Twenty-five years ago, NASA's Voyager 2 zipped past the planet Uranus on its way to Neptune. It was the first spacecraft ever to grab a close-up look at this bizarre world. Voyager 2 made its closest approach on Jan. 24, 1986, and since then we've only been able to gaze on the "ice giant" from afar.
However, Uranus might not be alone for too much longer if a group of 168 scientists from Europe and the U.S. have their way.
In a proposal submitted to the European Space Agency (ESA), a mission called Uranus Pathfinder has been short-listed to make the trek to the outer solar system, arrive in Uranian orbit and study the planet's unique chemistry, rings and its moons and investigate some of the planet's most enduring mysteries. This, in turn, will aid our knowledge of solar system history and how other star systems may form.
Chris Arridge, postdoctoral research fellow of University College London's Mullard Space Science Laboratory (MSSL) and project lead, told Discovery News that the motivation behind Uranus Pathfinder mission is to investigate a giant outer solar system planet that we know little about.
"To have a complete understanding of our solar system we must study all of its components," said Arridge. "It's like having a huge jigsaw and only having half the pieces — we need to get all those pieces to have a chance of being able to see the big picture."
Oddball Uranus
Uranus and Neptune are very different from their other gas giant cousins. Their thick clouds of atmospheric gases contain water, methane and ammonia, plus trace amounts of hydrocarbons. It is for this reason that Uranus and Neptune are often dubbed "ice giants" as they contain significant quantities of ices in their atmospheres. (In contrast, Jupiter and Saturn are composed mainly of hydrogen and helium.)
Whereas the Jovian and Saturnian systems have been studied in-depth by the Galileo and Cassini Equinox missions, the outer ice giants remain a mystery. And things don't get much more mysterious (and down-right bizarre) than oddball Uranus.
One of the most striking things about Uranus is the fact that it orbits the sun virtually on its side. The planet literally "rolls" around the solar system. During its 84-year orbit, each pole spends 42 years facing the sun and then 42 years in perpetual winter. Uranus' weather is driven by these extreme seasons.
"This strange polar orientation suggested that Uranus had been struck by a massive collision early in the history of the solar system," John Cooper, of NASA's Goddard Space Flight Center and Uranus Pathfinder scientist, told Discovery News. Cooper was part of the Voyager 2 team during the 1986 Uranus flyby.
But it doesn't stop there, not only does Uranus' axis of rotation have an extreme tilt, its magnetic poles are out of kilter too.
"Without a doubt the biggest surprise to me of the Voyager 2 encounter was the discovery that the magnetic poles of Uranus were tilted at 60 degrees to the rotational poles," said Cooper. "On Earth, this would be like having the magnetic north pole in Houston, Texas, instead of in the Canadian Arctic."
Christopher Russell, head of UCLA's Space Physics Center in the Institute of Geophysics and Planetary Physics, agrees that one of the biggest surprises to come from the Voyager 2 flyby was the nature of the planet's magnetic field.
"Uranus' magnetic field seems in some senses more like the solar (magnetic) field — generated in an outer layer and not deep in the interior," Russell said. "Uranian studies may lead to greater understanding of the solar magnetic field."
Moons and rings
Perhaps the magnetic field similarities between Uranus and the sun are a result of a set of common processes working in both solar system bodies, Russell speculates. The only way this can be tested is if a mission, like Uranus Pathfinder, is sent to the planet to study it up close.
In addition to Uranus' weird tilt, there's the question as to why the planet generates little heat. The gas giants are known to emit some heat from processes deep within their atmospheres, but Uranus, once again, is different.
Uranus Pathfinder could find answers to these oddities deep inside planet, but there's a huge wealth of science to be gleaned from the planet's 27 moons, 10 of which were discovered by Voyager 2. For example, do the Uranian satellites interact with the planet in a similar fashion to the moons of Saturn? Is there a radiation belt trapped inside the Uranian magnetosphere, like the Van Allen Belts surrounding Earth?
"I have published a theory that radiation belt particles at Saturn may provide chemical energy to drive the ice volcanoes of the Saturn moon Enceladus, and I wonder if there might also be such volcanoes from similar moon irradiation processes at Uranus," Cooper said.
Uranian logistics
If Uranus could reveal so much, providing us with a huge piece of the solar system puzzle, why haven't we already sent a probe?
Mark Hofstadter, planetary scientist at NASA's Jet Propulsion Laboratory and U.S.-lead investigator for Uranus Pathfinder, points out that before now the logistics of such a mission have been seen as too expensive when considering the science that can be gathered.
"Being farther away makes it more difficult (read more expensive) to get there than to, say, Jupiter or Saturn," Hofstadter said. It's for this reason that missions to the inner gas giants have been preferred. But, in light of technological advancements, the cost of sending a robotic mission to Uranus now is more manageable.
"This is due to past missions answering some questions at Jupiter and Saturn, but just as importantly, recent research — both theoretical and observational — has made us appreciate that the Ice Giants are very important if we are to understand the formation and evolution of planets both in our solar system as well as around other stars," he said.
A Solar System Gold Mine
As Uranus is 1.8 billion miles from the sun — or 19 times the distance from the Earth to the sun — solar panels would be useless to power the Uranus Pathfinder spacecraft, so like the Cassini Equinox mission currently orbiting Saturn, it would need a nuclear power source.
Also, like Voyager 2 that came before it, the mission would most likely use a series of gravitational assists (or "sling shots") by other planets in the solar system to propel Uranus Pathfinder to the outer solar system. Depending on the size of the spacecraft, the mission could take anywhere between 8 to 15 years to reach its destination, says Hofstadter. The team hopes to see Uranus Pathfinder launch in 2021.
"The only way to see how the solar system works in different places is to go there, or for a planet this far away, send an unmanned probe," Arridge concludes. "Uranus sits in quite a different position in the solar system, it's far from the sun, it doesn't appear to give off much heat, it orbits the sun on its side, it appears to have a very different magnetic field and its ring system is unique."
"Uranus is a gold mine to help us understand the planets."