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The Boeing Delta II launch vehicle was selected for the Mars Exploration Rover mission because it has the right liftoff capability for the weight requirements.
The Mars Exploration Rover 1, called Spirit was launched on June 10, 2003 on a three stage Delta ll 7925 launcher and the Mars Exploration rover ll, called Opportunity was launched on July 7, 2003 on a three stage Delta ll 7925H launcher.
Reason for the different launch Vehicles
The Spirit uses Delta ll 7925 launch vehicle, while Opportunity uses Delta ll 7925H launch vehicle, where The H in the Delta ll 7925H launcher stands for heavy. That is the second launch vehicle has more power.
One question you can ask yourself is, since Spirit and Opportunity are similar rovers, why do we separate launchers for both?
As the Earth and Mars orbit the sun, the two planets come into close proximity every 26 months. This is the best time to launch Martian missions as this saves a large amount of fuel. During the launch of The Spirit, the planets would be very close to each other. Thus the Rover will require minimum energy to get to Mars. Whereas, Opportunity is launched approximately one month after this cycle starts. So the distance between the two planets is comparatively more. Hence it will require more energy.
Now, at launch, the rocket weighs 285,228 kilograms of which, the spacecraft weighs only 1,070 kilograms.
The launch of the spacecraft consists of the following stages: