A last-minute scare on the launch pad had NASA engineers worried about Wednesday's launch of the space shuttle Discovery.

Even though Tuesday's problem was quickly solved, all of NASA is still on edge about the Discovery mission. It will be the 114th flight for the shuttle program, and the first since the Columbia disaster two-and-a-half years ago.

NASA has spent more than $1 billion US getting ready for this launch, determined to chart a new, safer course for the shuttle program.

In early 2003, a piece of foam the size of a suitcase broke loose from Columbia on takeoff, damaging the spacecraft's hull. The scope of the problem went unnoticed, and the shuttle burned up on reentry, killing all seven astronauts on board.

"It is utterly crucial for NASA, for the nation, for our space program to fly a safe mission," NASA Administrator Michael Griffin told a news conference. "We have done everything that we know to do."

He was speaking just before news of a setback: that a window cover, held on by tape, fell off the shuttle and damaged two thermal tiles near the tail. The entire aluminum panel was replaced in what was described as a minor repair job.

"The lessons we've learned are written in other people's blood," Griffin said. "And the minute we say we're good enough, we get bad again."

DiscoveryChannel.ca's Peter McMahon, covering Tuesday's news conferences at Cape Canaveral, said the only thing in the way of a smooth launch is the weather. The probability of a thunderstorm or showers halting the 3:50:53 p.m. ET launch rose from 30 to 40 per cent late in the day.

If all goes well, Discovery's seven-member crew, led by commander Eileen Collins, will fly to the International Space Station and run through a series of tests that are primarily designed to evaluate new safety procedures.

There have been about 50 safety improvements to the shuttle, including a redesigned external tank, new sensors and a boom that will allow astronauts to inspect the shuttle for any potential damage.

Astronauts Steve Robinson and Soichi Noguchi will venture outside the shuttle three times on spacewalks. On the first, they will demonstrate repair techniques on the shuttle's protective tiles.

During the second spacewalk, they'll replace a failed gyroscope that has plagued the space station since June 2002 and left it without steering power.

On the third outing, Robinson and Noguchi will install the External Stowage Platform, a sort of space shelf for holding spare parts during space station construction.

The crew is due to return to earth Monday July 25.

With files from Jed Kahane, CTV News and The Associated Press