WHEN it was first measured in the 1800s, Jupiter’s Great Red Spot was an enormous four times bigger than the Earth.

Then, when the space probe Voyager 2 hurtled past in 1979, it was seen to be a little more than twice the size of our planet.

Now under the prying eye of NASA’s Juno probe, it’s barely 30 per cent bigger than us.

“Nothing lasts forever,” Juno mission planetary scientist Glenn Orton told Business Insider.

“In truth, the GRS (Great Red Spot) has been shrinking for a long time ...

“The GRS will in a decade or two become the GRC (Great Red Circle),” Orton said. “Maybe sometime after that the GRM” — the Great Red Memory.”

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But NASA’s determined to have a good look at before it goes.

Juno sent back stunning images of the Great Red Spot last year as it dived close to the gas giant’s atmosphere. It passed 9000km above the 16,000km-wide storm.

It’s going to do so again in April.

But not quite as close.

None of its four future dives will bring it as close to Jupiter as July last year.

But they will still offer informative looks into the depths of the great red eye.

The first recorded instance of the Red Spot being sighted was in 1665. It’s been churning away at some 600km/h ever since.

The longest lasting known storm on Earth was recorded in 1994 — at 31 days.

Juno launched from Cape Canaveral, Florida in August, 2011, on a mission to learn more about Jupiter’s origins, structure, atmosphere and magnetosphere.

Like the Cassini probe’s mission to Saturn last year, NASA will deliberately crash Juno into Jupiter’s upper atmosphere as its life nears its end. This is to avoid any contamination of potentially life-supporting moons, such as water-spewing Europa.