NASA called for proposals for the rover's scientific instruments in April 2004,[21] and eight proposals were selected on December 14 of that year.[21] Testing and design of components also began in late 2004, including Aerojet's designing of a monopropellant engine with the ability to throttle from 15–100 percent thrust with a fixed propellant inlet pressure.[21]
By November 2008 most hardware and software development was complete, but testing continued