First rendezvous with the largest object in the asteroid belt separating Mars from Jupiter will reveal what Ceres is made of

Nasa scientists are making final preparations for a spacecraft to begin the first orbits around a dwarf planet in the planetary rubble on the far side of Mars.

Almost eight years after blasting off from Cape Canaveral in Florida, and travelling 3bn miles (4.8bn km), the $450m (£290m) Dawn probe is due to arrive at Ceres, the largest object in the main asteroid belt that separates Mars from Jupiter, on Friday 6 March.

A technical glitch that knocked out thrusters on the ship in September means that Dawn has already overshot Ceres and must now swing back towards the 590-mile-wide ball of minerals and ice to enter the right orbit. On Sunday, it was 30,000 miles away from Ceres.

Nasa probe spots mysterious shiny patches on dwarf planet Ceres Read more

If all goes to plan, the spacecraft will circle the dwarf planet for months, mapping its surface and measuring changes in its gravitational field, from which scientists can glean crucial insights about the body’s interior.



Early observations of Ceres already have astronomers enthralled. The dwarf planet is thought to have a rocky core encased in more than 62 miles of subterranean ice. If Dawn confirms this, it will mean the body holds more fresh water than the whole of the Earth’s surface.

On its approach to Ceres, Dawn’s camera revealed a bright patch on the otherwise dark and dull, carbon-rich terrain. But in higher resolution pictures released last week, the patch was found to be two spots: one very bright, the other dimmer.

What the spots are is unclear. They may be ice patches, shiny silicates on the surface, or reflective impact material left after a hit-and-run collision by another space rock. Ceres is not thought to be geologically active, but if its innards are still stirring, the bright spots may be eruptions from ice volcanoes.

The Dawn probe was flung into space in September 2007 to learn about the formation of the planets at the birth of the solar system 4.6bn years ago. The boulders drifting in the asteroid belt are the primordial material from which all planets are made. Some, like Ceres, became large by asteroid standards, but ultimately they are failed planets. Their growth was disrupted by the intense gravitational pull of Jupiter, which scattered the asteroids like billiard balls.

Facebook Twitter Pinterest Dwarf planet Ceres and the giant asteroid Vesta in comparison to Mars, Mercury, and Earth’s moon. Photograph: Nasa/JPL-Caltech/UCLA

When Dawn arrives in orbit around Ceres, at a distance of more than 249m miles from the sun, it will become the first spacecraft to have circled two different bodies in deep space. Four years into its journey, in 2011, Dawn caught up with Vesta, the brightest asteroid in the solar system and the only one visible to the naked eye.

Dawn’s 14-month survey of Vesta, a rock roughly half the size of Ceres, revealed it to be similar to the inner planets of the solar system: Mercury, Venus, Earth and Mars. Like Earth, Vesta has an iron core, surrounded by a mantle and crust. The surface is scarred with giant canyons and craters punch deep into its surface. One pit is 311 miles across and has at its centre a mountain that rises up more than twice as high as Mount Everest.



Ceres is a different beast. The first asteroid to be discovered, it was spotted by accident on new year’s day in 1801 by the Italian monk Giuseppe Piazzi of the Palermo observatory. Unlike Vesta, the makeup of Ceres is far more like the icy bodies in the outer solar system. About one-quarter of its mass is water, and some may be liquid under the surface.

The International Astronomical Union designated Ceres a “dwarf planet” in 2006, along with Pluto, which found itself demoted from full planetary status. Dwarf planets must orbit the sun and be massive enough to be shaped by their gravity, but, unlike proper planets, have not cleared a path through the solar system.

“We think that the building blocks of Earth were the siblings of Ceres and Vesta,” said Christopher Russell, the lead scientist on the Dawn mission at the University of California, Los Angeles. “Those like Vesta came to Earth and delivered the iron core, and other materials, while others, like Ceres, brought water.”

Last year, Michael Küppers, a planetary scientist at the European Space Astronomy Centre in Villanueva de la Cañada, Spain, reported observations for the Herschel space telescope, which found jets of water vapour coming from Ceres. Every second, the dwarf planet loses 6kg in steam.

Küppers hopes that Dawn will shed light on the source of the steam plumes. One possibility is that part of the crust was knocked off Ceres in a cosmic impact, exposing hard ice beneath, which vaporises in sunlight. But the interior of Ceres may still be active, and driving out gas from inside.

Andreas Nathues, the lead scientist on Dawn’s framing camera at the Max Planck Institute for Solar System Research in Göttingen, Germany, said the mission targeted Vesta and Ceres because they were so unlike each other. “Why is Ceres a body that contains water, and Vesta not? Why are they so different? Understanding that will help us understand how the planets formed,” he said.

Facebook Twitter Pinterest Two views of Ceres are seen in images acquired by Nasa’s Dawn spacecraft from a distance of about 52,000 miles on 12 February. Photograph: NASA/Reuters

The Dawn probe is powered by an ion thruster that forces xenon plasma out of the spacecraft at 77,670mph and can run continuously for years, accelerating the craft over time.

In September, the thruster failed when a highly energetic particle slammed into its electronics, forcing the probe into safe mode for four days. The loss of power means that, while Dawn will arrive at Ceres on time, it will take weeks to correct its orientation and point its cameras at the surface.

“We’ll be pointing in the wrong direction when we get there,” said Russell. “We have a period coming up where we won’t be taking much data, but we will get the next set of good pictures at the end of April.”

Some of those images could resolve the mystery of the bright spots on the surface of Ceres. Russell suspects that as the images from Dawn get sharper, the spots could become smaller and shinier, until they reflect nearly all the sunlight falling on them. “In a few months, the reflectivity may get to 100%, and that could mean water ice,” he said.

When the mission is over later this year, the Dawn probe will remain in orbit around Ceres at a safe enough distance to ensure it does not crash into the surface. Since Ceres has no atmosphere, there is no drag to bring Dawn spiralling down to the dwarf planet’s surface. “We are about to arrive. And when we do, we are at Ceres to stay,” said Russell.

