With the shuttle Discovery poised at long last on the launch pad, the U.S. is ready to resume its role as a space-faring nation after being grounded for nearly three years. That is, if all goes as planned.

Because of the tragic explosion of the shuttle Challenger in January 1986, with the loss of all seven crew members, the American space program is no longer taken for granted.Now, the world waits with nervous anticipation for Thursday's liftoff. The greatest gathering of news media for any space launch - even the very first ones - will observe the launch.

The atmosphere is reminiscent of the early days of the space program, a mixture of both trepidation and confidence. But make no mistake, there will not be any launch if there is even a whisper of doubt or the slightest hint of any problem. This may well be the most thoroughly prepared launch in the history of the entire space program.

In the aftermath of the Challenger disaster, NASA undertook the most massive re-evaluation, repair, and redesign program ever done by the agency. Every piece of hardware was re-examined and tested. More tests were done on the main engine and the shuttle booster rockets than were done before the shuttle first flew.

Among those waiting with considerable anxiety at launch time will be officials and workers of Morton Thiokol in Brigham City, the maker of the huge solid-fuel boosters. A flawed O-ring in one booster was blamed for causing the disastrous 1986 explosion.

The result has been a 32-month nightmare for Thiokol. The booster has been redesigned, full-scale tests fired five times, hundreds of smaller firings were done, and a flaws were deliberately introduced into one booster firing to see how the rocket would handle it. It was a test never tried before in the history of the space program.

Thiokol passed all these tough hurdles, but only when the shuttle has safely been fired into orbit - not once, but many times - will the shadow over the company finally be removed.

Because of the long delay in the space program, the U.S. has much catching up to do. But this flight is not directly concerned with all that. The goal and purpose of Discovery's launch into orbit is simply safety - to demonstrate that it can be done.

As launches become routine once more, the tension inevitably will dissipate. However, the care and concern for safety should remain higher than it was in pre-Challenger days.

Yet in any enterprise as inherently dangerous and difficult as space flight, there will be other tragedies. That is inevitable. But another disaster at this moment could mean the death of the space program. That is why so much is riding on the spacecraft Discovery.