After launch, each MER craft will be maneuvered into the proper trajectory to intersect the Red Planet's orbit. The best time to manage this is about every two years when Mars and the Earth move closer to one another. This alignment is called the "launch window" or "launch period." MER-A launched on June 10, 2003. MER-B launched on July 7, 2003.
Launch of Spirit gallery
MER-A The total time from launch to arrival in Earth orbit is just 11 minutes. The spacecraft coasts in Earth orbit for about half
an hour. A burn of the third stage, called a PAM-D will then push the spacecraft out of Earth orbit and on a trajectory to Mars.
Launch of Opportunity gallery
MER-B This launch will involve a longer coast time than MER-A to position the spacecraft properly to exit Earth's orbit. In contrast
to MER-A, which launches during the daytime, MER-B launches at night.