ATLAS-1, the first of the ATLAS series of Shuttle flights, was an important part of the long-term, coordinated research that made up NASA's Mission to Planet Earth. Two ATLAS-1 instruments, ACR and SUSIM, had direct counterparts aboard the Upper Atmosphere Research Satellite (UARS), while other instruments aboard each mission were closely related. Repeated flights of the ATLAS instruments, which were carefully calibrated before and after each flight, allowed for long-term calibration of UARS instruments. (After a successful mission, UARS reentered Earth's atmosphere on Sept. 24, 2011.)