SAN FRANCISCO — Planetary probes are usually named for people who go where none have gone before. Viking went to Mars. Pioneer went to Venus. Voyager went to Jupiter and beyond.
Ulysses is named for a man who took the long way home.
The name is appropriate because NASA and the European Space Agency, each of which contributes half to the spacecraft's operation, sent Ulysses on a rather roundabout path to the sun. The probe was launched by the space shuttle Discovery on Oct. 6, 1990. Instead of sending the probe straight to the center of the solar system, the shuttle launched it out toward Jupiter, which Ulysses encountered in February 1992.
Now Ulysses is headed toward the sun in a loop that will take it over the sun's south pole during the latter half of next year. In 1995, Ulysses will swing over the sun's north pole, and then its mission will be done.
But unlike the mythical hero Ulysses, who ultimately returned home, the spacecraft will never return to Earth. Instead, the 121-pound spacecraft will continue flying outward to, and perhaps beyond, the edges of the solar system.
Today Ulysses is 3.9 times as far from the sun as Earth is. Even though many probes have traveled many times farther into the solar system than that, Ulysses is in a part of the solar system where no other probe has ever gone.
Most probes travel in the same plane the planets occupy in their orbits around the sun - the ecliptic plane. But in its path toward the sun's south side, Ulysses is now 45 degrees below the ecliptic plane. Its position gives solar physicists an opportunity to answer many questions that have puzzled them for years.
Most of those questions concern the variability of the solar wind. In addition to observing the turbulent solar winds that pass by the planets, which several past probes have done, Ulysses has been able to better observe the high-speed streams of solar wind particles that originate near the sun's poles.
The probe also has helped solar physicists make sense of the interactions between fast solar wind, which generally comes from the polar regions of the sun, and the slow solar wind that comes from the equator. Most important, said John Phillips of Los Alamos National Laboratory in New Mexico, Ulysses is giving solar physicists their first look at a ''pure solar wind.''
The solar wind is a stream of particles given off by the sun's outer layer. Because of the geometry of the sun, no interplanetary probe has ever encountered a constant solar wind. But as of this May, Ulysses began to measure the steadiest solar wind ever observed.
The scientists have found new things because, in May, Ulysses escaped the heliospheric current sheet, an electrical ''tutu'' around the sun's midriff that stretches from the sun's equator outward into the plane occupied by the planets. The heliospheric current sheet is a thin band of electric current formed by the sun's magnetic field. It isn't very significant beyond the sun's immediate vicinity, but it is a marker of the solar wind.
The sun's magnetic field controls how fast that wind moves into outer space. Near the sun's equator, the magnetic field lets fairly slow-moving particles escape. But at higher latitudes, only faster particles escape.
In a simple world, this pattern would produce a slow-moving solar wind in the plane of the sun's equator and a fast-moving wind elsewhere. But since the sun rotates on an axis that is different from its magnetic axis, the slow solar wind moves away from the sun in a rippled pattern.
Those ripples make the solar wind an extremely complex phenomenon near the ecliptic plane. As the sun rotates, it throws fast-moving particles out after slow-moving particles. When the fast particles collide with the slow particles in front of them, they produce shock waves similar to the sonic booms that supersonic airplanes make.
The ecliptic plane, where Earth and the other planets orbit, is riddled with such shock waves. One of the main reasons for launching Ulysses was to get away from the ecliptic and thus beyond the shocks that come along with the heliospheric current sheet.
Now they've done that. At December's geophysics meeting, Ulysses scientists showed evidence that the probe had reached an area where the solar wind was blowing at a fairly constant 1.7 million mph.
''It happened quite early, about eight or nine months earlier than expected,'' Balogh said.
The early crossing into high-speed solar wind shows that the heliospheric current sheet wasn't quite where the Ulysses researchers expected it to be. That suggests that there are aspects of the sun's magnetic field that aren't quite understood yet, Balogh said.
By getting below the heliospheric current sheet, Ulysses has been able to answer one of the most fundamental questions about the solar wind. Solar physicists have always wanted to know whether there are shock waves outside of the messy region where the slow-moving solar wind ripples outward from the sun.