Replacement shuttle Rockwell International unveils Endeavour, the space shuttle that will replace the ill-fated Challenger, during a ceremony in Palmdale, Calif. Completion of Endeavour returns the shuttle fleet to a force of four orbitors, including Discovery, which is scheduled for liftoff Sunday morning. The Endeavour will fly on its maiden voyage in March 1992 in a mission to bolt a new rocket motor onto a failed communications satellite.