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Uranus is “a geometric nightmare”

Credits: Hubble/ NASA/ ESA/ L. Lamy/ Observatoire de Paris

The magnetosphere of Uranus, the seventh planet in our solar system, opens up and closes on a daily basis, according to a study published in The Journal of Geophysical Research. The researchers described it as “a geometric nightmare”.

When it is in “open” mode, solar winds circle around the planet. When it is in “closed” mode, the magnetosphere forms a protective barrier that deflects the wind up into space, if we are to believe the study led by Xin Cao and Carol Paty from the Georgia Institute of Technology in the United States. This is unusual from a terrestrial point of view, and appears to be repeated every day – in fact, around every 17 hours. The researchers relied particularly on data collected by NASA’s Voyager 2 probe, which crossed the icy giant in 1986.

On Earth, the magnetosphere aligns fairly tightly with the rotation axis, which means that it turns around like a spinning top, via magnetism. Turned towards the sun, the magnetosphere also swings fairly often between open and closed modes, but in an irregular manner, and always in response to strong solar winds. Visually speaking, the influx of particles from the solar wind generates intense auroras at the poles. It is therefore likely that such auroras are also produced on Uranus.

Uranus’s magnetosphere in fact has precise regularity in its changes between “open” and “closed” modes. “Uranus is a geometrical nightmare”, explains Carol Paty, coauthor of the study. “The magnetic field falls very quickly, like a child tumbling down a hill. When the magnetised solar wind meets this field falling in the right direction, it can reconnect and Uranus’s magnetosphere opens,  and then closes in order to open and close again on a daily basis.”

Uranus is fourteen times more massive that the earth, and is decidedly unusual. Contrary to other planets in our system, it turns almost at a right angle to its orbital plane around the sun. It therefore alternatively presents its North Pole, and then its South Pole to the sun, each of which periods lasts around 42 years. And an unusual planet means an unusual magnetic field. Contrary to the Earth, Uranus leans at a 59° angle to its rotation angle. On Uranus, this leaves everything highly disordered: “Depending on how much it moves, the magnetosphere changes in all sorts of directions”, explains Carol Paty.

The next challenge will be to send an orbiter to the area, over 3 billion kilometers away, aiming to collect new data, but also to compare existing data with established simulations. To this end, NASA are planning to send probes to Uranus and Neptune in 2030.

There are however a few obstacles in the way of this mission. The journey could in fact take fourteen years, and would need to use nuclear energy, as solar energy is largely ineffective at such a large distance from the sun. But the atomic batteries used by NASA to date, fuelled by plutonium-238, are insufficient, and international treaties have been preventing an increase in the amount of plutonium for several years.

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