STS-125, or HST-SM4 (Hubble Space Telescope Servicing Mission 4), was the fifth and final space shuttle servicing mission to the Hubble Space Telescope (HST) Launch occurred on 11 May 2009 at 2:01 pm EDT. Landing occurred on 24 May at 11:39 am EDTwith the mission lasting a total of just under 13 days.

Space Shuttle Atlantis carried two new instruments to the Hubble Space Telescope, the Cosmic Origins S and the Wide Field Cameractograph. The mission also replaced a Fine Guidance Sensor six gyroscopes, and two battery unit modules to allow the telescope to continue to function at least through 2014. The crew also installed new thermal blanketinsulating panels to provide improved thermal protection, and a soft-capture mechanism that would aid in the safe de-orbiting of the telescope by an unmanned spacecraft at the end of its operational lifespan The mission also carried an IMAX camera and the crew documented the progress of the mission for an upcoming IMAX movie.

The crew of STS-125 included three astronauts who had previous experience servicing Hubble.Scott Altman visited Hubble in 2002 as commander of STS-109, the fourth Hubble servicing mission John Grunsfeld, an astronomer, has serviced Hubble twice, performing a total of five spacewalks on STS-103 in 1999 and STS-109 Michael Massinino served with both Altman and Grunsfeld on STS-109, and performed two spacewalks to service the telescope.
managers and engineers declared the mission a complete success The completion of all the major objectives, as well as some that were not considered vital, upgraded the Hubble telescope to its most technologically advanced state since its launch nineteen years before and made it more powerful than ever.The upgrades will help Hubble to see deeper into the universe and farther into the past, closer to the time of the Big Bang

STS-125 was the first visit to the Hubble Space Telescope for Atlantis; the telescope had been previously serviced twice by Discovery and once each by Columbiaand Endeaour. The mission was the thirtieth flight of Space Shuttle Atlantis and the first flight of Atlantis in over 14 years (since STS-66) not to visit a space station.

The fifth servicing mission to Hubble, HST-SM4, was originally scheduled to launch in late 2005 or early 2006. On 16 January 2004, then-NASA Administrator Sean O'Keefe canceled the mission, as well as any future missions to Hubble, citing safety constraints imposed by the Columbia Accident Investigation Board. During the announcement, O'Keefe stated that it was his decision alone, and not a recommendation from any other departments. The decision was widely criticized by the media, the science community, and those in NASA. Maryland Senator Barbara Mikulski, a member of the Senate subcommittee that oversees NASA's budget, publicly accused O'Keefe of making a decision outside the transparency process against the wishes of the science community, and stated she would work to reverse the decision. In March 2004, Representative Mark Udall introduced a bill to the House of Representatives that requested an independent panel of experts review O'Keefe's decision to cancel the servicing mission. Also in March 2004, Space Telescope Science Institute (STScI) Director Stephen Beckwith released the results of the Hubble Ultra-Deep Field survey to the entire science community, which helped show the public how important Hubble was to science. The data showed the deepest images ever taken by a telescope and revealed approximately 10,000 galaxies, some of which most likely dated back to when the universe was just five hundred million years old. With Beckwith when he released the data to the scientific community was Mikulski, who said of the results, "I think it's just amazing... this is why I will continue to stand up for Hubble."

The mission added two new instruments to Hubble. The first instrument, the Cosmic Origins Spectrograph, is now the most sensitive ultraviolet spectrograph installed on the telescope.[NASA 6][NASA 7] Its far-UV channel is 30 times more sensitive than previous instruments and the near-UV is twice as sensitive. The second instrument, the Wide Field Camera 3, is a panchromatic wide-field camera that can record a wide range of wavelengths, including infrared, visible, and ultraviolet light.[NASA 7] Atlantis also carried the Soft-Capture Mechanism, which was installed onto the telescope.[NASA 10] This will enable a spacecraft to be sent to the telescope to assist in its safe de-orbit at the end of its life. It is a circular mechanism containing structures and targets to aid docking.