Voyager 2’s Flyby of Uranus in 1986 was During Anomalous Solar Event, New Study Suggests

When NASA’s Voyager 2 spacecraft flew by Uranus in 1986, it provided scientists’ first close glimpse of this ice-giant planet. Alongside the discovery of new moons and rings, baffling new mysteries confronted the scientists: the energized particles around Uranus defied their understanding of how magnetic fields work to trap particle radiation. According to a new study, the source of that particular mystery is a cosmic coincidence: it turns out that in the days just before Voyager 2’s flyby, Uranus had been affected by an unusual kind of space weather that squashed the planet’s magnetic field, dramatically compressing its magnetosphere.

Planetary magnetospheres — the region around a planet dominated by its magnetic field — affect the environment around a planet, and understanding their properties is crucial for mission planning.

Voyager 2’s flyby of Uranus revealed a unique magnetosphere that was strongly asymmetric, seemed to lack plasma — a common element of other planets’ magnetospheres — and had unusually intense belts of highly energetic electrons.

The characteristics from this single measurement have since been used as the basis of understanding Uranus’ magnetic field, but these anomalies have been difficult to explain without complex physics.

“If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus,” said Dr. Jamie Jasinski, a researcher at NASA’s Jet Propulsion Laboratory.

“The spacecraft saw Uranus in conditions that only occur about 4% of the time.”

Dr. Jasinski and colleagues reanalyzed the Voyager 2 data before the flyby and found that the probe met Uranus just after an intense solar wind event, during which a stream of charged particles was released from the Sun’s atmosphere.

This compressed the Uranian magnetosphere, leaving it in a state that only occurs 4% of the time.

This state would see a magnetosphere empty of plasma with highly excited electron radiation belts.

The authors suggest that owing to the variation of the solar wind at Uranus, there may be two magnetospheric cycles during the solar minimum.

Additionally, there may be a very low chance that Titania and Oberon — the outermost major Uranian moons — orbit outside the magnetosphere, which may allow scientists to detect subsurface oceans without interference from the magnetosphere.

“The 1986 flyby was packed with surprises, and we were searching for an explanation of its unusual behavior,” said Dr. Linda Spilker, also from NASA’s Jet Propulsion Laboratory.

“The magnetosphere Voyager 2 measured was only a snapshot in time.”

“This new work explains some of the apparent contradictions, and it will change our view of Uranus once again.”

The findings were published today in the journal Nature Astronomy.

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J.M. Jasinski et al. The anomalous state of Uranus’s magnetosphere during the Voyager 2 flyby. Nat Astron, published online November 11, 2024; doi: 10.1038/s41550-024-02389-3