The Defense Department last summer decommissioned a pair of missile defense satellites -- the Space Tracking and Surveillance System -- which delivered three times their original forecast service life but were nevertheless expected as recently as last year to remain in orbit supporting the U.S. military until 2023. Last May, Missile Defense Agency Director Vice Adm. Jon Hill said the STSS sensors would keep supporting MDA missions, suggesting they might overlap with plans to launch a next-generation capability -- the...