Space shuttle Discovery stands poised on launch pad 39B. Photo: NASA

With the shuttle Discovery poised for launch Thursday on the next space station assembly mission, senior NASA managers today attempted to downplay the potential impact of a projected $4 billion budget shortfall, calling an expected down-sizing a "minor adjustment."

"When all is said and done, when you see the international space station I would anticipate that you won't be able to tell the difference," said program manager Tommy Holloway. "We'll still build and assemble the international space station that you're familiar with."

Maybe so. But until a detailed bottoms-up program review is completed around May 1, the full impact of the projected budget shortfall will not be known.

In the meantime, NASA's sights are set on launching the shuttle Discovery on a space station crew rotation flight at 6:42:11 a.m. Thursday, the moment Earth's rotation carries the launch complex into the plane of the space station's orbit.

Forecasters are predicting generally good weather, but say there's a 30 percent chance low temperatures, humidity and wind speed could combine to cause potentially dangerous ice buildups on the shuttle's external fuel tank.

The forecast continues to call for a 90 percent chance of good weather Friday and an 80 percent chance of "go" conditions Saturday should launch be delayed for any reason.

The goal of the 103rd shuttle mission is to deliver the space station's second full-time crew - Russian commander Yury Usachev, James Voss and Susan Helms - along with nearly 10,000 pounds of equipment and supplies.

The station's current crew - commander William Shepherd, Yuri Gidzenko and Sergei Krikalev - will return to Earth aboard Discovery, resting on their backs in special recliners bolted to the floor of the shuttle's lower deck.

The official portrait of the Expedition Two crew. Photo: NASA

Shuttle program manager Ronald Dittemore said today Discovery is on track for launch on mission STS-102, as are downstream flights scheduled for takeoff in May and June. "STS-102 is the fifth flight in the last six months and if you think about it, it's the second flight this year of a planned five flights in the first six months of this year," he said.

"So these are busy times for us in the space business, but they're also very exciting times and perhaps the most rewarding of times."

Said Holloway: "It's a very exciting time for us and things are going well. People often ask me how things are going and I respond by saying it depends on which direction you're looking. If you look up, things are going extremely well.

"I think if you look at the history of human space flight you would not find a period when things have gone so well. ... Over the last eight or nine months, we have launched nine flights to the international space station: Four shuttle flights, three Progresses, a service module and a crew. And things have gone almost perfectly."

But there are clouds on the horizon.

Faced with a projected $4 billion crunch over the next four years, NASA is scaling back the size, capability and scientific scope and of the international space station, limiting full-time crews to just three for the foreseeable future and ending "core" construction years earlier than planned.

A $100 million propulsion module that would have eased reliance on Russian Progress supply ships and fuel tankers appears dead. The X-38 emergency crew return vehicle is in jeopardy and the station's main habitation module also faces the budget axe.

Instead of an eventual crew of seven with an emergency lifeboat capable of carrying them all back to Earth at once, the station NASA now envisions will be staffed by a crew of just three until - and if - current budget problems can be resolved.

At the same time, NASA is moving space station program management from the Johnson Space Center in Houston back to agency headquarters in Washington while a new "blueprint" for station construction is refined and priced out.

Despite the turmoil, Holloway said the current reassessment should not be compared to a redesign in the early 1990s when Russia was brought on board as a full partner.

"This time around is a minor adjustment compared to what went on in '92 and '93 in terms of the redesign," Holloway said today. "We are redirecting and rephasing the current space station, we are not redesigning it. I would consider it a minor adjustment compared to what went on in '93."

In the near term, NASA will press ahead with this year's schedule of six station assembly missions and, in all likelihood, meet next year's schedule as well. After that, however, all bets are off.

President Bush's 2002 budget request for NASA provides $14.5 billion, a 2 percent increase over 2001 funding levels.

The U.S. Habitation module as seen under construction in 1997 at the Marshall Space Flight Center's Space Station Manufacturing Facility in Huntsville, Alabama. Photo: NASA

A budget overview document said the budget provides "increased funding for international space station development and operations consistent with a strategy of constraining space station cost growth."

Translation: NASA must stop development of downstream station facilities to free up money to complete construction of a scaled back outpost still capable of supporting the planned European and Japanese research modules.

NASA's most recent space station assembly schedule called for completion of the orbital outpost in 2006. It now may be declared essentially complete, from NASA's standpoint at least, by the end of 2003, as soon as it is able to support the attachment of Japanese and European research modules.

NASA had hoped to eventually staff the station with seven-member international crews. But that assumed development of a habitation module to provide living quarters and a NASA-developed lifeboat that could ferry all seven crew members back to Earth in an emergency.

As it now stands, NASA relies on Russian Soyuz spacecraft for lifeboat duty. But the Soyuz only seats three, it is only certified for six months in space and only two can be docked to the station at the same time.

To support crews larger than three, NASA must develop a crew return vehicle of some sort or obtain additional Soyuz spacecraft from Russia.

Michael Hawes, a senior station manager at NASA headquarters, told reporters today it may be possible to obtain a second Soyuz through a barter arrangement of some sort. No matter what happens, NASA needs more than three crew members to carry out any kind of serious science agenda.

"What we need to do for crew is increased life support capability, increased volume so we can deploy the habitation racks," Hawes said, adding the racks, or crew sleep stations, could be mounted in various station modules.

"There are a number of different options we're looking at and what's the minimum set of hab racks you have to have. And then you need the ride home. Of those three big pieces, you have to have a set that all works together and fits in the budget and that's what we're off trying to assess."

Hawes said last week that a connecting module called Node 2 could be outfitted to serve as crew quarters in place of a dedicated habitation module that was to have been launched in September 2005. That still leaves the question of a crew return vehicle up in the air.

"You have to have habitation capability for more crew and you have to have a safe ride home for more crew," Hawes said last week. "The X-38, we certainly plan to continue that test program, although we may look at phasing it in a more effective way."

An artist's concept of the Crew Return Vehicle docked to the international space station. Photo: NASA

The X-38 is a precursor for the planned crew return vehicle, or CRV. Hawes said NASA may initiate additional design work to more accurately determine cost and how long it would take to implement such a spacecraft.

In the meantime, the project is on hold.

Hawes said the projected budget overrun became apparent after the Russians finally launched the Zvezda command module last summer after more than two years of delays.

NASA then began launching a stream of backed-up shuttle flights and spending levels began increasing. Unresolved technical issues, called "liens and threats," were reassessed and a major budget overrun because obvious. "Once the service module launched and we held our schedules for the next several flights, our spending rate increased and that led us down this exercise to go back and relook at all these liens and threats and give them another assessment," Hawes said.

"Those are the assessments the program has been doing down in Houston and we have a number of different estimates that have come out of that," he said. "We have numbers that are as high as $4 billion over the whole budget horizon and we have numbers that have been less.

"It's a matter of how many of these potential issues you want to capture in the budget estimate," he said.

One long-term threat to the station is the health of the Russian economy and the Russians' ability to finance, build and launch the six or so Progress fuel tankers needed each year to keep the station at a safe altitude.

NASA began work two years ago to design and build a U.S. propulsion module to carry the load should the Russians fail to meet their obligations.

But the so-called "prop module" appears doomed. Instead, NASA will rely on the Russians, on the shuttle's ability to reboost the station with spare propellant and the eventual development of a European refueling craft called the ATV.

"We've looked at where we are today, the support the Russians have been providing, the support the shuttle has been providing," Hawes said.

An artist's concept an ATV in orbit. Photo: ESA

"If you look over the last several flights, just shuttle reboost, which was not in our original plans, has already provided approximately a Progress M1's worth of equivalent propellant," he said. "We've also looked at the ATV development at ESA. ... They are making good progress.

"We think at this point we can continue to support (with) the shuttle, the Russians are supporting per their contributions and it's probably not the best use of our resources to also go pursue a prop module given those considerations."

Finally, given a reduced crew complement, the number of scientific experiments carried out aboard the station will be curtailed.

"The research needs to make sense with the capabilities that are in place on the space station with the limitation of three crew until we can solve options for increasing that," Hawes said.

"That is certainly a somewhat different research program than we had been planning. Our initial assessments indicate we will still be able to populate a significant number of the research racks.

"We are still working to prioritize science research areas to really specify what all of those racks may be."