Voyager found a mystery on Uranus. Decades later, NASA solved it.

NASA's Voyager mission beamed back unprecedented views. It also sent back some mysteries.

One of these came in 1986, when the Voyager 2 probe — one of a duo of Voyager craft sent into deep space — journeyed by the ice giant Uranus, a strange world rotating on its side. When the mission passed by, its instruments detected strong radiation around Uranus, yet, curiously, didn't find any source of energized particles to feed these zones of radiation.

For decades, the observation has been an enigma. But not anymore. Recent analysis of Voyager's old data found that extreme solar wind — a flow of particles shooting out from the sun — impacted the environs around Uranus and created the abnormal episode.

"The spacecraft saw Uranus in conditions that only occur about 4 percent of the time," Jamie Jasinski, a NASA physicist who led the new research published in Nature Astronomy, said in a statement.

The graphics below help demonstrate what happened. Like Earth, Uranus has a protective magnetosphere — the region or cavity around the planet home to its magnetic field (these magnetic fields are created by currents in the planets' metallic cores). Magnetospheres shield planets from solar storms and wind, but become compressed by this potent stream of solar particles.

When the solar wind hit Uranus' magnetosphere, it compressed the distant planet's magnetosphere, and squeezed out the plasma (hot gas composed of electrically charged particles) that naturally surrounds Uranus. Instead, the solar wind injected its own particles into radiation belts around Uranus. This explains why the Uranus environment was so irradiated — but didn't seem to have an obvious source of radiation.

On left: Typical conditions around Uranus, showing a rich area of plasma around the planet. On right: Particles from the solar wind compressing Uranus' magnetosphere and forcing out the system's plasma. Credit: NASA / JPL-Caltech

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These results also suggest that some of Uranus' five moons aren't dead, after all. The lack of plasma around the planet hinted that the moons weren't geologically active, because unlike other active moons of our solar system (like Jupiter's ocean moon Europa), it appeared Uranus' satellites emitted no charged water molecules. But that might not be the case.

There are no missions back to Uranus any time soon, though the planet, at 1.8 billion miles (2.9 billion kilometers) away, is considered a priority target for a future NASA mission.

Meanwhile, the Voyager probes continue their respective journeys through interstellar space, where they'll journey through the galaxy for billions of years.