Facing their toughest job yet, spacewalking astronauts floated outdoors Saturday to give the Hubble Space Telescope a better view of the cosmos by installing a new high-tech science instrument and fixing a broken camera.
It was the third spacewalk in as many days for the shuttle Atlantis crew, expected to be the most challenging ever performed because of the unprecedented camera repairs. Astronauts have never tried to take apart a science instrument at the 19-year-old observatory.
John Grunsfeld and Andrew Feustel got started on the daunting job Saturday morning as the joined shuttle and telescope soared 350 miles above the planet. Orbiting so high put Atlantis and its astronauts at an increased risk of being hit by space junk. NASA had another shuttle on launch standby in case a rescue was needed.
"It's dark out here," Feustel observed as Atlantis passed over Australia.
Before tackling repairs to the burned-out camera, the spacewalkers had to install the $88 million Cosmic Origins Spectrograph, which is designed to detect faint light from faraway quasars and provide greater insight into how planets, stars and galaxies formed.
"Hello, Hubble," Feustel said as the telescope door swung open.
To make room for the supersensitive instrument, Grunsfeld and Feustel had to remove the corrective lenses that restored Hubble's vision in 1993. The space telescope had been launched three years earlier with a flawed mirror that left it nearsighted. But the newer science instruments had corrective lenses built in, making the added contacts unnecessary.
The switch — taking out the 7-foot-long box containing the corrective lenses and putting in the cosmic spectrograph — was expected to be straightforward. It's exactly the kind of replacement work astronauts performed on four previous repair missions.
For the camera repairs, though, Grunsfeld and Feustel were going to have to open up the seven-year-old instrument and pull out fried electronic parts.
The instrument — called the Advanced Camera for Surveys — suffered an electrical short and stopped working two years ago. Ground controllers have since been able to eke out a minimal amount of science but hope to get it back into full operation.
Before it broke down, the surveys camera provided astronomers with the deepest view of the universe in visible light, going back in time 13 billion years.
NASA considers this repair job — and one set for Sunday on another broken instrument — to be the most delicate and difficult ever attempted in orbit. Neither instrument was designed to be handled in orbit by astronauts wearing thick, stiff gloves.
Grunsfeld, the chief Hubble mechanic, and Feustel were going to have to deal with more than 30 latches to get inside the camera and navigate a tricky corner to replace the burned-out power supply cards. NASA even designed new tools for the job.
The first two spacewalks of Atlantis' mission ended up running long because of unexpected difficulties. Hubble senior project scientist David Leckrone surmised late Friday that because the observatory went seven years without a tuneup, "it's gone wild again."
NASA hopes to keep Hubble working for another five to 10 years with all the improvements. No one will be back to Hubble, so everyone at NASA, the seven astronauts included, wants to squeeze in as much repair work as possible. Already, they have given Hubble an improved wide-field planetary camera, fresh batteries and gyroscopes, and a new science data unit to replace one that broke last fall.
If all goes well, the fifth and final spacewalk is set for Monday and the telescope will be released from Atlantis on Tuesday.
This last mission to Hubble cost more than $1 billion.