For the first time, scientists have been able to watch the process of two of Jupiter's giant "white oval" storms, each about half the size of Earth, colliding and merging to form an even bigger storm.

"Usually when we've seen two of them approaching each other, they bounce back away from each other," said Dr. Glenn Orton, senior research scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and member of a team of Spanish, French and American astronomers that used NASA's Hubble Space Telescope and ground-based telescopes to study the ovals this year. Dr. Agustin Sanchez-Lavega, an astronomer at Universidad del Pais Vasco, Bilbao, Spain, reported the team's observations this week at the meeting of the American Astronomical Society's Division of Planetary Sciences in Pasadena.

Photo: NASA/JPL

The researchers speculate that a similar merger took place centuries ago and may have built Jupiter's famous Great Red Spot, a storm that is twice as wide as the Earth and has persisted in Jupiter's southern hemisphere for more than 300 years.

Seeing the collision of two storms will help scientists understand more about the dynamics of Jupiter's atmosphere, Sanchez-Lavega said. One question has been how deeply the roots of a storm at Jupiter's cloud tops extend into lower layers. In this year's merger, the upper layer seemed to move differently than underlying clouds.

Three white oval storms, in a band of Jupiter's atmosphere farther south than the Great Red Spot, became active about 60 years ago. In the following decades until 1998, they sometimes approached each other but never collided. In early 1998, two of the ovals were approaching each other as Jupiter went out of sight from Earth, behind the Sun. When the planet came back into view, the two had become one.

"We weren't able to see how they came together that time," Orton said.

Last year, the oval resulting from the 1998 combination approached the remaining one of the original three ovals. Each was a swirling high-pressure vortex, upwelling at the center and spinning winds counterclockwise to about 470 kilometers per hour (290 miles per hour). One was about 9,000 kilometers (about 5,600 miles) across, the other slightly smaller.

A third, darker oval, swirling clockwise instead of counterclockwise, formed temporarily between the two white ovals. That type of interceding system may be what usually keeps white ovals from colliding, the team proposed. But in this case, the middle storm appears to have been pushed even farther south and torn apart as all three passed near the Great Red Spot last December.

The disappearance of the opposite-swirling storm from between them cleared the way for the two white ovals to meet.

Their collision dance began in March and lasted about three weeks. At the cloud tops, the storms circled around each other counterclockwise, then consolidated into a single oval about one- third wider than either of the ovals had been beforehand. In deeper clouds, the interaction did not include the storms circling each other, but it did produce complex cloud structures from stretching and contracting of the ovals and went through an intermediate phase as a single oval with a double nucleus.

The ovals' approach and merger was viewed in various wavelengths, showing events at different depths, with a planetary telescope at Pic-du-Midi in France, NASA's Infrared Telescope Facility in Hawaii, and the orbiting Hubble Space Telescope, a facility of NASA and the European Space Agency.

JPL is managed for NASA by the California Institute of Technology, Pasadena. The Hubble Space Telescope is a facility of NASA and the European Space Agency. It is operated by the Space Telescope Science Institute, Baltimore, Md., which is managed for NASA by the Association of Universities for Research in Astronomy in Honolulu, Hawaii.