Telescopic observers from the 19th century may not have had the technological wizardry available to modern-day skywatchers — but they apparently had an easier time spotting Jupiter's signature feature, its Great Red Spot. According to Amy Simon-Miller (NASA/Goddard Space Fight Center), today this giant cyclonic storm is only about half as big as it was in the 1880s. Simon-Miller and three colleagues confirmed the shrinkage during a careful comparison of historical records and contemporary images from the Voyager, Galileo, and Cassini spacecraft. She presented their results last month at a meeting of planetary scientists.

Astronomers have known since the early 1900s that the Great Red Spot's longitudinal extent has been decreasing. Late in the 19th century the spot was nearly 35° wide, which corresponds to about 40,000 kilometers, or more than three times Earth's diameter. By 1979, when Voyagers 1 and 2 swept past, it had shrunk to 21° (about 25,000 km), yet its latitudinal "height" remained essentially unchanged, about 12,000 km from top to bottom.

Simon-Miller has discovered that the contraction seems to have picked up steam since the Voyager visits: at its present rate of shrinkage (0.19° in longitude per year), the spot will become the "Great Red Circle" by the year 2040. However a perfectly round shape is unlikely, she explains, because the strong, opposing jet streams that confine the spot's northern and southern boundaries will always distort it into an oval.

No one knows why the not-so-Great Red Spot has shrunk — or, for that matter, why its color intensifies and fades over time. One clue is that the winds around its circumference are whirling 70 percent faster now (about 700 km per hour) than they were in the Voyager era. Some historical observations suggest that the Red Spot grows and shrinks in a decades-long sequence. "I'm not sure the behavior is really cyclical," Simon-Miller comments, "but I certainly would not be surprised in the least if this shrinking trend slowed or reversed."

One possible explanation is that deep-seated bursts of thunderstorm-like convection periodically energize the overlying cloud layers, causing the spot to bloat in size, then gradually contract as the turbulence subsides. "All of the weather on Jupiter seems to have sporadic increases in activity," she notes, "so whatever feeds the Great Red Spot likely will too."