Jupiter's trademark Great Red Spot - the most powerful storm in the solar system - has shrunken to its smallest observed size ever, and scientists struggle to figure out why.

Recent Hubble Space Telescope images of the storm show that it is now 10,250 miles (16,496 km) across, which is less than half the size of the storm in the late 1800s, Amy Simon of NASA's Goddard Space Flight Center said in a statement.

According to Space.com, at one point, scientists theorized that three Earths could fit inside the Great Red Spot (GRS), but today, only the width of one Earth could fit within the swirling anticyclonic storm.

And as the spot diminishes, it seems to be shrinking at an alarming rate.

The GRS's "waistline" is getting smaller by 580 miles (933 km) per year - a little less than the driving distance from New York City to Cincinnati - and its shape has morphed from a circle to an oval.

Scientists are not sure what is causing such dramatic changes.

"One possibility is that some unknown activity in the planet's atmosphere may be draining energy and weakening the storm, causing it to shrink," Hubble officials wrote in a news release.

Astronomers have been observing Jupiter's GRS since the 1600s, Space.com reported, but they failed to notice its significant downsizing until 1930, and especially so in 2012.

The spot was estimated at 25,500 miles (41,038 km) across in the late 1800s. A century later, NASA's Voyager 1 and 2 flybys of Jupiter in 1979 revealed the spot had shrunk to 14,500 miles (23,336 km) across.

"In our new observations it is apparent that very small eddies are feeding into the storm," Simon continued in his statement. "We hypothesized that these may be responsible for the sudden change by altering the internal dynamics and energy of the Great Red Spot."

This large storm is not the first to change or disappear in recent decades. In 1994, the Hubble telescope did not observe Neptune's "Great Dark Spot," which had been viewed in a Voyager 2 flyby in 1989.

Simon's team plans to study the motion of the small eddies and also the internal dynamics of the GRS to possibly determine what is causing Jupiter's GRS to shrink so drastically.