The international space station as seen from Atlantis. Photo: NASA TV/Spaceflight Now

Streaking through space at five miles per second, the 120-ton shuttle Atlantis gently docked with the 67-ton international space station today, capping a two-day orbital ballet that began with the shuttle's thundering liftoff Friday from the Kennedy Space Center.

With commander Terrence Wilcutt at the controls, the problem-free linkup occurred right on time at 1:51:37 a.m. EDT as the two spacecraft sailed high above western Kazakhstan.

"Houston, we have a capture light," astronaut Daniel Burbank called as the two docking systems engaged.

"Congratulations on a fine rendezvous and docking," astronaut Chris Hadfield said a few minutes later from mission control in Houston. "That was letter perfect, great to watch."

Over the next half hour or so, hooks and latches in the docking system closed, firmly locking the two spacecraft together. Leak checks indicated a tight seal across the docking interface and lead flight director Phil Engelauf said he was elated with the results.

"The rendezvous went extremely well from our standpoint," he told reporters at a morning briefing.

As expected, Wilcutt had no trouble carrying out the final stages of the rendezvous despite the failure of a star tracker Friday that normally would have been used to get a navigational fix on the station prior to the final rendezvous sequence.

Instead, Wilcutt rolled the shuttle 90 degrees around 10:30 p.m. Saturday, took a fix with Atlantis' other star tracker, and rolled back to the normal orientation. The simple maneuver only had to be done once and there were no problems.

"The crew flew flawlessly, the vehicle performed flawlessly and we go into tomorrow in better shape, really, than we started the day today in terms of the prospects for the remainder of the mission," Engelauf said.

A black and white camera on the international space station sees Atlantis moving in for docking. Photo: NASA TV/Spaceflight Now

He was referring to Wilcutt's fuel-stingy flying style, which saved needed propellant for a proposed one-day mission extension.

A final decision to extend the flight from 11 to 12 days will not be made until later in the mission, but flight controllers now project Atlantis will have enough propellant and electrical power to support the extra day.

"We successfully docked and in the process we managed to make some gains on our propellant margins and have continued to make some small gains on our power production margins, which give us a little bit more confidence in being able to add the 12th day to the mission," Engelauf said.

The only technical issue under discussion today concerns a brief power surge in one of the shuttle's main circuits shortly after the ship reached orbit. The spike did not trip any circuit breakers, but flight controllers want to understand what caused it.

In August 1999, the shuttle Columbia suffered a short circuit seconds after liftoff that left the crew one failure away from an engine shutdown. After the mission, engineers found scores of wiring defects across NASA's shuttle fleet, prompting extensive repair work.

But Engelauf said the current spike seen after Atlantis reached orbit does not raise any immediate concerns about more frayed wiring.

"This didn't raise any significant flags in the sense of thinking we had a big problem," he said. "We are off looking, obviously, because any time you see any kind of behavior on the vehicle that you don't completely understand we go off and try to work that."

The timing of the surge appears to be associated with the transition from ascent software to programs used for orbital operations.

"Different systems on the vehicle are actuated at the time of that transition and we've just been combing through that to make sure there wasn't a piece of equipment that operated during that time that wasn't supposed to or to rule out the possibility of a short," Engelauf said.

Looking up at the international space station from the payload bay of Atlantis. Photo: NASA TV/Spaceflight Now

"But we're not looking at any other specific concern like the wiring problems that we'd had on a previous mission."

Atlantis' crew, meanwhile, is pressing ahead with the flight plan and gearing up for a spacewalk early Monday to electrically connect the new Russian Zvezda command module to the rest of the station. The astronauts will not actually open hatches and enter the station until Monday evening.

They did, however, open a hatch leading into a pressurized mating adapter - a tunnel connecting the shuttle and the U.S. Unity module - in order to draw an air sample through Unity's hatch before the station's air purifiers were turned on.

Engineers on the ground want to find out how much material "out gases" from station components between shuttle visits.

"Like in your car when it's hot in the summer and you open the door and you smell some chemical type smells that you didn't when it's cooler or the windows are open, that kind of thing," said station flight director Mark Ferring.

"The materials inside the spacecraft out gas some of their chemicals into the air and over time, that acrues to certain levels to the point where it's above the point where our medical people want them to be," he said.

Each module is equipped with filters, or "scrubbers," to clean the air before astronauts arrive. Wilcutt and company had hoped to take a sample of the air in Unity before the scrubbers were activated to get a sense of just how much out gassing actually takes place between shuttle visits.

But the device needed to draw air through Unity's sealed hatch failed and Wilcutt was not able to obtain a sample.

The scene inside Mission Control after the successful docking. Photo: NASA TV/Spaceflight Now

"It looks like there may have been a problem with the interface from the air sample to the node itself," Ferring said. "It isn't critical for our entry. The scrub of the node happens anyway and our flight rules require the scrub, so it doesn't impact the mission at all that we've had this problem."

A sample presumably will be obtained by a future shuttle crew. For now, flight controllers are pressing ahead with the flight plan in anticipation of Monday's spacewalk and station entry.

"We're warming up the shell with our heaters so the internal temperatures are at the right temperature for the crew to ingress," Ferring said. "We are performing an air scrub in all the modules to make sure the air is of good quality at the time they enter."

And later tonight, U.S. station controllers will upload new software into Unity's flight computers that will enable the system to control a huge solar array package scheduled for attachment later this year.

For now, though, flight controllers are focusing on Atlantis' mission and work to outfit and activate the new Zvezda command module before arrival of the station's first full-time crew in November.

"It was really great to see that the station has changed since the last time we were there with the addition of Zvezda," Ferring said. "It really looks different."

"And I think what we're going to see in the next couple of missions is every time we fly one of these things, it's going to change. So the assembly is starting to pick up speed and it's very exciting for us to see."