The astronauts replaced the second of two 460-pound battery packs that keep the telescope running when its solar arrays are facing away from the sun. They also swapped in a replacement for one of Hubble's three Fine Guidance Sensors, which guide the telescope as it focuses on its targets, and replaced some of Hubble's thermal insulation.
NASA says that the mission's upgrades and repairs, which the astronauts completed over the course of five spacewalks in five days, will keep the telescope running until 2014, when it will be replaced by the new James Webb Space Telescope.
On this mission, the crew of the space shuttle Atlantis installed a new camera and spectrograph, replaced Hubble's batteries, replaced the telescopes positioning gyroscopes, and attached a docking ring so that a robotic spacecraft can eventually be sent to retrieve Hubble from orbit, among other tasks.
"This is a really tremendous adventure that we've been on, a very challenging mission," Grunsfeld said on NASA Television, according to Bloomberg News.
The mission generally went smoothly, although there were some delays in Sunday's spacewalk, when astronauts Michael Massimino and Michael Good faced equipment glitches. The problems took so long to resolve that astronauts scrapped the final part of the day's plan, to install the thermal insulation.
However, Monday's spacewalk was successful enough that the Grunfeld and Feustel were able to make up for lost time and install the insulation, which protects the telescope from its harsh environment.
The astronauts plan to release Hubble from the Atlantis dock on Tuesday, and to return to Kennedy Space Center in Florida on Friday. Although they discovered small dents on one wing's heat shield, NASA engineers have cleared the shuttle for a safe return.