 The tasks were to remove the obsolete Corrective Optics Space Telescope Axial Replacement (COSTAR), originally installed during STS-61 to correct the spherical aberration of Hubble's mirror, and install the Cosmic Origins Spectrograph (COS), and to repair the Advanced Camera for Surveys (ACS).The ACS failed in June 2006 due to an electrical issue, and after being restored partially, failed again in 2007 due to an electrical short.The ACS was not designed to be serviced or repaired in space, so the task was considered one of the most challenging of the mission.