Sen— While spacecraft continue to study the Moon, Mercury, Venus, Mars, Jupiter and Saturn, some space scientists are clamouring for missions to the neglected planets of the Solar System.
A NASA probe, New Horizons, is currently racing towards an ex-planet, Pluto, to speed past and tell us more about that world and its Kuiper Belt relatives. But ice giants Uranus and Neptune have not been properly studied from space since the Voyager missions of the 1980s.
There are no plans currently to return to the most distant planet Neptune. However, NASA has made a Uranus mission the third highest priority for planetary exploration in the 2020s, after another martian rover and further study of Jupiter.
The US financial crisis has put many future missions on hold due to budgetary restraints. But planetary scientists are pressing for such a mission to fly as soon as possible, especially after a similar proposal for a European mision, Uranus Pathfinder, was passed over in 2011.
The planet, discovered by William Herschel from Bath, England, in 1781, puzzles scientists because it is effectively lying on its side as it makes its 84-year journey around the Sun. Voyager 2 sped past in January 1986 to catch brief glimpses of its cloud tops and tenuous rings, but most of what we know has been learned using ground-based telescopes.
A leading expert on Uranus, which has an equatorial diameter of 50,700 km (31,500 miles), is Dr Chris Arridge of the Mullard Space Science Laboratory in Surrey, England. He told Sen why he considers it vital to send a mission to go into orbit around Uranus.
He said: "It is 27 years since the Voyager paid its fleeting visit. We know the planet's rings have changed since then and the atmosphere was different too when it reached equinox in 2007. We really need close-up studies to see what has changed.
An image of Uranus, its rings and moons from the Hubble space telescope in 1997. Credit NASA/ESA
"Uranus is quite unlike other planets, even though it and Neptune tend to get grouped with Jupiter and Saturn. But those gas giants are dominated by hydrogen and helium with small rocky cores. Uranus and Neptune contain a lot more rock and ice and have larger cores. There is also a lot more water in their atmospheres and a lot more methane.
"One of the big mysteries is why Uranus is so cold. It doesn't emit much heat at all, unlike all the other planets which release energy as they contract under their own weight."
Chris added: "It is thought that something the size of Mars or Earth may have hit Uranus early in the Solar System, tilting it onto its side. This means that for 42 years between equinoxes, one pole is in continual sunlight and the other is in constant darkness. This may cause seasonal effects on the planet's weather.
"Also, the ring system is quite different to those around Jupiter, Saturn and Neptune. They're very dense narrow arcs, wheras at Saturn you have a broad disk and at Jupiter you've got quite diffuse tenuous rings. We need to find out why Uranus's rings are so different and what role its moons play."
Scientists have further been puzzled by the nature of auroral displays observed on Uranus. Chris said: "Uranus's magnetic field is so strange and so different to the other planets. It is really going to challenge our ideas about these phenomena."