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planet itself at some 20 kilometers per second, the relative speed of two adjacent particles is very low, only some few centimeters per minute. Because of this relative motion, the particles can never stick together by their mutual gravity. As soon as they try, their slightly different orbital speeds pull them apart. If the ring were not so close to Saturn, this effect would not be so strong, and the particles could accrete, making small snowballs and eventually growing into satellites. So it is probably no coincidence that outside the rings of Saturn there is a system of satellites varying in size from a few hundred kilometers across to Titan, a giant moon nearly as large as the planet Mars. The matter in all the satellites and the planets themselves may have been originally distributed in the form of rings, which condensed and accumulated to form the present moons and planets.

For Saturn as for Jupiter, the magnetic field captures and accelerates the charged particles of the solar wind. When a charged particle bounces from one magnetic pole to the other, it must cross the equatorial plane of Saturn. If there is a ring particle in the way, the proton or electron is absorbed by this small snowball. As a result, for both planets, the rings clear out the radiation belts, which exist only interior and exterior to the particle rings. A close moon of Jupiter or Saturn will likewise gobble up radiation belt particles, and in fact one of the new moons of Saturn was discovered in just this way: Pioneer 11 found an unexpected gap in the radiation belts, caused by the sweeping up of charged particles by a previously unknown moon.

The solar wind trickles into the outer solar system far beyond the orbit of Saturn. When Voyager reaches Uranus and the orbits of Neptune and Pluto, if the instruments are still functioning, they will almost certainly sense its presence, the wind between the worlds, the top of the sun’s atmosphere blown outward toward the realm of the stars. Some two or three times farther from the Sun than Pluto is, the pressure of the interstellar protons and electrons becomes greater than the minuscule pressure there exerted by the solar wind. That place, called the heliopause, is one definition of the outer boundary of the Empire of the Sun. But the Voyager spacecraft will plunge on, penetrating the heliopause sometime in the middle of the twenty-first century, skimming through the ocean of space, never to enter another solar system, destined to wander through eternity far from the stellar islands and to complete its first circumnavigation of the massive center of the Milky Way a few hundred million years from now. We have embarked on epic voyages.

*Or, to make a different comparison, a fertilized egg takes as long to wander from the fallopian tubes and implant itself in the uterus as Apollo 11 took to journey to the Moon; and as long to develop into a full-term infant as Viking took on its trip to Mars. The normal human lifetime is longer than Voyager will take to venture beyond the orbit of Pluto.

*We even know what gifts they brought the Court. The Empress was presented with “six little chests of divers pictures.” And the Emperor received “two fardels of cinnamon.”

*In 1979 Pope John Paul II cautiously proposed reversing the condemnation of Galileo done 346 years earlier by the “Holy Inquisition.”

†The courage of Galileo (and Kepler) in promoting the heliocentric hypothesis was not evident in the actions of others, even those residing in less fanatically doctrinal parts of Europe. For example, in a letter dated April 1634, René Descartes, then living in Holland, wrote:

Doubtless you know that Galileo was recently censured by the Inquisitors of the Faith, and mat his views about the movement of the Earth were condemned as heretical. I must tell you that all the things I explained in my treatise, which included the doctrine of the movement of the Earth, were so interdependent that it is enough to discover that one of them is false to know that all the arguments I was using are unsound. Though I thought they were