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The Upper Stage contains fuel and oxidizer, and the vehicle's "brains." It fires twice: once to insert the vehicle-spacecraft stack into low Earth orbit and then again to accelerate the spacecraft out of Earth orbit and on its way towards Mars.
Two interstage adapters connect the first stage of the Atlas with its Centaur upper stage. The interstage adapter attached to the Centaur encloses the aft bulkhead carrier while the stages are linked.
The Centaur is 41.7 feet (12.7 meters) long and 10.2 feet (3.1 meters) in diameter. It has a restartable RL-10C engine made by Aerojet Rocketdyne, Sacramento, California.
This engine uses liquid hydrogen and liquid oxygen and can provide up to about 22,890 pounds (101,820 newtons) of thrust. The Centaur can control its orientation precisely, which is important for managing the direction of thrust while its engine is firing. It carries its own flight control computer and can release its payload with the desired attitude and spin rate.
The Centaur's aft bulkhead carrier holds the CubeSat deployment system that contained the two mini-spacecraft called Mars Cube One, or MarCO. During launch, these CubeSats were released after leaving Earth orbit, and they flew on their own path to Mars behind InSight.