Bound for the sun, by Jove.

Bound for the sun, by Jove

The Ulysses probe, launched by the space shuttle on Oct. 6, becomes the fifth spacecraft sent toward Jupiter. But its 16-month trek to Jupiter is just a side trip-and a means to an end: The European craft's actual objective is the sun, and the first study ever made of the solar poles.

Other spacecraft have flown sunward, including various U.S. and Soviet Venus probes, the German Helios 1 and 2 craft, and the U.S. Mariner 10 (whose orbit gave it three close encounters with Mercury). But these missions all approached the sun in orbits in a plane near the sun's equator. Accelerating sun-bound craft out of the plane and up over the sun's poles required a rocket more powerful than any the U.S. now possesses. As a result, previous sunward probes were limited to the same low-latitude window - near the solar equator - that restricts solar viewing by observers on Earth.

Ulysses, however, is scheduled to pursue a path around Jupiter that will harness the giant planet's gravity to twist the plane of the spacecraft's orbit so that it ends up nearly perpendicular to the solar equator.

After a scheduled rendezvous with Jupiter on Feb. 11, 1992, the new European Space Agency (ESA) probe should follow an orbit back toward the sun. On Aug. 6, 1994, Ulysses should pass about 330 million kilometers beneath the sun's south polar region, reaching a latitude of about 80[degrees]S. This orbit should carry it up and across the solar equator around Feb. 2, 1995, at a distance of 195 million km, then fling the craft over the solar north pole on July 12, 1995, again at a latitude of about 80[degrees] S and distance of roughly 330 million km.

Flying where no craft has gone before, Ulysses will study how the sun's poles differ from its lower latitude in such characteristics as flares, magnetic field, and the speed and composition of outflowing ions known as the solar wind.

Missing, however, is a camera to record the new views. When work on this mission began in 1978, scientists and engineers envisioned two spacecraft - one each from Europe and the United States. That original International Solar Polar Mission had also included a coronagraph. But plans for the camera died in 1984 when NASA, pressured by budget problems, canceled development of the U.S. craft that would have carried it. This decision not only caused anti-NASA feelings within ESA, but also eliminated one fundamental goal of the mission: the simulataneous crossing of opposite solar poles by the U.S. and ESA spacecraft.

Delays in the shuttle's development also forced NASA to postpone the original launch date of the European craft - initially for three years to 1986. Then, when safety concerns following the 1986 Challenger explosion caused NASA to cancel a powerful, upper-stage booster that had been under consideration for kicking shuttle payloads out of Earth's orbit, NASA then reassigned the ESA craft to a less potent rocket. This necessitated Ulysses' long swing by Jupiter - instead of a direct launch toward the sun.