Sun. It would be colder still except for the heat welling up from its interior. Neptune glides along the hem of interstellar night. It is so far away that, in its sky, the Sun appears as little more than an extremely bright star.

How far? So far away that it has yet to complete a single trip around the Sun, a Neptunian year, since its discovery in 1846.[14] It’s so far away that it cannot be seen with the naked eye. It’s so far away that it takes light—faster than which nothing can go—more than five hours to get from Neptune to Earth.

When Voyager 2 raced through the Neptune system in 1989, its cameras, spectrometers, particle and field detectors, and other instruments were feverishly examining the planet, its moons, and its rings. The planet itself, like its cousins Jupiter, Saturn, and Uranus, is a giant. Every planet is an Earthlike world at heart—but the four gas giants wear elaborate, cumbersome disguises. Jupiter and Saturn are great gas worlds with relatively small rocky and icy cores. But Uranus and Neptune are fundamentally rock and ice worlds swaddled in dense atmospheres that hide them from view.

Neptune is four times bigger than the Earth. When we look down on its cool, austere blueness, again we are seeing only atmosphere and clouds—no solid surface. Again, the atmosphere is made mainly of hydrogen and helium, with a little methane and traces of other hydrocarbons. There may also be some nitrogen. The bright clouds, which seem to be methane crystals, float above thick, deeper clouds of unknown composition. From the motion of the clouds we discovered fierce winds, approaching the local speed of sound. A Great Dark Spot was found, curiously at almost the same latitude as the Great Red Spot on Jupiter. The azure color seems appropriate for a planet named after the god of the sea.

Surrounding this dimly lit, chilly, stormy, remote world is—here also—a system of rings, each composed of innumerable orbiting objects ranging in size from the fine particles in cigarette smoke to small trucks. Like the rings of the other Jovian planets, those of Neptune seem to be evanescent—it is calculated that gravity and solar radiation will disrupt them in much less than the age of the Solar System. If they are destroyed quickly, we must see them only because they were made recently. But how can rings be made?

The biggest moon in the Neptune system is called Triton.[15] Nearly six of our days are required for it to orbit Neptune, which—alone among big moons in the Solar System—it does in the opposite direction to which its planet spins (clockwise if we say Neptune rotates counterclockwise). Triton has a nitrogen-rich atmosphere, somewhat similar to Titan’s; but, because the air and haze are much thinner, we can see its surface. The landscapes are varied and splendid. This is a world of ices—nitrogen ice, methane ice, probably underlain by more familiar water ice and rock. There are impact basins, which seem to have been flooded with liquid before refreezing (so there once were lakes on Triton); impact craters; long crisscrossing valleys; vast plains covered by freshly fallen nitrogen snow; puckered terrain that resembles the skin of a cantaloupe; and more or less