Dawn Arrives at Ceres, Makes History

Early this morning, at about 4:39 AM Pacific Time, NASA's Dawn spacecraft arrived at Ceres, making history as it swung into orbit around the dwarf planet. Dawn left Earth eight years ago, headed for the Asteroid Belt, located between Mars and Jupiter. The spacecraft spent a year photographing the asteroid Vesta, and then two-and-a-half years on the journey to its final port-of-call.

Over the last several months, scientists and the public have been growing steadily more excited as Dawn sent back photos of an ever-closer Ceres. For the average space enthusiast, Dawn's arrival feels like the discovery of a new world.

We've known Ceres existed since 1801, when it was discovered by Giuseppe Piazzi. But for scientists, this encounter means far more than seeing a mysterious object up close for the first time.

Indeed, the Dawn mission is not merely a geology field trip. It is the closest thing we have to a time machine. Dawn's exploration of Vesta and Ceres is like an archaeological dig or forensic investigation: the unearthing and reading of extant physical evidence to reconstruct what happened in our solar system's infancy, when the planets were being formed in an environment radically different from what we know today.

"Ceres has the potential to turn some of our old ideas about how planets formed completely upside down," says Dr. Britney Schmidt, Assistant Professor at Georgia Institute of Technology's School of Earth and Atmospheric Sciences. "If Ceres turns out to be icy in its interior, this would not only tell us that there were potentially lots of icy asteroids, but also that some of the 'classical' assumptions about the timing of planetary formation could be wrong."

Ceres is an example of a "protoplanet," an object that formed early in the solar system's history by accumulating smaller chunks of rock and ice and snowballing toward a planet-stature object—or at least a major building-block of another planet. But Ceres' development was arrested, and it has remained more or less unchanged from three or four billion years ago.

"Even though [Ceres] is likely refrozen now," Schmidt says, "with the gravity data from Dawn, we may be able to show that Ceres at one time had a subsurface ocean."

Dawn's leisurely approach over the past months has supplied us with a constant feed of images that have grown ever sharper and more detailed, peeling away layers of fuzzy mystery like the skin of an onion, and revealing new mysteries in the process. That's something astronomers are happy about, like getting an unexpected dividend on your investment. Mystery, after all, inspires science.

A week before Dawn's arrival, NASA whetted our appetites for the adventure by publishing a picture that revealed two small white spots nestled close together in a crater—and told us that the nature of the roughly Lake Tahoe-sized feature was as yet unknown.

What are these spots? Kids visiting Chabot Space & Science Center had some truly bright ideas, including giant pieces of reflective metal, huge chunks of ice, volcanoes, and, yes, aliens.

Dawn's previous subject of interest, Vesta, can also be classed as a protoplanet like Ceres, though Vesta was found to be composed mostly of rock. Ceres, on the other hand, may be as much as 25% water ice. In terms of the protoplanet accretion processes that formed the Earth, it is thought that dry Vesta-type objects may have built up Earth's rocky core and mantle, while icy "wet" protoplanets like Ceres may have contributed to the formation of our oceans. Certainly, Dawn's investigations in the Asteroid Belt have shown that the kitchen in which Earth was cooked up was stocked with both ingredients.