CAPE CANAVERAL, Fla. (Reuters) - As the International Space Station circles Earth, losing orbit, short on power and months away from being habitable, NASA prepared on Sunday to launch the space shuttle Atlantis on a mission that promised to be part progress and part rescue.

Atlantis and its crew of six Americans and one Russian are scheduled to lift off from the Kennedy Space Center in Florida at 4:15 p.m. EDT on Monday. The U.S. space agency said an approaching storm front was unlikely to pose a problem by launch time.

Inside the shuttle's cargo bay were more than 2,000 pounds (900 kg) of supplies, from smoke detectors to an exercise treadmill. Expeditionary crews will use them when they eventually take up residence, a step already two years behind schedule.

So far the $60 billion station is like a half-built house that has sat on a vacant lot too long. Some repairs will have to be made before the job can even be finished.

When completed, the station will have more pressurized air space than a Boeing 747, but now it is little more than storage space, still waiting for the long-delayed Russian service module, dubbed Zvezda, that will provide living quarters, power and navigational aid while construction continues on the remainder of the station.

Batteries Have Run Down

But first, the Atlantis crew will have to replace four of six batteries in the Russian-built Zarya module. Zarya has been in space since November 1998 and was never designed to operate alone for so long without help from Zvezda.

The crew will also try to buffer the noise inside Zarya. At 75 decibels, this is about as loud as a noisy office or normal city traffic, but through prolonged exposure, it can cause partial deafness.

They also will take plenty of air samples, hoping scientists on the ground can learn why an earlier supply crew became sick while working on Zarya. While space-walking astronauts put the finishing touches to a Russian construction crane outside the station, they will also repair a communications antenna that failed.

During more than five days of docked operations, Atlantis commander James Halsell and pilot Scott Horowitz will periodically fire the shuttle's engines to boost the station's orbit by about 20 miles (32 km). Without the fuel and thrusters on Zvezda, the station's orbit has been declining about 1.5 miles (2.4 kg) a week.

NASA defends its work on the station but says there is only so much the agency can accomplish until the Russians put Zvezda into orbit. That launch is scheduled for mid-July, but NASA managers are saying it may be August at the earliest.

Hardware Ready For Launching

"Today, 85 percent of the U.S. hardware (for the station) is down in Florida, waiting to be launched,'' said Tommy Holloway, space station manager for the Johnson Space Center in Houston. "We're ready for 10 flights of hardware. Five of those flights are in final preparations.''

Meanwhile, the Russian Space Agency, which is more than two years behind schedule on launching the service module, has come under increasing criticism from Congress and from the General Accounting Office, which in March complained that the Russians were delivering components, including Zvezda and Zarya, that did not meet specifications for safety and reliability.

Both modules were not adequately shielded from orbital debris from satellites and other space missions, which could collide with the station at 18,000 mph (29,000 kph) or faster, the accounting office found. It also said the modules would not operate if the station lost air pressure.

For their part, the Russians say that their international partners -- the United States, Canada, the European Space Agency and Japan -- lack the experience Russia has in building and orbiting space stations and sometimes see problems where the Russians have found solutions years ago.

"It's no picnic,'' said Valeri Alaverdov, First Deputy General Director of the Russian Space Agency. "I think the best thing in any argument is the evidence, and we have plenty of evidence that our hardware is reliable. We've been flying the Mir station for almost 15 years.''