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The biggest planetary storm in the solar system appears to be getting smaller. Jupiter’s iconic Great Red Spot, a massive anticyclonic storm observed for hundreds of years, is now only big enough to swallow one Earth. At its largest, 150 years ago, it spanned four Earth diameters.

A recent NASA study, which combines all consistent observations of the spot going back to the 1870s, suggests that it’s been shrinking since 1878, rallying only temporarily in the 1920s. In recent decades, the diameter has been shrinking by about 230 kilometres per year.

“Storms are dynamic, and that’s what we see with the Great Red Spot,” says study leader of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s constantly changing its size and shape, and its winds shift as well,” she says.

But what the spot is losing in girth, it’s gaining in height, according to observations by spacecraft that have visited Jupiter – from Voyager in 1979, to Galileo in 1995, up to the current, ongoing JUNO mission which recently sent back spectacular close-ups of the feature. The cloud tops in the storm are reaching higher than before, as seen in their higher reflectance over time.

“In an atmosphere, a spinning column has to conserve a type of momentum called vorticity. If it shrinks but doesn’t speed up, it must also stretch,” Simon says. She and her team relied heavily on data from the Hubble space telescope and high-resolution images from NASA’s Juno spacecraft to make observations of the spot.

Jupiter ascending

This stretching could also explain why the spot has become more orange between 2014 and 2017. It may be that higher altitudes gases change colour through exposure to more ultraviolet radiation.

Held at constant latitude by jet streams above and below spinning round the planet, the spot is known to gradually move westward over time, counter to the planet’s eastward rotation, but the observations show that its westward migration has speeded up. Simon says it’s likely related to conditions surrounding the storm, in the same way a hurricane on Earth is steered.

Simon says it’s possible that a similar type of storm was spotted in the 1660s. “That spot was smaller than the Great Red Spot was in the 1800s, so we suspect they saw a different storm that eventually disappeared,” she says.

It’s very likely Jupiter’s Great Red Spot could also disappear one day, she says, losing energy and dissipating through the generation of tiny atmospheric waves. “We see such a wave in the Juno data, but never saw anything like those in Voyager or Galileo data,” Simon says.

Read more: Jupiter’s Great Red Spot is making its atmosphere hotter