After voyaging 3 billion miles through space,
Dawn was launched in 2007 to study two very different asteroids and learn more about the building blocks of our solar system. Ceres is Dawn's second stop; its first was Vesta, which the spacecraft circled from July 2011 to September 2012.
FOR THE RECORD
An earlier version of this post said Dawn has traveled 2.4 billion miles. It has traveled 3 billion miles.
Now, after leaving Vesta and traveling through space for more than two years, the spacecraft is roughly 400,000 miles away from Ceres and speeding toward it at about 450 mph, with a rendezvous set for March 6.
Ceres is a dwarf planet, along with Haumea, Makemake, Pluto and Eris (whose discovery led to Pluto's infamous planetary demotion). At 590 miles across and holding roughly a third of the mass of the asteroid belt, Ceres is large enough that its own gravity pulls it into a spherical shape – which is part of why it was once considered to be very planet-like. In fact, Ceres was listed as a planet for decades after its discovery in 1801, and was briefly reconsidered during the 2006 debate surrounding Pluto's planetary status.
Ceres is the only dwarf planet in the inner solar system. Although it's much larger than Vesta, it's more of a mystery. Scientists have meteorites that they believe are from Vesta, which they can study and compare to Dawn's observations of the asteroid; but no such fragments found on Earth have been linked to Ceres. Until now, fuzzy images taken by NASA's Hubble Space Telescope have provided our best view of the icy dwarf planet. But that should change by the end of January, as Dawn approaches its target.
While Ceres and Vesta are both asteroids, they are very different – Vesta, the second most massive asteroid in the belt, is elliptical and very dry, while Ceres is spherical and thought to be so wet that it may harbor a subsurface ocean. Astronomers recently spotted water vapor rising from the dwarf planet.
Why are these two "protoplanets" so different? Scientists think Vesta formed earlier, when radioactive material was more abundant and produced more heat, leaving the asteroid very dry. Ceres may have formed later. Some researchers also think that Ceres may have originated much farther out in the solar system, before being yanked into its current position during a dramatic upheaval earlier in the solar system's development.
As building blocks of our solar system that never became full planets, these protoplanets' divergent life stories could shed light on the early solar system's history.
By the way, Ceres was discovered on Jan. 1, 1801, which means there are at least two events to celebrate this coming New Year's Day.