Once again, the shuttle Discovery is about to blast into space. And once again, it will dock with the International Space Station, and astronauts will continue the process of building the half-completed orbiting laboratory in a mission full of daunting challenges.

But the majesty of the first nighttime liftoff in more than four years, now scheduled for Thursday just before 9:36 p.m. Eastern time, will not dispel a question that has long been the subject of sharp debate among experts: What is the space station for?

In 1998, when its first components were launched as a replacement for the Mir, a worn-out Soviet-era relic, the station was billed as a manned science lab of nearly unlimited potential, with promises of advances in areas like pharmaceuticals thanks to the ultrapure crystals that could be grown in a microgravity environment.

It was to be finished by 2004, and it was to cost about $40 billion, shared by 16 nations, including the United States, Canada, Russia and the European Union.