The 4th and Final Hubble Servicing Mission
The Hubble Space Telescope Servicing Mission 4 (which is actually the fifth servicing mission, as Servicing Mission 3 was broken into two parts) is to be the final servicing mission, replacing batteries, gyros, insulation (see image), and scientific instruments to keep the telescope functioning for at least another five years. This mission was canceled once, after the 2003 Columbia explosion, but after a safety review, NASA rescheduled the mission. Only three days before the failure of the SDF, NASA announced that Servicing Mission 4, STS-125, would take place with the launch of Space Shuttle Atlantis on October 14, 2008.
The Hubble's technical problems delayed that mission until May 11, 2009. The timing of the anomaly, shortly before the last servicing mission, may have been fortuitous despite the delay. The Wide Field Camera 3 (WFC3) and a Cosmic Origins Spectrograph (COS) were able to be installed in the 4th and final Servicing Mission. Since no more servicing missions will take place, STS-125 really has to count, and ensuring another backup system in case Side B fails may extend the life of the invaluable Hubble Space Telescope.