Discovery's seven crew members put on their bright orange spacesuits and were strapped into the space shuttle for a practice countdown on the launch pad Thursday.

The mock countdown was not only practice for the astronauts' expected launch in July, but also a dry run for the flight controllers in the so-called firing room who will direct the liftoff.

Discovery's launch will be the first shuttle flight in almost a year and only the second one since the Columbia disaster in 2003.

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The shuttle's big external fuel tank has undergone modifications over the past year in an attempt by NASA to solve the problem that doomed Columbia — big pieces of foam insulation breaking off from the tank and hitting the spacecraft.

"We had a good test, a firing room full of console operators, getting familiar with their activities and launch-day events," said NASA spokesman Bruce Buckingham. "With the crew at the pad, it was a very successful test."

A glitch held the clock at 1:57 minutes before the three-hour countdown resumed.

The countdown was stopped with four seconds left on the clock, and the crew practiced an evacuation from the shuttle, climbing out of the vehicle and scurrying to an emergency wire basket about 50 feet away.

In a genuine emergency, the basket would slide down a rope to the ground like a ski-lift, taking the astronauts to safety.

On Saturday, top NASA officials are expected to make a final decision on a target launch date and whether the shuttle is safe to fly. The launch window is between July 1 and July 19.