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LOS ANGELES, Dec. 30-- Dawn spacecraft has entered an approach phase in which it will continue to close in on Ceres, a dwarf planet never visited before, NASA said on Monday.

On the far side of the sun, the spacecraft is closing in on Ceres, using its advanced ion propulsion system to match solar orbits with the dwarf planet.

The interplanetary explorer is currently 640,000 km from Ceres, approaching the asteroid at around 725 km per hour.

Dawn's arrival at Ceres orbit, which is scheduled on March 6, 2015, will mark the first time that a spacecraft has ever orbited two solar system targets.

The spacecraft, launched in 2007, previously explored the Vesta for more than a year, from 2011 to 2012, capturing detailed images and data about that celestial body.

"Orbiting both Vesta and Ceres would be truly impossible with conventional propulsion. Thanks to ion propulsion, we're about to make history as the first spaceship ever to orbit two unexplored alien worlds," Marc Rayman, Dawn's chief engineer and mission director, said in a statement.

Ceres, with an average diameter of 950 km, is the largest object in the main asteroid belt between Mars and Jupiter. By comparison, Vesta has an average diameter of 525 km, and is the second most massive body in the belt.

Ceres is of great interest to astronomers and scientists, who believe that the small dwarf planet may also be a large water reservoir in the inner solar system aside from the Earth. However, scientists are unsure how much of that water is actually liquid.

"Ceres is almost a complete mystery to us," said Christopher Russell, principal investigator for the Dawn mission, "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."