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Arthur T Knackerbracket has found the following story:

Eight and a half years into its grand tour of the solar system, NASA's Voyager 2 spacecraft was ready for another encounter. It was Jan. 24, 1986, and soon it would meet the mysterious seventh planet, icy-cold Uranus.

Over the next few hours, Voyager 2 flew within 50,600 miles (81,433 kilometers) of Uranus' cloud tops, collecting data that revealed two new rings, 11 new moons and temperatures below minus 353 degrees Fahrenheit (minus 214 degrees Celsius). The dataset is still the only up-close measurements we have ever made of the planet.

Three decades later, scientists reinspecting that data found one more secret.

Unbeknownst to the entire space physics community, 34 years ago Voyager 2 flew through a plasmoid, a giant magnetic bubble that may have been whisking Uranus's atmosphere out to space. The finding, reported in Geophysical Research Letters, raises new questions about the planet's one-of-a-kind magnetic environment.

[...] Readings from inside the plasmoid — as Voyager 2 flew through it — hinted at its origins. Whereas some plasmoids have a twisted internal magnetic field, DiBraccio and Gershman observed smooth, closed magnetic loops. Such loop-like plasmoids are typically formed as a spinning planet flings bits of its atmosphere to space. "Centrifugal forces take over, and the plasmoid pinches off," Gershman said. According to their estimates, plasmoids like that one could account for between 15 and 55% of atmospheric mass loss at Uranus, a greater proportion than either Jupiter or Saturn. It may well be the dominant way Uranus sheds its atmosphere to space.

How has plasmoid escape changed Uranus over time? With only one set of observations, it's hard to say.

"Imagine if one spacecraft just flew through this room and tried to characterize the entire Earth," DiBraccio said. "Obviously it's not going to show you anything about what the Sahara or Antarctica is like."

But the findings help focus new questions about the planet. The remaining mystery is part of the draw. "It's why I love planetary science," DiBraccio said. "You're always going somewhere you don't really know."

More information: Gina A. DiBraccio et al. Voyager 2 constraints on plasmoid‐based transport at Uranus, Geophysical Research Letters (2019). DOI: 10.1029/2019GL083909

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