The Mars Exploration Rover (MER) mission consisted of two spacecraft, MER-2 hardware which included the Spirit rover, and MER-1 hardware, which included the Opportunity rover. Spirit was launched June 10, 2003, on a Delta II 7925 launch vehicle. Opportunity followed 3 weeks later, launching July 7, 2003 on a Delta II 7925 Heavy vehicle.
urn:nasa:pds:context:investigation:mission.mars_exploration_rover
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