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25 Years After Voyager 2, Here’s What We Know About Uranus

Has it been a while since your last physical exam? Consider this: It’s been 25 years since anyone took a close look at Uranus.


Uranus, as seen by Voyager 2.

—Picture courtesy NASA/JPL

The Voyager 2 spacecraft, which launched in 1977, made its closest approach to Uranus on January 24, 1986, coming within 50,600 miles (81,500 kilometers) of the ice giant’s cloud tops.

Until that point, everything we knew about the seventh planet had been gathered by Earthly or Earth-orbiting telescopes. Since Uranus is a whopping two billion miles (three billion kilometers) away from the sun, such efforts gave us lots of information but couldn’t fill in some key details.

For example, we knew that Uranus had rings and moons, we just didn’t know how many, what they were made of, or whether they had any surface features.

We knew Uranus’s average temperature, but we had no idea how much the true temperature varied over time.

By the time Voyager 2 left Uranus in August 1989, we had counted 15 moons and 11 rings, and we had a better idea of what those features looked like, what was going on in the Uranian atmosphere, what was driving the planet’s magnetic field, and more.

Since then, no human-made craft has visited the distant planet, and no space agency currently has plans to do so, although European scientists are submitting a proposal for a Uranian mission.

The Hubble Space Telescope has helped a lot, and has even allowed astronomers to spot new moons and rings that Voyager 2 missed.


A false-color view of Uranus from Hubble.

—Picture courtesy NASA/JPL/STScI

It makes one wonder what a dedicated orbiter might reveal about the Uranian system, considering the success we’ve had with Venus, Mars, Saturn, Jupiter, and Mercury.

In the meanwhile, here are a few key facts about Uranus:

  • Uranus was discovered by William Herschel in 1781.
  • Herschel tried to name the planet Georgian Sidus, after Britain’s King George III.
  • The planet was officially named for Uranus, Greek god of the sky.
  • Uranus is 31,764 miles (51,119 kilometers) wide.
  • The planet has 27 known moons, all named after characters from Shakespeare or Alexander Pope.
  • Titania is the largest moon of Uranus, at 1,577.8 miles (2,539.2 kilometers) wide.
  • Uranus has 13 rings divided into two sets: 9 inner rings and 2 outer.
  • A year on Uranus lasts 84 Earth years.
  • A day on Uranus lasts 17.2 Earth hours.
  • The planet orbits on its side; in other words, the rotational axis is almost parallel to the orbital plane.
  • Due to this odd alignment, the planet spends a quarter of its year with one pole aimed directly at the sun and the other releasing heat into the blackness of space.
  • Uranus mostly has hydrogen and helium in its atmosphere, but it looks blue due to significant amounts of methane.