Probe reaches mystery dwarf planet
A US spacecraft has started orbiting a mysterious "mini-world" in the asteroid belt after a journey through space lasting 7.5 years.
The Dawn probe, powered by futuristic ion propulsion, was captured by the gravitational field of the dwarf planet Ceres at 12.39, UK time.
Dawn was 38,000 miles (61,000 kilometres) from the 590 mile (950km) wide object when the orbital insertion occurred.
Over the coming weeks the craft will spiral slowly down towards Ceres until it is just a few hundred kilometres above the planet's surface.
Chris Russell, principal investigator of the Dawn mission at the University of California at Los Angeles, said: "We feel exhilarated.
"We have much to do over the next year and a half, but we are now on station with ample reserves and a robust plan to obtain our science objectives."
Mission controllers at the American space agency Nasa's Jet Propulsion Laboratory in Pasadena, California, received a signal from the spacecraft at 1.36 pm, UK time, confirming that all was well.
Even before reaching Ceres, Dawn had already uncovered a mystery. Images from the spacecraft's cameras revealed two bright white spots sitting inside a wide crater.
They could be the plumes of ice volcanoes, or sites where impacts have dug out surface deposits exposing the planet's icy interior, scientists believe.
Future images from Dawn are expected to show the features in more detail and possibly reveal their origin.
Experts think Ceres could be covered in an icy crust, perhaps encasing a subterranean ocean.
Ceres is the largest object in the asteroid belt which circles the Sun between the orbits of Mars and Jupiter.
It was first spotted by Sicilian astronomer Father Giuseppe Piazzi in 1801. As more objects were discovered in the same region they became known as asteroids, or minor planets.
Initially classified as a planet, Ceres was later categorised as an asteroid. In 2006 it was re-classified as a "dwarf planet" along with Pluto and Eris, which lies near the edge of the solar system.
Dawn was launched in September 2007, making its first port of call the giant asteroid Vesta, which it explored for 14 months, capturing detailed images and data.
It is the first spacecraft ever to orbit two mission targets and has so far travelled 3.1 billion miles (4.9 billion kilometres). The two-stop tour was made possible by Dawn's three ion engines, which are far more efficient than conventional chemical rockets.
The engines work by using charged metal grids to accelerate ions - electrically charged particles - of xenon gas.
At maximum thrust each engine produces about the same amount of force as that exerted by someone holding a piece of paper.
But they use a fraction of the fuel consumed by chemical rockets and can be kept firing for long periods of time, building up high speeds in frictionless space.
Dawn began its final approach to Ceres in December. Since January 25 the spacecraft has been delivering the highest resolution images of the dwarf planet ever captured.
Jim Green, director of Nasa's Planetary Science Division at the agency's headquarters in Washington D.C., said: "Studying Ceres allows us to do historical research in space, opening a window into the earliest chapter in the history of our solar system.
"Data returned from Dawn could contribute significant breakthroughs in our understanding of how the solar system formed."
Ceres and Vesta are the two most massive bodies in the asteroid belt and distinctly different, despite both being roughly spherical in shape.
Vesta is the smaller of the two with an average diameter of 326 miles (525 km) and is very dry, having formed earlier than Ceres.
In contrast, a quarter of Ceres is believed to consist of water.