Astronomers have spotted a mysterious, mysterious storm on Neptune as it suddenly reverses its direction using NASA’s Hubble Space Telescope, something they say has never been seen before.

The storm, which is wider than the Atlantic Ocean, has been raging in the northern hemisphere of the gas giant since at least 2018, and has been slowly drifting toward Earth’s equator where such storms are expected to dissipate.

But instead of gently walking to death, and to the surprise of observers, the vortex suddenly changed direction by August 2020 and doubled back to the north.

This is the first time that Hubble, the only telescope with the sharpness and sensitivity of the visible light spectrum to track these elusive features, has seen this kind of atmospheric behavior.

Adding to the mystery, Hubble discovered another, smaller dark spot in January of this year that appeared near the larger storm. Scientists suspect it may have been a piece of the giant vortex that split off and drifted away before disappearing.

“We are excited by these observations because this little dark patch is likely part of the dark spot inactivation process,” said Dr. Michael Wong of the University of California, Berkeley.

Dr. Wong added, “This is a process that has never been noticed. We have seen some other dark spots fade, and are gone, but we have never seen anything disruptive, although it was expected in a computer simulation.”

The large storm is 4,600 miles (7,400 km) wide – more than the distance from London to Miami – and is only the fourth dark spot that Hubble has observed on Neptune since 1993.

The Voyager 2 spacecraft detected two more storms in 1989, but they had vanished by the time Hubble orbited to observe them.

And NASA said: “It is still a mystery how these storms form, but this recent giant vortex is the best studied so far.”

“The dark appearance of a storm may be caused by the height of the layer of dark clouds, and astronomers may tell of the vertical structure of the storm.

NASA added: “Another unusual feature of the dark spot is the absence of shiny accompanying clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018.”

“Apparently, the clouds disappeared when the vortex halted its southward journey. Shiny clouds form when the airflow is disrupted and turns upward over the vortex, causing the gases in the methane ice crystals to solidify.

The researchers added that “the lack of clouds may reveal information about how the spots evolved.”