Dawn Circles in on Ceres
When NASA’s Dawn spacecraft was captured into orbit around the dwarf planet Ceres on Friday, March 6, 2016, there was no fanfare in mission control. In fact, the spacecraft couldn’t even be in radio contact. There’s no need, because Dawn’s path was set–this is a spacecraft unlike any other.
What makes Dawn unique is its ion propulsion system, which gives the spacecraft incredible maneuverability. Instead of using large bursts of thrust to get where it’s going, Dawn used the slow and steady approach. Its ion engine delivered a tiny but continuous thrust that could last for days or weeks at a time.
For two-and-a-half years, Dawn slowly reshaped its trajectory to bring it near Ceres and, most importantly, to match the dwarf planet’s speed – Ceres travels around the Sun at nearly 64,000 kilometers per hour.
For other planetary missions, entering orbit is make or break. It’s an intense moment that hopefully ends in jubilant celebration when all goes as planned and the spacecraft momentously falls into orbit. But Dawn’s slow approach meant that it was perfectly on course to guarantee capture by Ceres’ gravity.
Ceres is the second object that Dawn has orbited. Between July 2011 and September 2012, Dawn was in orbit around Vesta, which like Ceres, resides in the Asteroid Belt located between Mars and Jupiter.
This marks the first time that one spacecraft has been able to orbit two different planetary objects. And it’s only possible because of Dawn’s ion engine.
A spacecraft powered in the usual way using chemical propellant, would require ridiculous amounts of fuel to carry out such a mission. And even if it was possible for a spacecraft to carry that much fuel on-board, the cost of the mission would be astronomical.
The planets of the Solar System formed by a method of accretion. Starting out as specs of dust that collided and stuck together, thy then grew bigger and formed rocks until eventually they were large enough to draw in enough material to form planets. Vesta and Ceres seemed to have halted midway through this process. This is most likely due to the formation of Jupiter and its gravity may have prevented objects in the asteroid belt from coming together to finish off the planet building. As a result, Vesta and Ceres provide a unique opportunity for understanding the early formation of the planets because they came so close to be4come planets themselves.
 See Astronomy (January 2016), p. 48