CAPE CANAVERAL, Florida (CNN) -- After nearly 20 years of missions around and around the Earth, the Space Shuttle soon will have a destination -- the international space station, construction of which is finally about to begin.

The launch of the shuttle Endeavour is scheduled for Thursday at 3:57 a.m. ET. Weather forecasters say that storms could threaten the launch.

"One of the important things that the space station does is give the space shuttle somewhere to go," says John Pike of the Federation of American Scientists.

But giving the shuttle a port of call isn't cheap. When completed, the new station is expected to cost U.S. taxpayers more than $50 billion. That includes the cost of the 35 scheduled shuttle flights to build the station, something NASA says the shuttle is well suited to do.

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"The shuttle was designed to support something like this, so we are long overdue to have a destination such as the station," says ISS Deputy Program Manager Frank Culbertson.

In fact, the station, with a 30-year life expectancy, guarantees the shuttle will have a job for several more decades. It's a case of one hand washing the other. Without the shuttle, there couldn't be a station; without the station, the shuttle appeared on the verge of extinction. As one former NASA historian says, the shuttle program was drying up.

"It is a victim of its own success," says Alex Roland. "They're not actually doing anything up there in space, or at least, they're not doing anything that they weren't doing 35 years ago.

The shuttle was originally supposed to fly at least 25 times a year -- putting satellites in orbit for a few dollars per pound and providing inexpensive access to space. Instead, each mission costs $800 million, and cargo costs $20,000 a pound. And the Challenger disaster called into question the shuttle's reliability.

Despite its history, the shuttle has provided plenty of drama and achievements -- the Hubble repair mission, satellite rescues and the recent return to space of John Glenn, to name a few. Now it faces its biggest challenge, and the success of the space station is on the line.

Correspondent John Zarrella contributed to this report.