As the shuttle Discovery is being readied on launch tower 39B at Cape Canaveral, Fla., the U.S. space program is finally showing signs of life. If nothing else goes wrong - an uncertainty at best - the spacecraft could be headed into orbit in less than two months.

Nearly two-and-a-half years have passed since the shuttle Challenger blew up shortly after lift-off, killing all seven aboard and effectively putting an end to America's space program.The long delay has been criticized, considering the fact that the problem that led to the blast was a fairly straightforward one - a leaky O-ring in a booster joint that allowed flame to escape. It seems that NASA could have been back in space a little quicker.

Yet it should be recognized that the changes in the shuttle program have been more than just redesigning an O-ring. In many respects, the Discovery and its boosters have been remodeled from top to bottom.

Between 400 and 600 modifications have been made in the orbiter, the main engines, the external fuel tank, the solid fuel boosters, and support equipment on the ground. The cost has been more than $2 billion.

Engineers at Morton Thiokol have worked hard to redesign the boosters, including some 145 changes. Clearly, the boosters are more reliable under serious stress than they were before.

This massive overhaul of the entire shuttle presumably has made the spacecraft a safer vehicle, but it also raises some problems, namely, how will all the new parts work in flight?

Two more tests remain. One is a 20-second engine firing while the Discovery sits on the launch pad July 24. A final test firing of the boosters will take place July 25 in Utah.

However, the public should not forget that it is impossible to design and build a perfectly safe spacecraft. The possibility for something going wrong will always be there. The element of risk cannot be removed entirely.

It has taken a long time to produce a new shuttle system that is safer than the ill-fated Challenger. It was worth doing, but the job must come to an end at some point. If the tinkering never stopped, the space program would remain firmly rooted to the ground.

There will be another accident with a spacecraft sometime in the future. It may be years in coming, but it is the price for exploration, for reaching into space, for taking chances.

Knowing that, it's still worth doing.