An unmanned probe with a futuristic propulsion system is closing in on a distant mini-world never before visited by a spacecraft.
The American space agency NASA's Dawn orbiter is about 643,600km from the Texas-sized "dwarf planet" Ceres and is expected to reach its destination in March.
Measuring 949.31km across, Ceres is the largest body in the asteroid belt, the rocky region between Mars and Jupiter.
Dawn's arrival on March 6 will mark the first time a spacecraft has orbited two Solar System targets. The probe previously explored another large asteroid belt object, the "protoplanet" Vesta, for 14 months from 2011 to 2012.
Christopher Russell, the mission's principal investigator from the University of California at Los Angeles, said: "Ceres is almost a complete mystery to us. Ceres, unlike Vesta, has no meteorites linked to it to help reveal its secrets. All we can predict with confidence is that we will be surprised."
The mission is breaking new ground because of the spacecraft's ion propulsion system. Instead of being propelled by a chemical rocket, the craft is driven by accelerated electrically charged particles of xenon gas which exert a small force over a long period of time, allowing it to build up speed.
Dawn has completed five years of accumulated thrust time, far exceeding that of any other spacecraft.
"Orbiting both Vesta and Ceres would be truly impossible with conventional propulsion," said chief engineer and mission director Marc Rayman, from NASA's Jet Propulsion Laboratory in Pasadena, California.
"Thanks to ion propulsion, we're about to make history as the first spaceship ever to orbit two unexplored alien worlds."
The probe recently emerged from solar conjunction, in which it was hidden from Earth on the opposite side of the Sun, limiting communication.
Since its launch in September 2007, it has crossed 2.9 billion miles of space.
Scientists hope studying Ceres will provide valuable clues about the early history of the solar system.
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