STS-125, or HST-SM4 (Hubble Space Telescope Servicing Mission 4), was the fifth and final Space Shuttle mission to the Hubble Space Telescope (HST). The launch of the Space Shuttle Atlantis occurred on May 11, 2009, at 2:01 pm EDT. Landing occurred on May 24 at 11:39 am EDT, with 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 Spectrograph and the Wide Field Camera 3. 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 blanket insulating panels to provide improved thermal protection, and a soft-capture mechanism that would aid in the safe de-orbiting of the telescope by a robotic spacecraft at the end of its operational lifespan. The mission also carried an IMAX camera with which the crew documented the progress of the mission for the 2010 IMAX film Hubble.

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 Massimino served with both Altman and Grunsfeld on STS-109, and performed two spacewalks to service the telescope.

NASA 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. The upgrades helped Hubble to see deeper into the universe and farther into the past, closer to the time of the Big Bang.

STS-125 was the only visit to the Hubble Space Telescope for Atlantis; the telescope had been previously serviced twice by Discovery and once each by Columbia and Endeavour. The mission was the 30th flight of Space Shuttle Atlantis and also the first by Atlantis in over 14 years not to visit a space station, the last one being STS-66.

The fifth servicing mission to Hubble, HST-SM4, was originally scheduled to launch in late 2005 or early 2006. However, the loss of Columbia in 2003, in which the orbiter disintegrated on re-entry into the atmosphere, had negative effects on the Hubble program as well as other NASA missions. On January 16, then-NASA Administrator Sean O'Keefe canceled the mission, as well as any future missions to Hubble, citing them as too risky, that all future shuttle missions would dock with International Space Station if an in-flight problem were to develop, as well as 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 response to the criticism, O'Keefe said that he would review his decision to cancel the servicing mission.

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." Joining Mikulski as an advocate for servicing Hubble was NASA's Chief Scientist, physicist John Grunsfeld, who was present at the meeting when O'Keefe announced the cancellation of the mission. A veteran astronaut of four shuttle missions, including two Hubble servicing missions, Grunsfeld had devoted years to Hubble, and was very disappointed when O'Keefe canceled the mission. He briefly considered retiring from NASA, but realized if he stayed, he could continue to advance physics in other ways. Instead, Grunsfeld dedicated himself to finding alternate ways to service the telescope, possibly by sending a robot into orbit to do the job. When O'Keefe announced his resignation as Administrator in December 2004, five days after a National Academy of Sciences committee opposed O'Keefe's position regarding servicing Hubble, the media and science community saw hope for the telescope's servicing mission to be reinstated.

O'Keefe's replacement, Michael D. Griffin, took just two months after his appointment to announce that he disagreed with O'Keefe's decision, and would consider sending a shuttle to repair Hubble. As an engineer, Griffin had previously worked on Hubble's construction, and respected the discoveries the telescope brought to the science community. He agreed with the National Academy of Sciences that a robotic mission was not feasible, and said that in light of the "Return to Flight" changes made following the Columbia accident, a shuttle mission to repair Hubble should be reassessed. After the successes of the Return to Flight STS-114 and STS-121 missions, and the lessons learned and improvements made following those missions, managers and engineers worked to formulate a plan that would allow the shuttle to service Hubble, while still adhering to the post-Columbia safety requirements.