It's been around for centuries, but Jupiter's Great Red Spot is shrinking, and doing so at an accelerating rate. Astronomers can still impress people by talking of a single storm larger than the whole Earth, but maybe not for much longer.

The Great Red Spot remains the largest storm in the solar system. It is a high pressure (anticyclonic) storm system at latitude 22° in Jupiter's southern hemisphere. How long it is has been there no one knows, but something similar was described by Cassini in 1665 and even Galileo thought he saw something that might have been a predecessor. A century ago it stretched roughly 40,000km east to west and 14,000km north-south. The north-south axis hasn't changed all that much, but the long axis is now 16,500km long leading to suggestions the spot will soon be round, 

For most of its history the spot has been useful as a timing device to see when it crosses Jupiter's edge or midpoint, and a way for amateurs to test out the capacity of their telescopes, but its nature was mysterious. The Voyager Spacecraft gave us much more insight into the nature of Jupiter's weather, and told us the Spot is colder and 8km higher than the surrounding clouds, but we still have very little idea what is causing it. Even the source of the color is a mystery, with theories including phosphorus and complex organic compounds. To add to the confusion, the distinctive cloud cover that gives the Spot its name does not exactly coincide with the vortex that causes it or any features.

Similarly, the Spot's survival remains unexplained – nowhere else in the solar system do storms last anywhere near this long. On Earth the presence of continents tends to be disruptive, but storms also come and go on the other gas giants with nothing like the same staying power. Smaller spots on Jupiter risen and disappeared, most in the summer hemisphere. The next longest lasting storm observed has racked up 80 years, but in that time changed from white to red, meanwhile the Great Red Spot rolls on.

However, that doesn't mean it will be here forever. There has been evidence of spot shrinkage since the 1950s, but the rate is not constant. Recently it accelerated. In the decade finishing 2006 the Spot lost 15% of its long axis length, The Spot's rotation, once occurring around every 6 Earth days, appears to have slowed. Amateur astronomers have been reporting faster shrinkage for some years, but NASA devoted some of the Hubble Space Telescope's precious time to a more precise measurement.

"In our new observations it is apparent that very small eddies are feeding into the storm," said NASA's Dr Amy Simon. "We hypothesized that these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot." 

The spot and other Jovinian features are discussed by NASA scientists here: