Animation of Atlantis docking to ISS. Photo: NASA TV/Spaceflight Now

The shuttle Atlantis closed in on the international space station today while the astronauts checked out their rendezvous computers, activated the shuttle's docking system and tested the spacesuits that will be used Monday during a planned spacewalk.

If all goes well, commander Terrence Wilcutt will guide Atlantis to a gentle docking with the space station at 1:52 a.m. EDT Sunday (0552 GMT) as the two spacecraft sail over Russian ground stations. Read our detailed story on how Atlantis will dock.

The only technical problem of any significance is a balky star tracker, used to collect navigation and orientation data for the shuttle's inertial measurement units. Atlantis is equipped with two such star trackers, one that looks straight up along the so-called minus Z axis and another that looks out parallel to the left wing along the minus Y axis.

After launch Friday, engineers noticed suspect data from the minus Z tracker, the one that would normally be used to "look" at the station for a navigation update just before the terminal initiation, or TI, rocket firing that begins the shuttle's final approach.

Tests early today confirmed the minus Z tracker is acting erratically and it will not be used for future IMU alignments.

But the failure will add only a slight complication to the crew's final rendezvous sequence, forcing the pilots to roll the orbiter 90 degrees to one side so the station will be visible to the minus Y star tracker. Lead Flight Director Phil Engelauf said the crew has trained for just that contingency and no problems are expected.

Flight controllers are optimistic the crew will have enough electrical power to support a one-day mission extension, giving the crew six days to outfit and activate the station instead of five.

But Engelauf said an official decision to extend the flight probably will not be made until next week. While there appears to be enough oxygen and hydrogen to power the shuttle's electrical generators an additional day, propellant margins for the ship's forward rocket pod are tight.

That could be a factor in any decision to extend the mission because the shuttle's maneuvering jets will be used periodically to maintain the orientation, or attitude, of the station.

"The risk here is that if docked attitude hold for the stack requires more propellant than we've projected, we could actually run out of forward propellant prior to the last docked day," Engelauf said.

But at this point, he added, "we really don't think that's going to be a problem. ... We believe we're on track to protect the 12th day."

The space shuttle flight control room at Mission Control. Photo: NASA TV/Spaceflight Now

Engelauf also provided a bit of insight into the mood at mission control as NASA embarks on its most ambitious launch schedule since the pre-Challenger era.

The agency plans seven to eight shuttle missions a year for the foreseeable future with nine flights - eight station construction missions - in the next 12 months, the most ambitious launch schedule since 1985.

"The whole team, on both the shuttle side and the station side, are extremely excited to be really kicking off a long string of assembly missions," Engelauf said early today.

"We do have a very aggressive string of missions here that we have to put together and it's going to be real challenging from a manpower standpoint, from a training standpoint."

"Rather than working in a very serial fashion, we're starting to work very parallel, many of us are working two or three or four missions in parallel with each other and the station team is starting to gear up for a lot more around the clock operations," he said.

"It's a very exciting time for us, it's something people came here to do and they've been training for it with anticipation. We're really looking forward to it."