The United States and Russia have pledged to keep the International Space Station operating, despite the moratorium on U.S. shuttle flights following the Columbia disaster.

One of the original arguments in favor of U.S. space shuttles in the 1960s and 1970s was to use them to service a space station. But it was not until the mid-1990s, nearly a decade-and-a-half after the first shuttle flight, that shuttles actually fulfilled that vision. They began visiting the Russian station Mir, and have been the mainstay for the construction of the current international outpost.

But now that the remaining three U.S. shuttles are grounded after Columbia's loss, questions remain about how the station will be serviced, restaffed, and maintained. The NASA official overseeing the shuttle and station programs is Michael Kostelnik.

"You can trust that we are behind the scenes looking very hard, to start thinking about, if and when we get things ready to go, we will go back to fly in a timely fashion to support the international space station," he said.

Mr. Kostelnik says the near term is not a problem. Thanks to a previously scheduled Russian cargo launch on Sunday, the two U.S. astronauts and one Russian cosmonaut aboard will have supplies until late June. They were supposed to exchange places with a new crew during a shuttle visit in March, but the NASA official says they will now stay longer.

"The international space station is pretty solid right now," he said. "Obviously, there are going to be some demands on the crew, because we're going to have to extend their time in orbit."

In fact, NASA officials say the station crew has offered to remain aloft as long as necessary, while the Columbia investigation proceeds. But the space agency prefers not to keep astronauts in weightlessness for more than six months because of its weakening effects on bones and muscles.

If the shuttle flight moratorium is lengthy, an obvious way to exchange the space station crew would be on Russian Soyuz spacecraft, which have been used to ferry crews and Russian-sponsored space tourists to the station. One is always attached to the outpost as an emergency escape vehicle.

But NASA international affairs spokeswoman Debra Rahn says, no decisions have been made yet on crew exchange or any other operational issues.

"That's something we need to look at. We just don't have any answers yet," she said. "I mean, the accident just took place on Saturday. The program will have to have discussions internally with our partners to see where we go from here."

Planned Russian space visits to the station this year include two more by supply rockets and two by cosmonaut crews on Soyuz craft to swap escape vehicles. The official Itar-Tass news agency quotes a member of Kazakhstan's Aerospace Committee, Meirbek Moldabekov, as saying, the number of Russian takeoffs from the Kazakhstan launch site might even increase to accommodate station needs.

The Russian contribution might be sufficient to keep the outpost supplied and staffed, but NASA's Michael Kostelnik says that, only shuttles can advance station construction, meaning that building is on hold until further notice.

"The space shuttle becomes the primary assembly support, because it is the only heavy lift vehicle that can take these large structures up to space, and it is the only vehicle we have to bring things down from space, both of which - up mass and down mass - are required to do this," he said.

NASA cannot predict when shuttles might return to flight and continue station assembly. After the 1986 explosion of the shuttle Challenger, the agency grounded the fleet for two years and eight months, while it incorporated safety redesigns into the remaining orbiters. The top NASA official for space flight, Bill Readdy, says the latest moratorium could be lengthy, while the cause of Columbia's calamity is sought.

"Let me assure you, we're going to take however long it takes to get to the bottom of it, to identify the root cause, and fix it, and then get back to flying," he said. "So whether it takes three months or three years, so be it."

Columbia was scheduled to make one flight to the space station before next February, which was when NASA had hoped to complete the core outpost, before other nations' laboratory modules are added. Mr. Readdy says that, despite the orbiter's loss, the remaining three shuttles are sufficient to meet station needs.