The Zvezda service module under construction. Photo: NASA

The Russian command module Zvezda, a 44,000-pound (20,000-kilogram) spacecraft that will provide crew quarters, propulsion and flight control for the international space station, is finally poised for liftoff on a make-or-break mission.

Running more than two years behind schedule because of Russian funding problems and, more recently, trouble with Russia's workhorse Proton rocket, Zvezda is scheduled for launch July 12 at 4:56:28 GMT (12:56:28 a.m. EDT) from the Baikonur Cosmodrome in Kazakstan.

If all goes well, Zvezda and the international space station, made up of the U.S. Unity node and a Russian-built NASA-funded propulsion module called Zarya, will dock July 26 while the spacecraft are flying above Russian ground stations.

With Zvezda in place, station assembly will finally take off with a steady stream of U.S. and Russian flights. A Progress supply ship is scheduled to dock around Aug. 2 and a crew aboard the shuttle Atlantis will pay a visit in September to activate critical systems.

Stabilizing gyroscopes and a major truss element are scheduled for delivery in October and the first full-time crew - commander William Shepherd, Yuri Gidzenko and Sergei Krikalev - should arrive aboard a Soyuz ferry craft in early November. From that point on, the outpost will be permanently staffed with international crews.

If all goes well, that is.

If Zvezda (pronounced za-VEZ'-dah) fails to reach orbit or is unable to dock with the space station for some reason, the future of the $100 billion project would be in jeopardy. At the very least, Russian participation would be in doubt.

"This is one of the single most important launches in the history of the space age to date," said John Pike, a space policy analyst with the Federation of American Scientists.

An artist's concept of Zvezda's two solar arrays being deployed shortly after launch. Photo: NASA

In a very real sense, he told Spaceflight Now, "the future of human presence in space is riding on this."

With a working space station in orbit, NASA could possibly absorb a catastrophic shuttle failure down the road, which most insiders believe is inevitable sooner or later. But without a permanently manned station, which requires the Zvezda module in its initial stages, a shuttle failure could kill the manned space program outright.

"They're always betting the program every time they launch," Pike said of the shuttle. "If I've got a space station, I can explain how we get back in the saddle. If I've got no space station to fly to, it becomes rather more difficult to explain what the program is after the next shuttle accident."

But NASA managers and their Russian counterparts are optimistic about the prospects of successfully launching Zvezda to open a new era in space station construction.

"I look forward with great anticipation to the service module launch," said Kirk Shireman, the NASA manager who oversees Russian components. "We've put a lot of blood, sweat and tears into the manufacture of this vehicle."

Said Yuri Malenchenko, a Russian cosmonaut who will visit the station in September aboard shuttle Atlantis: "This is going to be a new station of a new type and I'm waiting with impatience for the time when I'm going to step on the international space station."