Europe Asks If It Can Probe Uranus

This past September the giant planet Jupiter made its closest approach to Earth since 1951, briefly becoming the brightest object in the night sky, aside from the moon.

And not too far from that brilliant dot, sky-watchers with even modest binoculars could easily spot one of Jupiter’s distant relatives: the icy gas planet Uranus.


Uranus’ moon Ariel (white dot) crossed the face of Uranus in a Hubble Space Telescope picture.

—Image courtesy NASA/Space Telescope Science Institute

At that time, Jupiter and Uranus were dancing through the second of three closely timed conjunctions—when multiple planets seem to snuggle together in the night sky, as seen from Earth. (See a picture of a December 2008 conjunction of Jupiter, Venus, and the moon that made a smiley face in the sky.)

The first Jupiter-Uranus conjunction was on June 6, when the pair were huddled a mere half-degree apart (half a degree is about the same as the apparent width of the full moon).

Yesterday Jupiter and Uranus were in their third and final conjunction, the last until April 2024.

Without a bright companion such as Jupiter, Uranus can be fairly hard to spot. The blue-green planet is just about at the limit of what the naked eye can see even in very dark, rural areas.

That’s because Uranus is not only less than half the width of Jupiter, it’s 1,300,256,614 miles (2,092,560,180 kilometers) farther away from the sun.

Even the best telescopes on Earth or in Earth orbit have a tough time studying something so far away. And so far Voyager 2 is the only spacecraft that’s paid Uranus a visit, snapping a few pictures back in 1986 on its way to the outer reaches of the solar system.

(See “Hubble Reveals New Moons, Rings Around Uranus.”)

But the European Space Agency (ESA) wants to change all that, according to a recent article on the astronomy enthusiast website Skymania.

Their writers snagged an interview with Chris Arridge of University College London’s Mullard Space Science Laboratory, leader of the team of scientists who submitted a proposal to ESA in December for the Uranus Pathfinder mission.

According to the mission website: “All the major components of the solar system are being actively explored in situ by spacecraft apart from the ice giant planets Uranus and Neptune. Yet the ice giants are an important and essentially unknown part of the solar system, they have a unique place in planet formation, and are crucial in understanding exoplanetary systems.”


A false-color pictures of Uranus, its rings, and ten of its moons taken by Hubble in 1998.

—Image courtesy NASA/JPL/STScI

For the bargain price of 400 million U.K. pounds, the Uranus Pathfinder team would launch a nuclear-powered probe in 2021 designed to study the planet and its moons, Skymania reports. The mission, which would be a joint effort between ESA and NASA, already has support from at least 120 scientists in 11 countries.

“One of the big mysteries about Uranus is that it doesn’t emit much heat at all,” Arridge told Skymania. “It is thought that something the size of Mars or Earth hit Uranus early in the solar system and tilted it into its side, and that may have caused a massive loss of primordial heat.”

And that’s just one of the planetary curiosities begging to be explored. For example:

  • Uranus has the most powerful known winds in the solar system, blowing at more than 500 miles (805 kilometers) an hour;
  • Uranus is the only planet with a rotational axis that’s tilted almost parallel to its orbital plane—in other words, the planet appears to be rolling through space on its side;
  • Uranus has two sets of rings, and the outer set is brightly colored;
  • Uranus has 27 known moons, some of which may be captured asteroids.

Considering the vast distances, we’ve got a decent start on accumulating facts about the seventh planet. But according to Arridge, “we’ve only really scratched the surface of Uranus.”

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