Nasa has launched one of its most advanced missions after a spacecraft called Dawn blasted off.
The probe has been built to explore a part of space between the planets Mars and Jupiter, where huge rocks float in a giant ring called an asteroid belt.
Dawn's mission is to take a close look at the two biggest asteroids in the belt, called Vesta and Ceres.
Scientists hope that by studying them closely they will learn more about how the planets were created.
Launch - Summer 2007
Mars arrival - March 2009
Vesta arrival - September 2011
Vesta departure - April 2012
Ceres arrival - February 2015
End of mission - July 2015
Some scientists think Vesta and Ceres could have turned into planets themselves, but didn't get the chance.
It's going to take eight years for the spacecraft to complete its mission, and in that time it will travel for 4.8 billion kilometres (three billion miles).
The mission is unusual because we don't often get a close look at asteroids, and no spacecraft has ever orbited one before.
Nasa also say Dawn will be the first space mission to orbit two different objects in space once it leaves Earth.