New images from the Hubble Space Telescope reveal that Saturn's Great White Spot, a monster hurricane first observed by ground-based astronomers in September, is unexpectedly rich in roiling, ruffled cloud formations resembling those around the storm known as the Great Red Spot on Jupiter.

The storm on Saturn has grown into an impressive cataclysm 150 miles high and at least 6,000 miles wide that is whirling eastward around the planet's equator at 1,000 mph like a hula hoop, scientists said. It has galvanized astronomers on Earth, who are enjoying a rare chance to study such a disturbance in detail as it evolves.

Using ground-based astronomers' predictions of the storm's progress, the $1.5 billion orbital observatory was programmed to photograph the ringed planet on Nov. 9 and 11, using its Planetary Camera. One image from that session was released yesterday.

But the storm changed so rapidly during the two days that scientists and NASA officials decided they needed more. They mounted a full-court press to use the Hubble again, with the cooperation of colleagues who agreed to give up observing time and of teams who worked overtime to reprogram the complex instrument.

Last weekend, the space telescope took pictures of Saturn continuously, bringing the total to some 400 images, according to James Westphal of the California Institute of Technology, leader of the Planetary Camera team.

The storm "has turned out to be something very special," he said. "All those filigrees, and eddies -- nobody's ever seen anything like this from the ground."

Scientists believe the storm is an upwelling, or "burp," of hydrogen, helium and ammonia, possibly propelled by Saturn's intense internal heat, to high altitudes where the ammonia freezes into clouds of bright crystals.

But they said they do not understand why such storms occur -- or, more to the point, why they only occur occasionally and why the planet appears so smooth much of the time.

"It has to be some sort of internal, intermittent thing. If you like, Saturn burped," said Andrew Ingersoll, a planetary scientist from the California Institute of Technology. "Why it should wait so long to burp, I don't know."

He noted that both Jupiter and Saturn, the solar system's largest planets, are "bubbling cauldrons of liquid and gas" with no solid crust that would cause pressures to build up and then burst out in the manner of volcanic eruptions, as happens on Earth.

It is now summertime in Saturn's northern hemisphere, and Ingersoll said it is possible that the changing of seasons somehow triggers giant storms like the White Spot.

The storm is sometimes called the Wilber Spot after its discoverer, amateur astronomer Stuart Wilber of Las Cruces, N.M., who saw it in late September when it was less than 10,000 miles long, or only slightly larger than the diameter of Earth.

The Great Red Spot of Jupiter, which would blanket three Earths, has existed for at least 300 years. The storm on Saturn is much larger, circling the planet's 200,000-mile circumference. Such storms are seen occasionally on Saturn, but those around the equator seem to occur only about every 30 Earth years, or the length of one year on Saturn, and usually last only a few weeks, the scientists said. The last one detected in that region was in 1933.

The two Voyager probes each spent a week viewing Saturn eight years ago during their grand tour of the outer planets, but no great storms were in progress. The new images "tell us Saturn is much more active than we knew," Ingersoll said.

The Hubble data of this "storm of the century" will be converted into a short film showing how the tempest evolved, according to NASA's chief astrophysicist, Charles Pellerin.

The Saturn observation tested the network of humans and computers that operates the Hubble, which is still in its checkout phase, scientists said.

"A wondrous and fun thing happened," Westphal said. "Everything worked! . . . We got it {the imagery} all back. We're a little overwhelmed."

During the Saturn sessions, the team experienced none of the technical problems that previously afflicted the telescope, such as a jitter caused by its solar panels. And images of such close objects as Saturn are little affected by the manufacturing flaw in the telescope's main mirror.