Hubble was built to be tuned up in orbit. But it wasn't designed for the major overhaul NASA astronauts undertook during its fourth servicing mission, 3B, in March 2002. They delved into the telescope's guts during long space walks and replaced parts that the original designers never thought they'd need to.
Installation of a new power-control unit forced an unprecedented and nerve-wracking shutdown of the entire satellite—a move comparable to a surgeon stopping a patient's heart during surgery, says Anne Kinney, NASA's director of astronomy and physics. Astronaut John Grunsfeld raced to finish the task before the temperature of the switched-off telescope dropped far enough to damage it. Would it power back up? "When you run a computer for 12 years, you don't know what kind of ghosts you have in the system," Kinney says. When all systems reactivated as planned, the astronauts, as well as astronomers and mission controllers on the ground, breathed a collective sigh of relief. The rest of the mission went like clockwork, including installation of a new cooling system for Hubble's near-infrared camera—NICMOS—useful for surveying dusty and cold areas of space, and installation of new solar panels and other science equipment.
It was the most challenging service mission ever attempted in space, and its success elated astronomers. Chief among the wonders was the long-awaited ACS, or Advanced Camera for Surveys. It essentially made Hubble into a new telescope. "ACS has roughly ten times the discovery power of the previous camera," says Mario Livio, astronomer at the Space Telescope Science Institute in Baltimore. Translation: Hubble can now see twice as much with five times more light sensitivity.
Tragically, this mission would be the last successful voyage for space shuttle Columbia. The disaster in February 2003 grounded Discovery, Atlantis, and Endeavour, the three remaining shuttles, and will delay plans to bring Hubble a spectrograph and a new wide-field camera with ultraviolet and infrared capability.