A Storm for the Ages
For at least 200 years, and possibly more than 350, astronomers have observed a colossal, crimson-coloured storm churning in Jupiter’s southern hemisphere. The Great Red Spot is an anticyclone, a high-pressure system spinning counter-clockwise, with winds
far more ferocious than any hurricane on Earth. At its largest observed size in the late 1800s, this single storm was wide enough to fit three Earths side-by-side, measuring over 40,000 kilometres across. Its sheer scale and longevity have made it a subject of fascination and intense study, a permanent-looking feature on a planet defined by its chaotic, ever-shifting cloud belts.
The Incredible Shrinking Spot
Despite its long history, the Great Red Spot is not as permanent as it once seemed. For decades, scientists have noted that the storm is shrinking. Observations from NASA’s Voyager probes in 1979 measured it at about 23,300 kilometres across. By the time the Hubble Space Telescope turned its eye on the planet, the trend was undeniable. In 1995, it was down to about 21,000 kilometres. By 2009, it was smaller still. Recent observations confirm the storm is now the smallest it has ever been measured, roughly 16,500 kilometres in diameter — still larger than Earth, but significantly diminished. The shape has also changed, morphing from a distinct oval into a more circular form.
Tornadoes Within a Tempest
The latest headline isn't just that the spot is shrinking, but how it appears to be doing so. Recent, highly detailed images from telescopes like Hubble have revealed that the storm's dynamics are more complex than a simple, steady contraction. Scientists have observed smaller vortices, or eddies, being drawn into the main storm system. These smaller storms, like cosmic tornadoes, appear to be feeding into the Great Red Spot, potentially altering its internal energy and momentum. One hypothesis is that these interactions could be sapping the great storm's strength, contributing to its accelerated shrinkage. In addition to these small eddies, observers have noted strange, wispy filaments stretching and twisting within the storm's core, hinting at complex processes at work in its less colourful centre.
The View From Above
Our understanding of this planetary marvel is thanks to a fleet of powerful observatories. The Hubble Space Telescope, through its Outer Planets Atmospheres Legacy (OPAL) program, provides yearly check-ups on the outer planets, allowing scientists to track changes in storms and winds over time. More recently, the James Webb Space Telescope (JWST) has used its incredible infrared sensitivity to peer into the upper atmosphere above the Great Red Spot. Where scientists once expected a relatively calm region, Webb discovered a host of intricate structures, including dark arcs and bright spots, revealing an unexpectedly active and complex interface between the planet’s atmosphere and its magnetic field. Furthermore, data from NASA's Juno spacecraft, which has made several close passes, revealed the storm is incredibly deep, extending over 320 kilometres below the cloud tops — 50 to 100 times deeper than Earth's oceans.
