It was supposed to be the hardest job of Atlantis' mission. Instead, it looked like the easiest.
Spacewalkers John Grunsfeld and Andrew Feustel, working on the Hubble Space Telescope, had practiced for nearly two years to repair the Advanced Camera for Surveys, a sensitive instrument never designed to be opened by astronauts.
The camera's electronics had shorted out. Engineers decided it would be better to repair them than replace the entire camera.
It was exacting work, with Grunsfeld opening tiny screws, cutting through a metal cover and pulling four fried circuit boards from the inside of the camera.
"You're ahead of the timeline," mission control in Houston told the space shuttle astronauts. "We never expected this."
A camera on Grunsfeld's helmet showed him using specially designed tools to get the camera open. He did not actually hold the circuit boards in his gloved hands; instead, he attached a clamp and slid them out by turning a knob.
The circuit boards have sharp edges, and astronauts have been told to be careful with their spacesuit gloves. Twice in recent years, spacewalking astronauts have found, to their surprise, that they had somehow cut through their gloves' outer layers, and their spacewalks had to be cut short for safety's sake in case their suits began to lose air pressure.
Grunsfeld checked his gloves regularly. They were fine.
Earlier, Grunsfeld and Feustel removed a large unit, about the size of a phone booth, from the inside of the telescope. It was a set of corrective optics, installed by astronauts in 1993 after the Hubble's main mirror -- much to NASA's embarrassment -- had turned out after launch to be slightly misshapen.
Newer cameras have corrective optics of their own, and the old unit is no longer needed. Its place was taken by a spectrograph, an instrument that measures the chemical composition of distant objects from the light they give off.
The astronauts spent most of their time outside on detail work, but there was a little time for sightseeing.
"I can see Hawaii!" called Feustel at one point. At another, the astronauts said, "We seem to be flying right over Houston. Tell everyone down there they look great."
Two spacewalks remain. Grunsfeld and Feustel have been alternating with two other crewmates, Mike Massimino and Mike Good.
Inside the shuttle, Scott Altman is the commander, and Greg Johnson is the pilot, second in command. Megan McArthur is flight engineer, operating the shuttle's robot arm on which spacewalkers often perch.
Among other jobs, astronauts have been told to install new insulation on the outside of the telescope. For lack of an insulating atmosphere 350 miles up, the Hubble can go through temperature swings of a few hundred degrees everytime it passes between daylight and darkness.
Weather permitting, the shuttle Atlantis is expected home on Friday.