STS-125 (Atlantis / final Hubble servicing)

Background

By the late 2000s, the Hubble Space Telescope had become one of the most scientifically productive instruments in the history of astronomy — and one of the most visibly aging. Launched in 1990 and subsequently repaired and upgraded on four earlier shuttle missions, Hubble was showing its years. Its primary imaging workhorse, the Advanced Camera for Surveys, had suffered an electronics failure in 2007. Two of its six gyroscopes had failed, leaving the telescope operating near the minimum needed for stable pointing. NASA had originally cancelled a fifth servicing mission in the wake of the Columbia disaster, judging the risk too high: unlike missions to the International Space Station, a flight to Hubble's orbit offered no refuge if the shuttle sustained critical damage. Public and scientific pressure, combined with a rigorous engineering review, eventually reversed that decision. A rescue shuttle — Endeavour — was placed on a second launch pad and kept ready throughout the mission as a contingency. STS-125 would be Hubble's last house call.

The crew assembled for the mission reflected deep experience in spacewalking and telescope operations. Commander Scott Altman was making his fourth spaceflight; Pilot Gregory Johnson his second. Mission specialists John Grunsfeld and Michael Massimino were Hubble veterans, each having worked on the telescope during the third servicing mission a decade earlier. Rounding out the team were Megan McArthur, who would operate the shuttle's robotic arm to capture and berth the telescope, and mission specialists Michael Good and Andrew Feustel, both making their first flights. Five spacewalks were planned, each targeting a different set of repairs and upgrades — an extraordinarily compressed schedule of precision work demanding some of the most demanding EVA choreography NASA had ever attempted.

The Flight

Atlantis lifted off from Kennedy Space Center's Launch Complex 39A on 11 May 2009. The ascent was clean, and the orbiter reached its operational orbit roughly eight and a half minutes after leaving the pad. Because Hubble circles Earth at an inclination and altitude that takes it well out of reach of the ISS, Atlantis climbed to an orbit that was, in terms of rescue options, an island. Engineers scrutinized imagery of the thermal protection system with unusual intensity during the on-orbit inspection, aware that any damage found at this altitude would leave limited options.

Two days into the mission, McArthur captured Hubble with the robotic arm and berthed it in Atlantis's payload bay, approximately fifty hours after launch. The five spacewalks that followed were conducted on consecutive days and represented the densest program of extravehicular activity ever flown on a Hubble servicing mission. Grunsfeld and Feustel performed the first and third EVAs; Massimino and Good handled the second and fourth; Grunsfeld and Feustel returned for the fifth. Together the teams replaced all six gyroscopes, installed a new main computer, and fitted fresh batteries. They also replaced one of Hubble's fine guidance sensors, a critical component for the telescope's pointing precision.

The camera work was equally ambitious. The Wide Field Camera 3 — a versatile instrument capable of imaging across ultraviolet, visible, and near-infrared wavelengths — replaced the older Wide Field and Planetary Camera 2, which had been aboard since 1993. The Advanced Camera for Surveys, whose electronics failure had darkened two of its three channels, was repaired in orbit — a procedure not originally designed to be performed in space, requiring the removal of more than 100 tiny screws from an instrument never intended to be opened by gloved hands. The Cosmic Origins Spectrograph, a new instrument sensitive to ultraviolet light and designed to probe the large-scale structure of the universe, was installed in the bay previously occupied by COSTAR, a corrective optics device that had become redundant once all other instruments were built with their own corrections. After approximately 77 hours and 46 minutes of mission-elapsed time, the final planned servicing activities were complete, and Hubble was released back into its independent orbit — upgraded, refueled in terms of its operational reserves, and in most respects more capable than at any prior point in its life.

Challenges and Contingencies

The mission did not pass without tension. During the fourth spacewalk, Massimino found that a handrail on the Advanced Camera for Surveys was obstructing access to the instrument. After consultations with engineers on the ground, the decision was made to break the handrail free — an improvised solution to a problem that had no scripted answer. It worked. The repair of the ACS was completed, restoring capabilities that had been lost for two years and substantially expanding Hubble's scientific reach.

Throughout the mission, Endeavour sat ready on Pad 39B, the first time since the early shuttle years that two orbiters had been prepared for simultaneous flight. The rescue scenario was never needed, but its availability was itself a statement about how seriously NASA treated the hazards unique to this mission.

Legacy

Atlantis performed its deorbit burn late in the thirteenth day of the mission and touched down at Edwards Air Force Base in California approximately 309 hours and 37 minutes after launch, closing out a flight that the scientific community had fought hard to see happen.

The investment proved profound. Wide Field Camera 3 quickly became one of the most productive astronomical instruments ever built, contributing to discoveries ranging from exoplanet atmospheres to the most distant galaxies yet observed. The repaired Advanced Camera for Surveys resumed its survey work. The Cosmic Origins Spectrograph probed the cosmic web of gas that connects galaxies across cosmic time. Hubble, given up for aging by some before STS-125 was approved, went on to operate for well over a decade after the mission and continued generating landmark science.

STS-125 stands as a testament to what human spaceflight can accomplish when precision engineering and trained human judgment are brought to bear in an environment where automated repair is not possible. It was the end of an era of direct human contact with one of history's great scientific instruments — and the beginning of Hubble's most powerful chapter.