A giant, years-long storm on Neptune is disappearing — and it’s being caught on camera for the first time

_{A picture of Neptune taken by the Voyager 2 spacecraft, showing a since-vanished dark storm.}

• A huge storm on Neptune is disappearing, and the Hubble Telescope is documenting it for the first time.
• The storm has led scientists to question their understanding of the prevailing dynamics of Neptune’s atmosphere.
• The storm appears to be moving in the opposite direction than scientists predicted.

A dark storm on Neptune is big enough to stretch from Boston to Portugal on Earth – but it’s fading away as the Hubble Telescope watches.

Neptune’s giant storms were first discovered by NASA’s Voyager 2 spacecraft in the late 1980s. Since then, the storms have “played a game of peek-a-boo” with NASA’s Hubble Telescope over the years, the agency said.

The latest storm was spotted in 2015 but is being photographed for the first time. In some ways, it’s similar to Jupiter’s famous Great Red Spot, which is also disappearing.

When Neptune’s storm was first detected, it was estimated to be 3,100 miles across. It’s now down to 2,300 miles. Here’s what it looks like in a series of recent Hubble images:

_{This series of Hubble Space Telescope images taken over 2 years tracks the demise of a giant dark vortex on the planet Neptune. The oval-shaped spot has shrunk from 3,100 miles across its long axis to 2,300 miles across, over the Hubble observation period}

How long a storm lasts varies greatly from planet to planet. Storms on Neptune typically last for a few years. Jupiter’s Great Red Spot has been around since perhaps the 1600s. Earth’s longest recorded storm, by comparison, lasted 31 days – that was Hurricane John in 1994.

Scientists think Neptune’s storm, which appears as a dark stain on the planet, may be composed of hydrogen sulfide, but they still don’t have a full understanding of it.

“We have no evidence of how these vortices are formed or how fast they rotate,” Agustín Sánchez-Lavega, a scientist at the University of the Basque Country in Spain who worked on the project, said.

The researchers initially thought that because of Neptune’s prevailing wind patterns, the storm would drift toward the planet’s equator and break up with a “spectacular outburst of cloud activity,” according to Michael Wong of the University of California at Berkeley.

But that’s not what Hubble observed.

The storm actually traveled in the opposite direction – towards the South Pole – and has slowly faded away, rather than gone out with a bang. According to NASA, Neptune’s storm swirls in an anti-cyclonic direction and dredges up material from deep inside the planet’s atmosphere. That gives researchers a unique opportunity to study the icy giant’s wind patterns.

These insights wouldn’t be possible without the Hubble Telescope training its powerful lens on Neptune.

“For now, only Hubble can provide the data we need to understand how common or rare these fascinating Neptunian weather systems may be,” Wong said.