Dawn Spacecraft is Ceres-ly Orbiting a Dwarf Planet
It has been announced that the Dawn Spacecraft has successfully transitioned to orbiting around the dwarf planet Ceres. Someone thought it amusing to label this accomplishment as a first, as Dawn is now orbiting a dwarf planet, rather than orbiting some other type of celestial rock, which we have orbited before. But seriously, it wasn't until 2006 that Ceres was even referred to as a dwarf planet. Before then, it was called an asteroid, and an orbit of an asteroid was already achieved on February 14th, 2000 (Mission was NEAR, the asteroid was 433 Eros). With any luck, the IAU will redefine Ceres as something like a dormant, mega-comet at some point, and a new NASA mission can be sent there to claim yet another astronomical first.
The real first is that Dawn has accomplished is to achieve orbit at two separate celestial destinations during one mission (Vesta and now Ceres), and this is due to its use of ion propulsion. Ion engines are about ten times more efficient than conventional bi-propellant engines, when it comes to using fuel. As a result, they allow spacecraft a greater wander range than a conventional engine would allow. In this case, increased engine performance results in much smaller fuel tanks, which results in a smaller spacecraft with more range.
Ion engines put out a miniscule amount of thrust -- it takes the Dawn spacecraft 4 days to accelerate from 0 to 60 miles per hour, but unlike chemical propulsion, Dawn can maintain that thrust continuously for months at a time. The Ion engines are remarkably simple and sturdy, and can run for upwards of five years at full thrust without breaking.
Now that Dawn has arrived at Ceres, we will get the chance to investigate this peculiar planetoid. So far, scientists have speculated that Ceres has a thick crust of ice surrounding a rocky core. It appears as though Ceres' surface is coated with a fairly thick layer of dust, and there appears to be some activity that has revealed the underlying ice layer. We should be in for an exciting several months of science ahead.