Their mission nearing an end, the two space station astronauts installed antennas and released a baby Sputnik during a spacewalk Monday amid heightened safety procedures and multiple breakdowns.

Commander Leroy Chiao and his Russian crewmate, Salizhan Sharipov, left the international space station empty for the second time in just a couple months, floating outside to perform the 220-mile-high maintenance work. They wrapped up the spacewalk in 4 1/2 hours, more than an hour early, earning praise for their quick action.

NASA (search) and the Russian Space Agency (search) instituted extra safety measures to avoid a repeat of the problem that occurred during the men's spacewalk in January. Chiao got too close to firing thrusters during that first outing because of miscommunication, and so flight controllers put unambiguous rules in place for Monday's outing.

"Please don't rush things," Mission Control urged the men.

Complicating the spacewalk this time was a seriously hampered stabilization system.

Two weeks ago, a circuit breaker popped open and cut power to one of the gyroscopes needed to keep the space station stable and pointed in the right direction. The breakdown left the station running on only two gyroscopes, the bare minimum.

The two functioning gyroscopes eventually became overloaded, and the space station ended up drifting around Earth and pitching over, almost like a partial slow-motion cartwheel, for 17 minutes — far less than the two hours anticipated by flight controllers.

Engineers have yet to identify the source of the mysterious force that tilts the space station during spacewalks and causes the American-made gyroscopes to lose steering control. Each time, Russian thrusters have to take over, potentially exposing the crew to toxic fuel. This time, flight controllers were careful not to fire the thrusters until the spacewalkers were at a safe distance.

Chiao and Sharipov — marking their 165th day in orbit — plugged in four antennas for a new type of cargo carrier due to fly next year. They also carried out a 1-foot-long, 11-pound satellite called Nanosputnik, designed for experimental maneuvering by ground controllers.

Sharipov let go of Nanosputnik on the count of two as Chiao photographed the event. "Off it goes," Sharipov said as the satellite floated away with a spin. Minutes earlier, he commented: "Everything is like in the movies, and it's hard to believe."

The spacewalkers ignored the failed circuit breaker; visiting shuttle astronauts will tackle that job in two more months.

The two station residents have spent the past several weeks dealing with an assortment of breakdowns, including an oxygen generator that still isn't working. Over the weekend, they replaced a pump panel that is part of a critical cooling system.

A fresh two-man crew will relieve them next month.

The space station has been home to only two astronauts at a time since 2003, one fewer than usual because of the grounding of the shuttle fleet. The cutback means no one is inside overseeing station systems during spacewalks.

NASA hopes to launch Discovery to the space station in mid-May on the first shuttle flight since the Columbia tragedy. A milestone was set for later in the day with Discovery's much-delayed move from the hangar into the Vehicle Assembly Building for hookup to its booster rockets and redesigned external fuel tank.

Foam from the tank led to Columbia's loss.