Servicing Mission 4

Servicing Mission 4 (SM4), was the last scheduled shuttle mission (STS-125) for the Hubble Space Telescope in May 2009.The servicing mission was first planned for October 14, 2008.However on 27 September 2008, the Science Instrument Command and Data Handling (SI C&DH) unit on HST failed. All science data passes through this unit before it can be transmitted to Earth. Although it has a backup unit, if the backup were to fail, the Hubble's useful life would be over.Therefore, on 29 September 2008, NASA announced the launch of SM4 was postponed until 2009 so this unit could be replaced as well.SM4, with a replacement SI C&DH unit,was launched aboard Space Shuttle Atlantis on May 11, 2009.

On SM4 astronauts, over the course of five spacewalks, installed two new instruments, Wide Field Camera 3 (WFC3), and the Cosmic Origins Spectrograph (COS). WFC3 will increase Hubble's observational capabilities in the ultraviolet and visible spectral ranges by up to 35 times due to its higher sensitivity and wider field of view. The telephone-booth sized COS assembly replaced the Corrective Optics Space Telescope Axial Replacement (COSTAR) that was installed in 1993 to correct Hubble's spherical aberration problems. (COSTAR was no longer needed after the replacement of the last two instruments which did not have the necessary correction built in.) The COS will do observations in the ultraviolet parts of the spectrum, complementing the measurements done by the repaired STIS system. The service mission repaired two instruments that had failed, the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS). They also performed other component replacements including: all three Rate Sensor Units (each containing two gas-bearing gyroscopes); one of three Fine Guidance Sensor (FGS) units used to help keep pointing accuracy and increase platform stability; the SI C&DH unit; all six of the 125-pound (57 kg) nickel-hydrogen batteries used to provide all Hubble's electrical power to support operations during the night portion of its orbit; and three New Outer Blanket Layer (NOBL) thermal insulation protective blankets. The batteries had never been replaced and were more than 13 years over their original design life.[71] Assuming testing and calibration of the new equipment goes well, the Hubble should resume routine operation in September 2009.These efforts should keep the telescope fully functioning at least into 2014 and hopefully longer.

Hubble was originally designed to be returned to earth on board a shuttle. With the retirement of the shuttle fleet this will no longer be possible. NASA engineers developed the Soft Capture and Rendezvous System (SCRS), a ring-like device that was attached to Hubble’s aft bulkhead which will enable the future rendezvous, capture, and safe disposal of Hubble by either a crewed or robotic mission.Atlantis released the Hubble Space Telescope on May 19, 2009 back into space after all repairs were successfully made. The next mission will be to deorbit the Hubble at the end of its service life.