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 640,000 kilometres from the Texas-sized ‘dwarf planet’ Ceres and is expected to reach its destination in March.

Measuring 950km 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.

Dawn’s arrival on March 6 will mark the first time a spacecraft has orbited two solar system targets

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.

“Thanks to ion propulsion, we’re about to make history as the first spaceship ever to orbit two unexplored alien worlds,” added Rayman from Nasa’s jet propulsion laboratory in Pasadena, California. 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 4.7 billion kilometres of space. Scientists hope studying Ceres will provide valuable clues about the early history of the solar system. Ceres is the smallest body to be classified as a dwarf planet and the only one in the inner solar system.

The object may have formed later than Vesta, and have a cooler interior. Evidence suggests Vesta only retained a small amount of water because it formed at a time when radioactive material was more abundant, generating more heat.

Ceres, in contrast, has a thick mantle of ice and may even possess an ocean under its surface.