The Ulysses spacecraft, having logged 242 million miles, began the second leg of its journey to Jupiter, traveling at 53,000 miles per hour on a voyage that will take it the sun.

Deployed from the shuttle Discovery on Oct. 6, the boxy Ulysses passed the halfway mark of its 484 million-mile trek at 10:40 a.m. PDT Sunday and was expected to reach the solar system's largest planet Feb. 6, 1992.Once it reaches the gaseous giant, Ulysses will use the planet's gravity to propel it to the sun.

Ulysses is expected to enter a polar orbit around the sun in June 1994, scientists at NASA's Jet Propulsion Laboratory in Pasadena said.

The spacecraft's mission is to study the sun's poles and interstellar space beyond them.

While knowledge of the equatorial regions of the sun is extensive, no instruments have ever been launched to look down on the sun's polar regions because of the tremendous energy required to overcome the gravitational bonds that hold the planets in equatorial orbits around the giant star.

If all goes well, the tiny 850-pound instrument-loaded Ulysses, which was built by the European Space Agency, will revolutionize understanding of the sun and how it influences Earth's fragile environment.