Moving toward Jupiter at more than 40,000 miles an hour, the Voyager 2 spacecraft was inward bound today on the last leg of its two-year journey to the largest planet in the solar system.

Less than 800,000 miles from Jupiter at 10 p.m. EDT tonight and picking up speed each minute, the silver and black, 1,800-pound spacecraft was due to fly by Jupiter at a distance of 404,000 miles at 7:21 p.m. EDT on Monday. Before encountering Jupiter, Voyager 2 was to fly past three of its four large moons, Callisto, Ganymede, and Europa. After leaving Jupiter it was to begin a 10-hour watch of the volcanic multi-colored moon io.

"The spacecraft pointing has been absolutely superb, we've had no drift at all," Voyager Project Manager Ray Heacock said today at the Jet Propulsion Laboratory where the flight is directed. "Everything is in excellent shape."

Voyager's exploration of Jupiter and its four Galilean moons began today when the spacecraft passed within 132,00 miles of Callisto at 7:13 a.m. Voyagers 2 will pass 37,000 miles from Ganymede on Monday at 4:06 a.m., 127,000 miles from Europa at 2:43 p.m. and start its "io watch" from a distance of 701,000 miles at 8:09 p.m.

In the next three days, Voyager will take more than 1,000 pictures of the four large moons and the huge planet they orbit. More than 180 pictures will be taken of Io, whose yellow, orange and red surface is covered with no fewer than eight active volcanoes spewing black ash up to 220 miles into space.

Photographs taken of seven of the volcanoes in the last few days show that the largest is now dormant but that the other six are not. The ash plume of one of the erupting six was up to 160 miles above the surface.

The volcanoes were discovered when Voyager 1 flew by Jupiter last March. All eight volcanoes were erupting then.

The six erupting volcanoes that Voyager 2 has seen are within 30 degrees of Io's equator but the spacecraft has also seen a string of burned-out out volcanoes near the moon's south pole.

"None of these is active," said Dr. Laurence Soderblom of the U.S. Geological Survey. "These vents are not ancient either."

Soderblom said that Io's south polar region is strewn with desert-like mesas that give it a look something like Monument Valley in Arizona and Utah. Soderblom said he had no solid explanation for the erosion process that makes mesas on Io. He speculated that sulfur dioxide gas poured from the vents froze into snow when it left the warmth of Io's interior and then carried entire cliffs with it when it fell like an avalanche from its own weight.

The eight volcanoes seen on Io run the gamut of every type seen on earth, Harold Masursky of the U.S. Geological Survey said. There are volcanoes producing regular dark lava, some exploding ash into space and others pumping out a mix of lava and ash not nearly as dark as the black lava seen around more conventional volcanoes.

"This implies a considerable change of chemistry, one that is at least as varied as the chemistry on Earth," Masursky said. "It may be that Io has undergone an even longer evolution than the Earth has."

Geologists disagree on the mechanism that produces volcanoes on Io that are still erupting after what may be billions of years of activity. The heat generated by Io probably comes from two sources: the radioactive rock in its interior and the tidal motion that forces it to wobble up and down when it passes its nearest neighbors, Ganymede and Callisto.

"There has got to be some radiogenic heat on Io, Soderblom said, "but the tidal effects from the other moons make it much more active."

Geologists agree that the yellow, red and organge colors on Io's surface come almost entirely from sulfar that has poured from the interior through the volcanic vents. So much sulfur has come to the surface that it has covered up just about every crater punched by meteorites into Io's surface since geologic time.

The mechanism that explodes sulphur ash and lava into space from Io's interior could come from a liquid sulfur ocean that lies less than a mile below the surface. Dissolved into this liquid sulfur is sulfur dioxide gas under pressure just like the carbonates and minerals in an artesian water system on Earth.

Soderblom speculated today that the sulfur dioxide gas explodes out of the liquid sulfur when it suddenly reaches a point below Io's crust where the pressure drops. The exploding gas either finds a vent in the surface or creates one, Soderblom suggested carrying with it tons of rock, hot ash and liquid sulfur to the surface and out into space.