HOUSTON — Hoping to save a ripped space station solar wing, astronauts ventured outside today for one of the most difficult and dangerous spacewalking repairs ever attempted.

Flight controllers and the astronauts in orbit worked day and night to plan every detail of the momentous repair mission. The snagged panel must be fixed before space station construction can continue.

“Go out there and fix that thing for us,” a station commander, Peggy Whitson, said just before spacewalkers Scott Parazynski and Douglas Wheelock floated out of the hatch.

“We will,” Mr. Wheelock responded.

Mr. Parazynski installed a work platform at the end of a 90-foot robotic arm and boom extension and strapped himself on for a 45-minute ride to the damage site.

After traveling about half the length of a football field away from the pressurized compartments where the astronauts work and live, he will tinker with what amounts to a damaged electrical generator. The solar panel will be teeming with more than 100 volts of electricity, possibly as much as 160 volts.

To save the solar wing, Mr. Parazynski needs to clear whatever snagged the panels and caused the wing to tear in two places while it was being unfurled Tuesday.

The commander of the docked shuttle Discovery, Pamela Melroy, peered at the snarled mess through binoculars as Mr. Parazynski prepared for his robotic ride. It looked to her like two hinge wires, a guidewire, and a guidewire grommet had gotten tangled.

“Wow, that sounds like we have to do the whole enchilada for the repair, huh?” Mr. Parazynski said.

“It doesn’t look like an easy just rattle it and shake loose the grommet kind of situation,” Ms. Melroy replied.

Without repairs, the wing poses a structural hazard for the international space station. The damage could worsen and the wing could become unstable, possibly forcing NASA to cut it loose and lose a vital power source for future laboratories.

Once Mr. Parazynski has cleared the snag — possibly by cutting the guidewire and letting it wind up at the base of the tower — he plans to install several homemade braces so astronauts can deploy the wing to its full 115 feet. It is about three-quarters deployed now.

Astronauts made the braces from aluminum sheets, wires, and insulated tape aboard the linked shuttle-station complex.

Mr. Wheelock was going to position himself at the base of the wing to guide Mr. Parazynski and the astronauts operating the robotic arm. Italian astronaut Paolo Nespoli choreographed the outing from inside the complex.

It was Mr. Parazynski’s fourth spacewalk this mission and the seventh of his 15-year astronaut career.