Ulysses Turns 'Down' Into Space

International spacecraft makes historic swing by Jupiter as it heads for complex look at sun

ULYSSES, the international sun explorer, is boldly going where no spacecraft has gone before.

It is heading steeply downward toward the ecliptic plane - the plane where planets orbit. Ulysses will cross that plane at the end of February, entering unexplored solar-system territory.

Ulysses' main mission is to explore regions of space above the north and south poles of the sun. No rocket can give a spacecraft the impulse needed to send it north or south of the ecliptic plane - roughly the plane of the sun's equator. So Ulysses is following a course that took it through the Jupiter orbit switching junction on Feb. 8.

Like a train passing through a railroad junction, the giant planet's gravity switched the 370-kilogram (816-pound) spacecraft onto an orbit that bends down 80 degrees out of the ecliptic plane and then passes back around the sun. This took Ulysses through one of the most hostile environments in the solar system - the region where Jupiter's magnetic field traps and guides fast-moving electrons and other electrically charged particles. Ulysses, which was "hardened" to survive this ordeal, came through unsc athed.

Ulysses has a long cruise back toward the sun, looping around it at distances between 1.3 and 2.3 times Earth's radius. It will pass over the south polar region June-October 1994 and over the north polar region a year later. But that's not idle time for the Ulysses spacecraft.

"We seem to have something to do all the time," says Edward Smith, Ulysses project scientist at the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, which manages the mission for the National Aeronautics and Space Administration and the European Space Agency.

Ulysses sampled the interplanetary medium as it cruised out to Jupiter and will continue this sampling. However, Dr. Smith says, it now is beginning "a very interesting" study - a search for gravitational waves. If that search is successful, the discovery probably would overshadow all else that Ulysses does.

According to Einstein's theory of general relativity, large masses should generate detectable ripples in space time when they accelerate. Such waves should permeate the universe. Yet they generally would be hard to detect.

Smith says Ulysses is an ideal gravitational wave sensor. Passing gravitational waves would show up as small frequency variations in the spacecraft's radio transmissions. He explains the waves should produce a very distinctive signal with three pulses.

Ulysses also measured Jupiter's environment as it swung around the planet. It found that the magnetosphere - the space dominated by the planet's magnetic field - has changed configuration since Voyagers 1 and 2 sampled it in 1979. The new data will help scientists better understand how that magnetosphere behaves.

Ulysses swung around and left Jupiter on the dusk side of the planet. This, too, is unexplored territory. Thus, Smith says, the spacecraft has actually been in unexplored space since Feb. 8. At that time, Jupiter was a little above the ecliptic plane.

S the spacecraft moves well below that plane and back toward the sun, it will give scientists new information on the sun's sphere of influence - a three-dimensional volume dominated by the sun's magnetic field and its "wind" of charged particles.

The full extent of that volume is unknown. It probably extends billions of miles beyond the farthest planet along the ecliptic plane. but in regions over the sun's poles, the solar magnetic field exerts less influence. The heliopause - the boundary between sun-dominated space and interstellar conditions - may be relatively close.

The spacecraft should finish its primary mission in 1995. However, its navigation has been so accurate, it has used relatively little maneuvering rocket fuel. If all continues to go well, the mission may be extended to sample even more unexplored space.