When storms collide on Jupiter.

For the first time, astronomers have witnessed giant storms colliding on Jupiter. Each storm, imaged with the Hubble Space Telescope and ground-based detectors, resembles a white oval and covers an area half the size of Earth. These swirling vortexes carry winds that rotate some 470 kilometers per hour

Observers 60 years ago first noticed three white-oval storms in Jupiter's southern atmosphere. For nearly 6 decades, the trio occasionally drew close together but never merged. In 1998, however, two of the storms approached each other just as Jupiter was moving behind the sun. When the planet came back into view, the two ovals had already become one. Astronomers had missed out on seeing the merger.

Last year, the oval resulting from the 1998 collision approached the other member of the original trio. A third, darker oval--swirling in the opposite direction from the other two--temporarily formed between them.

Last December, the middle, darker oval sped south and was apparently torn apart as it passed Jupiter's famous Great Red Spot, a much larger vortex. The disappearance of the reverse-rotating storm seems to have cleared the way for the two remaining vortexes to meet. In March, astronomers monitoring the upper layers of the ovals saw the storms swirl about each other. After 3 weeks, these layers consolidated into one oval. In deeper layers, the storms didn't circle each other, but instead the ovals stretched and contracted, producing a single oval with two nuclei.

Glenn S. Orton of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues suggest that a similar merger may have formed the Great Red Spot. This storm, twice as wide as Earth, has been a fixture in the planet's southern hemisphere for more than 300 years.

Orton and his colleagues reported their findings last month in Pasadena at a meeting of the American Astronomical Society.