Nasa's Dawn Spacecraft Begins Approach to Dwarf Planet Ceres

Launched in 2007, Dawn is scheduled to enter Ceres orbit in March 2015.

Dawn recently emerged from solar conjunction, in which the spacecraft is on the opposite side of the Sun, limiting communication with antennas on Earth.

Now that Dawn can reliably communicate with Earth again, mission controllers have programmed the manoeuvres necessary for the next stage of the rendezvous, which they label the Ceres approach phase.

Dawn is currently 640,000 kilometres from Ceres, approaching it at around 725 kilometres per hour.

The spacecraft's arrival at Ceres will mark the first time that a spacecraft has ever orbited two solar system targets.

Dawn previously explored the protoplanet Vesta for 14 months, from 2011 to 2012, capturing detailed images and data about that body.

"Ceres is almost a complete mystery to us," said Christopher Russell, principal investigator for the Dawn mission, based at the University of California, Los Angeles.

"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," Russell said.

The two planetary bodies are thought to be different in a few important ways. Ceres may have formed later than Vesta, and with a cooler interior.

Current evidence suggests that Vesta only retained a small amount of water because it formed earlier, when radioactive material was more abundant, which would have produced more heat.

Ceres, in contrast, has a thick ice mantle and may even have an ocean beneath its icy crust.

Ceres, with an average diameter of 950 kilometres, is also the largest body in the asteroid belt.

The spacecraft uses ion propulsion to traverse space far more efficiently than if it used chemical propulsion.

In an ion propulsion engine, an electrical charge is applied to xenon gas, and charged metal grids accelerate the xenon particles out of the thruster.

These particles push back on the thruster as they exit, creating a reaction force that propels the spacecraft.

Dawn has now completed five years of accumulated thrust time, far more than any other spacecraft.

The next couple of months promise continually improving views of Ceres, prior to Dawn's arrival. By the end of January, the spacecraft's images and other data will be the best ever taken of the dwarf planet.