Several of Jet Propulsion Laboratory’s future missions, including its next Mars rover, might be delayed or cut to compensate for other NASA priorities, the agency administrator said Thursday. Michael Griffin, NASA’s new administrator, told a Senate subcommittee the space agency will have to revise its spending plan for the year in order to offset costs associated with the space shuttle’s return to flight, possible human servicing of the Hubble Space Telescope and growth in upcoming missions.