On a clear night, you can see the continuing pageant of Jupiter and Saturn marching across the sky within a couple of degrees of each other. For half a year, they have appeared to be keeping such close company you could imagine a physical connection between them. In a certain sense, space scientists now find that to be the case.
Voyager 2, heading for a late August rendezvous with Saturn, has picked up the magnetic tail of Jupiter almost 482 million kilometers distant from that giant planet. By the time the spacecraft reaches Saturn, that planet also is likely to be immersed in what planetary physicists call Jupiter's magnetosphere.
Jupiter's reach, it seems, extends all the way to Saturn.
The two planets are undergoing a triple conjunction, in which three times within roughly half a year they appear to come close together. Actually, they remain over 600 million kilometers apart. However, they are lined up in such a way that, as seen from Earth, their separation on the vault of the sky narrows dramatically. For the past six months, they have been separated by only a few degrees. Last Dec. 31 and again on March 4, the separation narrowed to 1.05 degrees. On the night of July 23-24, the third and last of the conjunctions will narrow the gap to 1.06 degrees. Then they will begin to diverge. Earthlings won't see another such triple conjunction of these planets until the years 2238-39 in the 23rd century.
This astronomical spectacle reflects a rare orientation of planets which makes the Voyager multiplanet missions possible. For much of the past decade, the outer planets have been moving in a spacial relationship in which a spacecraft can be sent on a course to Jupiter such that the giant planet will then deflect it to Saturn. Saturn, in turn, can deflect it on to Uranus, as will be done with Voyager 2. At the same time, Saturn is moving relative to Jupiter so as to pass through Jupiter's magnetic tail.
Any planet with a magnetic field -- such as Earth, Jupiter, or Saturn -- has such a magnetospheric tail. It is like a comet's tail. A solar wind of particles linked with magnetic fields blows outward from the sun. A planet's magnetism deflects this wind, creating a buffer zone around the planet which is the magnetosphere and which the solar wind stretches into a long tail.
Earth's magnetospheric tail extends for millions of kilometers. The moon passes through it as it orbits our planet. Jupiter's magnetosphere is much more extensive. Voyager scientist Frederick L. Scarf of TRW Inc. has suggested it could reach to Saturn's orbit. Now he and his colleagues Donald Gurnett and William Kurth of the University of Iowa have detected it in Voyager 2 data, confirming his intuition.
Saturn probably passes through Jupiter's tail once every 13 years.This time, however, a spacecraft will be there to observe th e event.