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Jupiter's famous red spot is not so great as before

Paul Sutherland, News Editor
17 May 2014, 5:00 UTC

Sen—Jupiter’s Great Red Spot, a favourite target for amateur astronomers’ telescopes, is shrinking, images from the Hubble Space Telescope confirm.

The feature is a huge storm that has been raging in Jupiter’s cloudtops since at least 1831 and possibly a lot longer. It is a twisting anti-cyclone whose edge is rotating at around 360 km (225 miles) per hour and which may have been first recorded by Italian astronomer Giovanni Cassini in 1665.

The Great Red Spot has been known to vary in colour and intensity from year to year. But the storm has shrunk to its smallest size ever measured, Hubble’s measurements show, stretching about 16,500 km (10,250 miles) across.

Jupiter

Images of Jupiter's Great Red Spot, taken by the Hubble Space Telescope over a period of 20 years, show how the striking feature has decreased in size. Image credit: NASA/ESA

In the late 19th Century, the spot was a lot bigger, and judged to be 33,000 km (25,500 miles) across on its greater axis, while the NASA spaceprobes Voyager 1 and Voyager 2 flew by in 1979 and measured it at 23,300 km (14,500 miles).

Then in 1995, a Hubble image showed the long axis had reduced to around 21,243 km (13,020 miles) across, and then in a 2009 image, it measured 17,912 km (11,130 miles).

The rate at which it is shrinking was revealed to increase from 2012, at a rate of 935 km (580 miles) per year, with amateur observations suggesting its shape was also changing from an oval to a circle.

The latest study has been made by a team of planetary scientists led by Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. So what is causing the spot’s decline? That is still something of a mystery.

Simon said: “In our new observations it is apparent very small eddies are feeding into the storm. We hypothesized these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot.”

Simon’s team plans to examine the motions of these small eddies plus the internal dynamics of the raging vortex to determine whether the eddies can feed or sap momentum entering the it, resulting in this yet unexplained shrinkage.

Incidentally, the latest magnificent Hubble images of Jupiter were processed for the team by one of the world’s leading amateur imagers of the planets, Christopher Go, who lives in Cebu City in the Philippines.

An excellent example of professional-amateur (Pro-Am) collaboration, it is nevertheless something of a challenge because Hubble takes three separate images through red, green and blue filters (RGB), with a delay between each, and the planet is rotating quite rapidly.

Go told Sen: “I was the one who processed their full disk images. I have collaborated with those involved so I was able to get the raw data. The tough part was aligning the RGB which is tricky because it takes Hubble three minutes to change filters. You need to derotate the channels.”

Go, who takes his own excellent pictures of the planets too with a Celestron C14, added: “I have actually been collaborating with this group for the last eight years. Pro-Am collaboration is strong in planetary science.”

In June 2010, Go imaged a bright flash in Jupiter’s cloudtops that was simultaneously phoographed by another amateur, Anthony Wesley at Broken Hill in New South Wales, Australia. It is thought to have been caused by an asteroid or small comet entering the planet’s atmosphere.

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