Cape Canaveral, Fla. ? Two spacewalking astronauts gave the Hubble Space Telescope a new central power unit in a risky “heart transplant” Wednesday that ended with the patient surging back to life.

In order to pull off the surgery, NASA had to turn off the 12-year-old observatory for the first time in orbit. A tense four hours and 25 minutes after Hubble went dead, electricity was sent to the newly installed unit.

Astronaut John Grunsfeld is shown in an image from television as he takes the new power control unit up to the Hubble Space Telescope. The old power control unit, which was used for 12 years on the telescope, was replaced on Wednesday.

“A postoperative report: We have a heartbeat,” Mission Control informed space shuttle Columbia’s relieved astronauts.

The telescope’s old power control box had a loose screw that caused it to malfunction on and off. But putting in a new unit amounted to the riskiest repair job ever attempted on Hubble: The unit was never meant to be replaced. And once NASA turned off Hubble, there was no guarantee it could get the telescope working again.

As a result, the seven-hour spacewalk was a nail-biting experience for NASA.

“A lot of emotion has been invested into this, in both the hopes that it will work and the fears that it might not,” flight director Bryan Austin said. “I can tell you right now, those emotions are coming out in all ways.”

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“Now that we’ve given Hubble the power to see to the edge of the universe, tomorrow we’ll give it its eyes so it can see just that,” said astronaut John Grunsfeld, the chief repairman.

During a spacewalk scheduled for early today the fourth excursion for Columbia’s astronauts in as many days the crew planned to install a camera that should increase Hubble’s discovery capability tenfold.

Wednesday’s spacewalk got off to a shaky start. A half-hour before Grunsfeld was to float outside, his spacesuit began leaking. A popped valve caused as much as a gallon of water to saturate the upper half of his suit. He pulled off his soaked top and got into a dry one.

He and Richard Linnehan made it out the hatch two hours late.

The spacewalkers took turns removing 36 connectors from the telescope’s original power-control unit. Grunsfeld pushed in the new unit and hooked all 36 connectors back up. He used a long spatula and a specially designed wrench to tighten the hard-to-reach connections on the 4-by-2-by-1-foot black box.

“What I need,” Grunsfeld said, pausing to take a breath, “is to have talked about this in 1975 and laid out this bay.”

The astronauts had to race against the clock. The telescope’s systems could have been damaged by the cold if left unpowered for eight hours.

With the repairs completed, power was sent coursing through the telescope and, one by one, the multitude of systems slowly awakened.

If a critical system failed to come back on, the $2 billion-plus telescope could have been rendered useless.

NASA ultimately decided the risk of surgery was less than the danger that the malfunctioning power unit would cripple Hubble.