Today is the day that the Dawn Mission completes a long 7.5 year long journey that has taken it past the orbit of Mars and into the asteroid belt, studying the second largest asteroid Vesta before heading toward the dwarf planet Ceres, where it has now injected itself into orbit, as of 7:39 am EST.
This marks the first time in history a spacecraft has seen a dwarf planet up close, and with New Horizons passing Pluto in July, Dawn won the race in an astronomical photo finish.
The Story So Far
Launching on September 27th, 2007, Dawn orbited the Sun and used its Ion propulsion thrusters to accelerate slowly out to the orbit of Mars. Ion propulsion is a highly efficient form of thrusting in space, using minimal fuel but resulting in a slow and nearly continuous acceleration. Dawn used its thrusters for approximately 80% of the time it was orbiting the Sun, before using Mars as a gravity assist in 2009 to push it further out into the asteroid belt.
Two years later, Dawn arrived at Vesta, slowing down in order to insert into orbit around the ~500 Km asteroid. It spent 14 months snapping photos and mapping the surface of Vesta in detail before leaving Vesta on September 4th, 2012, to head for Ceres.
Now that Dawn has begun its orbit of Ceres today, it will map the entire surface in detail, and begin studying surface features. Over time it will determine if the surface features are changing, giving evidence of any current geological activity. Ceres is also about 25% water by mass, so astronomers on Earth hope to gather some insight into how water behaves on the dwarf planet, in contrast to the earlier observations of the much drier Vesta.
10:07 EST UPDATE: Mission Engineers received confirmation of the orbit injection at 8:39 am EST, with indication that the spacecraft had entered orbit as planned.