The dwarf planet Ceres in crescent phase as seen by NASA's Dawn spacecraft

NASA's Dawn spacecraft captured this image of a crescent Ceres from 48,000 kilometers away as it prepared to enter orbit around the dwarf planet. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Shortly after 7:30 am Eastern time this morning, a seven-year space voyage at last reached its final destination: NASA’s Dawn mission entered orbit around Ceres, a small, icy world orbiting the sun between Mars and Jupiter. Confirmation of Dawn’s arrival came about an hour later, via the spacecraft’s radio signal to NASA’s Jet Propulsion Laboratory in Pasadena, California.

“Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls Ceres home,” said Marc Rayman, Dawn’s mission director and chief engineer at JPL.

Since its discovery in 1801, Ceres has mystified astronomers and defied easy categorization. It’s been called a planet, then an asteroid, and most recently a “dwarf planet” akin to Pluto. Unlike an asteroid, which is typically misshapen and lumpy, Ceres is round like a planet. It is small as far as worlds go: at nearly 950 kilometers in diameter, it’s approximately the size of Texas. Even so, it contains a third of all the mass in the asteroid belt, and is probably an ancient planetary building block left over from the solar system’s infancy some four and a half billion years ago.

Dawn is the first spacecraft to visit a dwarf planet, beating out NASA’s New Horizons mission, which will arrive at Pluto in July. Dawn is also the first spacecraft to orbit two worlds after leaving Earth. After its launch in September 2007, Dawn first visited the large asteroid Vesta before disembarking for Ceres. No conceivable conventional rocket could perform such a feat without stopping to refuel. Dawn’s multiple orbital destinations are possible thanks to its fuel-efficient ion engine, which can maintain a weak thrust for years on end using only solar power and a small reservoir of xenon propellant. At peak thrust, Dawn’s engine produces only as much force as a single falling sheet of paper. That thrust has built up in the years since Dawn left Earth, boosting Dawn’s speed by a record-breaking 39,600 kilometers per hour.

Studying Ceres up-close will help researchers better understand the dwarf planet’s history, and with it how our planetary system formed and evolved. Those studies could also uncover a new frontier in the search for extraterrestrial life. Telescopic measurements suggest Ceres has a large mantle of water ice on top of a dense, rocky core—enough water, in fact, to make some scientists think Ceres-like objects were how Earth got its oceans. Early in its life Ceres may have had an ocean, too. That ocean would have frozen as Ceres slowly cooled, and its icy surface would have gradually sublimated in the sunlight, leaving behind gunky, briny deposits of organic minerals. Astronomers have already used large ground- and space-based telescopes to glimpse what seem to be carbonates, clays, and other water-altered minerals on the dwarf planet. Ceres’s dust-covered, crater-scarred face may actually be the frozen surface of an ancient abyss, and some researchers speculate there could still be reservoirs of liquid seawater lurking within it.

Dawn could deliver answers soon as it settles in for at least a year and a half of exploration. Already, it has spied strange bright spots on the dwarf planet’s surface that may be deposits of highly reflective minerals salts or even water ice. As Dawn gets closer to Ceres’s surface and gains a better view, the nature of these mysterious spots will become clear.

In late April, after its gentle ion jets bring it into a polar orbit some 13,500 kilometers high, Dawn will begin making a world map of the dwarf planet. A survey orbit at 4,400 kilometers will come next, followed by a high-resolution mapping orbit at 1,470 kilometers. Finally, Dawn will slip into a 375-kilometer-high orbit to snap very high-resolution images and to map the distribution of elements at and just below the surface.

By then, fuel will be too low to shift orbits again. Dawn’s primary mission will end. For a while, Dawn keep circling Ceres in its lonely, low orbit. If Ceres proves half as interesting as some astrobiologists suspect it to be, other visiting orbiters, landers, and rovers won’t be far behind.