Less than 4 million miles from Jupiter, an American spacecraft named Voyager sees dozens of cyclones and hurricanes in the giant planet's upper atmosphere that dwarf most of Earth's tropical storms.

The smallest atmospheric storm Voyager has photographed in the last few days, Dr. Bradford Smith of the University of Arizona told a news conference today at the Jet Propulsion Laboratory, is 85 miles across.

Most of these storms -- and Smith said dozens are scattered across the face of Jupiter -- measure about 600 miles across. The largest is the planet's Great Red Spot, which is a permanent hurricane the size of Earth.

"Before this mission we thought these bright-and dark-colored halos were all giant thunderheads," said Smith, who is the lead Voyager photographic scientist. "Now we're bewildered, because what we see in many cases could not be thunderheads. We don't know what they are."

When a thunderhead forms on Earth, it is fed by heat sucked off the surface by upwelling air currents that carry the energy to it and keep it intact, even though it's nothing more that a bag of hot air.

At least twice, the storms on Jupiter have split in two, with one of the sections moving in one direction and the other in the opposite, each of them trailing atmospheric debris for thousands of miles. Each time, one of the sections has reformed itself a few days later to look exactly as it had before it split.

"It's as if each of these things has a life of its own," Smith said. "You can stretch them, deform them and even break them apart, and they still have an inner cohesion that keeps them together."

As the 1,800-pound Voyager moves toward Jupiter at more than 27,000 mph, its cameras show most of these storms to be dark-colored circles surrounded by white halos.At times, the halos disappear, and at other times they engulf the dark circles and appear as white globes of frozen ammonia.

Most of the halo shapes move from west to east at speeds ranging from 125 to 250 mph, caught up in the many jetstreams (rapidly moving atmospheric currents) that move the clouds around the planet. The face of the planet is striped into 12 regions, six dark-colored belts and six light-colored ones.

"We still don't see any more than 12 zones and belts," Smith said today, "but we do see quite a few more atmostpheric curents carrying clouds and gas of different composition at different speeds around the planet. These currents are far more complex than we thought they'd be."

Nowhere on the planet are the atmospheric currents as complicated as they are around the Great Red Spot, which is more a brownish-yellow right now than the brick red it was in 1973 and 1974, when Pioneer 10 and 11 photographed it.

Voyager's photographs have shown changes in shape as well as color, but the spot appears nearly stationary as the clouds to the north and south speed by. It drifts a little westward as almost everything moves eastward.

Streamers of frozen ammonia clouds swirl counterclockwise around the red spot at more than 125 mph, creating what still is apparently the largest and most permanent hurricane in the solar system.

Close enough now to Jupiter that it can clearly see the four largest of its 13 known moons, Voyager today photographed two of them, Ganymede and Callisto.

The photo of Ganymede, the largest of the moons, shows a bright spot near the center that looks like one of the haloed craters on the Earth's moon that have been made by meteorites in the recent past.

The picture of Callisto shows a darker surface, covered with bright spots that could be holes punched in its icy crust by recent meteorites.

Early this morning, Voyager crossed what scientists call a "bow shot" that marks the beginning of Jupiter's magnetosphere, where the planet's strong magnetic field traps charged particles pouring off the sun. This is the spot where the supersonic solar wind streaming off the sun abruptly slows down as it comes in sudden contact with the Jovian magnetosphere.

The encounter took place today a little more than 3.8 million miles from Jupiter, which placed the bow shot closer to the planet than the time Pioneer 10 encountered it more than five years ago.

The reason is that the solar wind is more intense and faster-moving this year than in 1973. The great speed of the solar wind compresses the magnetosphere of Jupiter and moves it back closer to the planet.