Astronauts Jeffrey A. Hoffman and F. Story Musgrave tackled the Hubble Space Telescope's nearsightedness early today, replacing its most sensitive and versatile camera with a new model designed to correct for the error built into the telescope's big primary mirror.
Ground controllers quickly confirmed that the new camera was "alive" and functioning, and if further checks and calibrations go smoothly over the next six to 12 weeks, astronomers should have Hubble's busiest camera working as it was originally designed.
A fourth spacewalk tonight and early tomorrow has been set to install more corrective optics to sharpen the focus of light reaching three other astronomical instruments.
"Boy, you guys are doing great," capsule communicator Greg Harbaugh said after telling the astronauts that the newly installed camera was working.
The installation was the last of the Endeavour crew's chores that NASA planners had cited as vital to a successful mission, and leaders of the much-criticized agency were clearly beginning to feel vindicated.
"I think what you're witnessing is what the men and women of this agency can do with sufficient resources to do the job," said lead flight director Milton Heflin.
But space telescope program scientist Dr. Ed Weiler cautioned that, despite all the testing and independent verifications to assure that the corrective mirrors would work, it will be weeks before astronomers see the images that will prove that Hubble's troubles are over.
"Let's all think about this and not declare total success until the optics are there," he said.
Later in this morning's spacewalk, the two astronauts rode the robot arm 40 feet up to the top of the space telescope, where they attached two new magnetometers to take over from two that have worked only intermittently. The devices help Hubble orient itself in space by sensing the Earth's magnetic field.
Both proved later to be working, but the astronauts found that metal plates covering two sides of one of the old devices had come loose, exposing a foam interior material. NASA officials feared the material -- located close to the telescope's aperture door -- might generate gases that could contaminate Hubble's sensitive optics.
Plans were being made to send the astronauts back to the devices during a later spacewalk, to install protective covers over both of the disconnected magnetometers. The old magnetometers cannot be removed.
Today's six-hour, 47-minute spacewalk ended just before 5:30 a.m. It was the third of five planned during the current shuttle mission, and it appeared to go as flawlessly as one Sunday to replace the telescope's failed gyroscopes and a second yesterday morning to replace two solar panels that had developed the shakes.
Late tonight and early tomorrow, astronauts Kathryn C. Thornton, 41, and Tom Akers, 42, are scheduled to install COSTAR, a package of robot arms and mirrors that give three other astronomical instruments on the telescope a cure for the mirror flaw.
The design of COSTAR -- which stands for Corrective Optics Space Telescope Axial Replacement -- was the brainchild of Dr. Jim Crocker of the Space Telescope Science Institute in Baltimore and a Colorado optical scientist.
Early Thursday morning, Dr. Thornton, a physicist, and Mr. Akers, a mathematician, are to take the mission's final spacewalk and replace the electronics that control telescope's solar arrays. Endeavour is to land in Florida Monday.
Aside from some brief problems attaching the magnetometers, and a failed power tool, the astronauts seemed to encounter no problems. It was no accident. Dr. Hoffman, 49, an astrophysicist, and Dr. Musgrave, 58, a surgeon, spent more than 350 hours in a NASA water tank rehearsing.
Dr. Hoffman rode on the end of the shuttle's robot arm, pulling the Wide Field/Planetary Camera from the side of the telescope like a giant drawer, while Dr. Musgrave, standing in foot restraints attached to the telescope, helped align the camera and protect it from damage.
Although the old and new cameras are each roughly the size and shape of a baby grand piano, and weigh 610 pounds on Earth, they seemed to handle effortlessly in orbit.
"I'm not even pulling it, just coaxing it with my fingertips, nice and smooth," said Dr. Hoffman at one point as he drew the new camera from its storage cabinet in the shuttle's payload bay. "You can do things in zero-G [gravity] that you could never dream of."
WF/PC2 (pronounced whiff-pick two) is the space telescope's workhorse instrument, performing more than half its observations. It can see in wavelengths well beyond those of visible light, from the ultraviolet to the infrared, and is far more sensitive than any other Hubble device.
But it has been handicapped by Hubble's misformed mirror. Scientists had to postpone half the observations originally scheduled for WF/PC, and accelerate other research to take advantage of the camera's remaining capabilities.
"Hubble wasted most of the light it received" because of the mirror defect said Dr. David Leckrone, the telescope's project scientist at the Goddard Space Flight Center in Greenbelt.
With the camera's new corrective optics, astronomers should be able to see starlight 10 to 15 times fainter than that visible with the old camera. That also means they will be able to see three to four times deeper into space. That should begin to help them answer fundamental questions about the origin and fate of the universe, the evolution of galaxies and the existence of black holes at the center of galaxies.