Washington

The future of the International Space Station — the US$25-billion project to place a permanent scientific laboratory in orbit — is hanging by a thread this week after the loss of the space shuttle Columbia cut its main lifeline to Earth.

If investigations into the accident (see next item) drag on for more than a few months without a conclusive answer, or uncover pervasive problems that keep the remaining three shuttle vehicles on the ground, construction of the station will stall and the prospect that it will ever be fully crewed could permanently recede, experts on the project say.

On the other hand, if NASA can quickly locate and remedy the problem that caused the accident, the station could still be completed. And, either way, the overall impact of the tragedy on the agency's broader space-science programme may be small — in contrast to the retrenchment and disarray that followed the 1986 Challenger accident.

When Challenger exploded just seconds after launch, dozens of spacecraft, from the Hubble Space Telescope to the Galileo Jupiter probe, were stranded for years. But now, nearly all of NASA's astronomical and planetary spacecraft — including the Space Infrared Telescope Facility and two Mars landers, scheduled for launch in April, May and June, respectively — go into space on expendable rockets instead.

That leaves the shuttle almost exclusively dedicated to building and servicing the space station, the US segment of which had been scheduled for completion next February. The next shuttle flight, originally scheduled for March, would have delivered research equipment to the existing Destiny laboratory — including a freezer for storing specimens and a high-quality window for Earth observation. That was to have been followed by six more flights, spaced out over a year, to deliver large pieces of the station's backbone-like structure and more solar arrays for electrical power.

That schedule has now been scrapped. But if the shuttles are flying again within a few months, the station's research programme may not be greatly affected, says Milburn Jessup, a professor of oncology at the Georgetown University Medical Center in Washington DC and chairman of the space station scientific users' committee. Only one of next year's assembly flights was to involve Columbia, and NASA might well be able to use another vehicle in its place.

Delays beyond that could pose problems, particularly if the sequence in which research equipment is delivered to the station has to be rearranged, says Jessup. A lengthy delay, like the almost three-year downtime after the Challenger accident, could be devastating, forcing scientists to stop work on experimental equipment bound for the station.

NASA is under pressure to get the shuttles flying quickly. The station's current three-man crew can remain onboard at least until June, their time being limited by factors such as onboard supplies. If the shuttle is not cleared to fly by then, NASA could, in a worst case, be forced to power down the outpost and leave it abandoned until a new shuttle crew can be sent up.

Russian Soyuz craft are currently the only vehicles besides the shuttles that can ferry crews to the station. But the cash-strapped Russian government is barely able to produce the two vehicles a year required to fulfil its obligations to the international project. And US law bars NASA from buying more Soyuz vehicles from Russia, which it accuses of selling arms to unfriendly states.

Watching the situation warily are the station's partners from Europe, Japan and Canada. Even if the shuttles resume flying soon, NASA still has no short-term plan for a vehicle that could safely evacuate a crew of six or seven — the minimum number required to make full use of the laboratory, according to scientists. NASA's newly proposed Orbital Space Plane, designed to dock with the station after launching on a conventional rocket, could take up to ten years to design and build.

Critics of NASA, such as Congressman Ralph Hall (Democrat, Texas), the senior Democrat on the House Committee on Science, argue that this would be far too little, too late, to salvage the utility of the space station. As long as the crew 'lifeboat' issue remains unresolved, they charge, the station will be next to useless as a scientific laboratory — whether the shuttles are flying again or not.