Countdown Is On

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NASA confirmed the start of its launch countdown for space shuttle Atlantis on Tuesday, following new security guidelines intended to thwart a terrorist attack.

The countdown clocks in the launch control center had actually started the previous evening for Atlantis' liftoff Thursday afternoon on a space station assembly mission.

Under the 3-week-old policy, NASA won't announce the start of the countdown until after the fact and won't announce the shuttle launch time until 24 hours in advance. In addition, the astronauts' movements on launch day will not be televised live as usual.

NASA test director Jeff Spaulding said the policy has not hampered countdown activities.

"There really hasn't been a change to any of the internal operations," Spaulding said. "There have been some discussions with the folks just to remind them of what the guidelines and policies are."

NASA says liftoff will happen sometime between 2 p.m. and 6 p.m. Thursday.

CBS Space Consultant William Harwood reports that based on independent analysis of the international space station's orbit, launch is believed to be targeted for around 5:13 p.m. Thursday.

Spaulding noted that engineers are not working any major problems at pad 39B. Forecasters, meanwhile, have upgraded the weather outlook for Thursday from 60 percent "go" to 70 percent. The outlook for Firday improves to 80 percent go and drops to 60 percent go on Saturday. Because launch is targeted for the afternoon, during NASA's normal work day, Spaulding said shuttle managers likely would consider making three attempts in a row if problems developed that prevented a launch on Thursday or Friday.

Atlantis is loaded with a 44-foot girder for the international space station that weighs nearly 27,000 pounds. It's not just a basic metal beam: This $600 million truss has 475,000 parts, including a myriad of electrical cabling and plumbing and four sets of computers. It also has a $190 million railcar and track for eventual use by the space station's robot arm.

Additional girders will be attached to this component on future flights, and the entire framework ultimately will stretch more than 350 feet with the railroad track running the full length. The track is needed to move the robot arm from one end of the space station to the other, to build up the orbiting outpost.

In a space shuttle first, Atlantis will be propelled by three new-style main engines during liftoff. The high-pressure fuel turbopump in each engine was built without welds, making it more reliable and safer.

The seven astronauts assigned to Atlantis' 11-day flight will be the first visitors for the three men who have been living on the space station since December. The shuttle crew is taking up fresh food and clean clothes for the space station residents.