The Hubble Space Telescope has a new, more powerful set of solar wings and a surer steering system after maintenance work by spacewalking U.S. astronauts. It was part of a week of overhaul chores by a team from the space shuttle Columbia.

Astronauts Jim Newman and Michael Massamino ventured outside Columbia for seven hours to attach the second of two new solar panels, copying procedures that an alternate team used to assemble the first array Monday.

The smaller, more rigid arrays promise to cause less drag and deliver more power to ensure the continued ability to use all Hubble instruments at once.

The two spacewalkers also replaced one of four rotating wheels that provide the spin momentum to turn and point the observatory. It had malfunctioned briefly in November and Hubble controllers did not want to take the chance it would do so again.

The official who is overseeing the operation, shuttle flight director Bryan Austin, says the spacewalks have been flawless so far. "When things go well like they do, you can sit back like as director of an orchestra," he said. "When things are going well, the music plays itself, and I think that's really what it felt like."

Astronauts will conduct three more spacewalks this week to complete the telescope's overhaul and extend its vision deeper into space with a powerful new camera.

On the next spacewalk Wednesday, the alternate team of John Grunsfeld and Rick Linnehan will replace a faulty power control unit, or PCU. U.S. space agency Hubble scientist David Leckrone compares the procedure to a heart transplant. "You can think of the PCU as the central pump that circulates electricity through the spacecraft," he said.

The task is the most risky of the observatory's overhaul. Flight directors must shut off the telescope's entire electrical system for the first time, raising the specter of freezing delicate components and ruining them if difficulties require the astronauts to work longer than the planned seven hours.

Despite that risk, Hubble program manager Preston Burch says the PCU hardware must be exchanged because its troubling internal resistance to electricity could worsen and ultimately burn out half of the telescope's six batteries. "As with any beloved relative, you are worried about sending them in for bypass surgery or even a heart transplant, but you realize that the risk of not doing it is severe," said Preston Burch. "In this case, the risk of not changing the PCU is severe."

This is the fourth maintenance visit since Hubble's deployment from a shuttle 12 years ago. The U.S. space agency plans to stop servicing the observatory in 2004 and return it to earth six years later.