Uranus has always been left out when it comes to missions to our near neighbours. But now there are serious attempts to visit this toxic gas giant, writes Richard Hollingham.

The butt (snigger) of countless jokes, Uranus is almost certainly the most unloved planet in our solar system. It always seems to get overlooked when the mission invitations go out.

Spacecraft have been sent to Mercury, Mars, Venus, Saturn and Jupiter. There is even one on its way to non-planet Pluto. Uranus has only ever qualified for the planetary equivalent of a presidential brush-by, when Voyager 2 sped past on its way to the edge of the solar system in 1986.


But Uranus (pronounced “yur-an-us” in polite astronomical circles) does not deserve its dull, or comic, reputation. In fact it is one of the most interesting, exciting and downright weird planets we know of.

“Uranus really stands out,” says University of Oxford planetary scientist, Leigh Fletcher. “It’s the oddball amongst the collection of planetary types we have.”

With a volume 60 times that of Earth, Uranus is a compressed mass of toxic gases, including methane, ammonia and hydrogen sulphide, surrounding a small rocky core.

“We don’t have a solid surface on any of these giant planets,” says Fletcher. “There’re no sharp boundaries, nothing to stand or sail on, but there’s a continuous progression from gas to liquid to some sort of solid.”

‘Interesting’ atmosphere

Circled by 26 small moons, a few faint rings and a weak magnetic field Uranus appears to be tipped over on its side. Every planet has a slight tilt when it spins – it gives us our seasons – but unlike every other planet in the solar system, Uranus rotates on an axis pointing almost directly at the Sun. Something that Fletcher describes as “really weird”.

“Imagine a world where winter lasts 42 Earth years and you don’t see the Sun once during that time,” he says. “You have this situation where the atmosphere isn’t heated for decades and that can lead to some really interesting atmospheric properties.”

Fletcher is part of an international team that believes Uranus has been neglected for too long. This group of space scientists and engineers from Europe, the United States and several other nations, including Japan, is working on a $600m mission proposal for the European Space Agency (ESA) with the aim of sending out a space probe, within the next 10 years, to discover why Uranus is so odd. The mission will investigate the atmosphere, magnetic field and capture detailed images of this strange world.

By comparing the ancient soup of preserved gases in Uranus’ atmosphere with Earth or Jupiter, they also hope to get a better understanding of what conditions were like when the Solar System was being formed.

“Think of Uranus as the missing link,” says Fletcher. “A mission that was able to probe the internal structure of the planet, to sniff out the atmospheric composition and understand how the atmosphere evolves would allow us to put together the puzzle about how planets form.”

“I’d argue,” he adds, “that if we can’t understand how planets in our own solar system formed, it’s going to be rather difficult to do the same for planets around other stars.”

Epic journey

However, there is a good reason why, in the entire history of space exploration, only one mission has visited Uranus: it is extremely difficult.

For a start the planet is almost three billion km (1.8 billion miles) away from the Sun – 20 times further than the Earth. As a result, any spacecraft will take up to 15 years to get there.

Because sunlight is so weak at that distance, rather than solar panels, the mission will need to employ a nuclear power source, which is trickier to build and operate.

There is also the issue of how you communicate and get the data back from a spacecraft that is so far away. Do you fit a giant dish on the side or build a huge receiver on Earth? Or both?

Another major hurdle is the challenge of keeping the mission, engineering and operations teams together in the decade or so between launch and arrival at the planet.

All that is even before you start to consider what instruments to put onboard.

‘Global momentum’

Despite a Uranus mission being considered a priority by the space agencies, previous ESA and Nasa proposals have sunk without trace, including a plan by a European team in 2010 known as Uranus Pathfinder. So what makes them think this latest one is any different?

“In 2010 we hadn’t worked out all the details,” admits Chris Arridge from University College London, one of the leaders of the Uranus team, speaking to me from a mission planning meeting in Washington DC.

“This time we have a very well-developed understanding of the science we want to do and the instruments we want to take with us.”

The scientists have until January 2015 to submit a detailed mission proposal to ESA. “There’s a tremendous amount of work involved – we need to work out everything from what sort of rocket we launch to what orbit we go into and what instruments we take with us,” says Arridge. “However, there’s a growing global momentum and a real sense of excitement.”

Even if the mission is accepted it will not be launched until at least 2020 and it would only reach Uranus after more than a decade’s travel, in the mid-2030s. However, for Fletcher, it will still be a dream come true. “I’m sat in my office now as a 30-something researcher,” he says. “I’ll hopefully still be sat in my office as a 60-something researcher when the spacecraft gets there.”

“Planetary exploration hasn’t ended,” Fletcher adds. ”There are still exciting ideas like this on the table.”

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