A rare event during 1986 Uranus flyby may have skewed the way scientists understand the planet, study finds

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When the Voyager 2 spacecraft became the first and only mission to fly by Uranus in 1986, it defined the way astronomers understand the ice giant. But the data collected by the probe also introduced new mysteries that have continued to puzzle scientists in the decades since the historic flyby.

Now, a new look at the data has revealed that Voyager 2 happened to zoom by the distant planet during a rare event, which suggests that scientists’ current understanding of the planet may have been shaped — and skewed — by an unusual stellar coincidence.

The findings of the study, published Monday in the journal Nature Astronomy, may have solved some of the riddles created by Voyager 2’s odd Uranus readings.

“The spacecraft saw Uranus in conditions that only occur about 4% of the time,” said lead study author Jamie Jasinski, space plasma physicist at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement.

The study results could also reinforce the idea that Uranus remains a largely misunderstood world, given that astronomers’ basic knowledge of the planet stemmed from an extraordinary anomaly.

A planet of oddities

Voyager 2’s flyby of the sideways-rotating Uranus revealed previously unknown rings and moons around the planet.

But the spacecraft’s observations of Uranus’ magnetosphere were wildly different from astronomers’ expectations, and scientists deemed the planet an outlier among the other large planets in our solar system, such as Jupiter, Saturn and Neptune.

Magnetospheres are the protective bubbles around planets like Earth that have magnetic cores and magnetic fields, and they are driven by the planet’s magnetic field. These magnetic bubbles shield the planets against solar wind, a stream of energetic particles and gas that flows continuously from the sun.

Understanding how magnetospheres function around other planets not only aids scientists in planning exploratory missions, it also provides insight about how Earth’s magnetosphere operates.

Voyager 2’s data showed that Uranus’ magnetosphere was home to unexpectedly powerful electron radiation belts. Their intensity was similar to the massive bands of radiation found around Jupiter.

As the largest planet in our solar system, Jupiter has a magnetic field 20,000 times stronger than Earth’s, according to NASA. The magnetic field traps charged particles and accelerates them to high speeds. The rapidly moving particles release energy in the form of intense radiation that bombards Jupiter’s closest moons.

However, there was no apparent source for energetic particles to drive and boost the intensity of the belts seen around Uranus because there appeared to be a lack of plasma, or ionized gas, which was strange because plasma is a common element in magnetospheres around other planets.

Voyager 2’s observations of Uranus’ magnetosphere defied the way astronomers understand how magnetic fields trap energetic particles and their radiation.

The astronomers were puzzled by the lack of plasma because five of Uranus’ icy moons exist within the magnetosphere, and they should have been producing ions within the magnetic bubble surrounding Uranus and some of its moons. This odd discovery led the Voyager scientists to conclude that the moons must be entirely inactive.

But a new analysis of the Voyager 2 data showed that Uranus experienced a rare cosmic occurrence just before the flyby.

Solar interference

Days before the flyby, an intense solar wind event was released from the sun, stirring up space weather throughout the solar system. Solar wind hit Uranus and dramatically compressed its magnetosphere, likely pushing plasma out of it. But the solar wind also made Uranus’ magnetosphere more dynamic by feeding it with electrons, which boosted the planet’s radiation belts, according to the new study.

“If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus,” Jasinski said.

It’s likely that Uranus’ magnetosphere would have looked similar to the magnetic bubbles around the other giant planets in our solar system without any anomalies, the study authors said.

The findings also suggest that some of Uranus’ moons could be geologically active, as they were probably releasing ions into the magnetosphere before the solar wind temporarily whisked the particles away.

“We highlight that our understanding of the Uranus system is highly limited, and our analysis shows that any conclusions made from the Voyager 2 flyby are similarly tentative,” the authors wrote in their study. “We suggest that discoveries made by the Voyager 2 flyby should not be assigned any typicality regarding Uranus’s magnetosphere.”

The Uranus flyby “was packed with surprises” and researchers immediately began to search for a way to explain the unexpected data, said Linda Spilker, project scientist for the twin Voyager probes at JPL, who served as one of the mission scientists for Voyager 2 during that time. Spilker was not involved in the new study.

“The magnetosphere Voyager 2 measured was only a snapshot in time,” Spilker said in a statement. “This new work explains some of the apparent contradictions, and it will change our view of Uranus once again.”

If astronomers’ knowledge of Uranus is based on a flyby conducted under rare circumstances, it suggests that there may be good reason to revisit the ice giant. The James Webb Space Telescope has already helped reveal new information about Uranus, including highlighting its typically hidden rings, moons, weather and atmosphere.

Fortunately, sending a dedicated mission to study Uranus in the future has become a priority for NASA, according to a report released in 2022.

The planetary decadal survey recommended the first dedicated Uranus Orbiter and Probe as the next large NASA mission. After launching as soon as the early 2030s, the proposed spacecraft would conduct an orbital tour of the ice giant during flybys and deliver a probe to explore the atmosphere.

Meanwhile, the long-lived Voyager 2 has continued its journey and is currently almost 13 billion miles (21 billion kilometers) from Earth and exploring interstellar space, contributing its unique perspective to help astronomers understand the uncharted territory beyond our solar system.

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