NASA put shuttle launches on hold following the Columbia disaster, clouding the future of missions including assembly of the international space station, which has three astronauts aboard.
Even with its shuttles grounded, NASA can easily retrieve the astronauts using Russian vehicles. But if the space agency's remaining shuttles are out of service for an extended period in the wake of Saturday's catastrophe, as seems likely, it could prove difficult to maintain the station's operations.
"This is clearly a big setback for the station because during the rest of this year shuttles were supposed to carry up lots of big pieces of hardware for assembly,'' said John Logsdon, director of the Space Policy Institute at George Washington University.
NASA is extremely unlikely to build a new space shuttle to replace Columbia, according to experts, leaving the space agency with the three remaining orbiters as its entire fleet for the foreseeable future.
The next generation of reusable space vehicles is at least 10 to 15 years off, said Donald H. Emero, who served as the shuttle's chief engineer from 1989 to 1993.
"I think the country will not invest in any more shuttles,'' Emero said Saturday.
Until a few years ago, NASA was exploring several designs for vehicles to replace the space shuttle. But NASA's new administrator, Sean O'Keefe, has shelved those designs and committed to operating the space shuttle for the next 10 to 15 years.
NASA's three remaining shuttles are from 10 to 18 years old.
NASA's shuttle fleet was grounded for nearly three years following the 1986 Challenger disaster, as investigators struggled first to determine what had caused it to explode with seven astronauts on board and then to fix the problem. In the hours after that accident, few could have guessed that the cause would be a rubber "O-ring'' - stiffened and cracked by low temperatures.
Emero said the investigation of Saturday's accident could take as long as that inquiry, but doubted it would because Challenger was destroyed by such a minor defect that was difficult to find.
The next shuttle mission on NASA's flight schedule is a March 1 trip to the space station by the Atlantis orbiter.
Russian space agency officials said they were ready to pick up some of the slack for the space station if NASA grounds its shuttle fleet. But with Russia's ability to launch supply vehicles already compromised by budget problems, the loss of U.S. space shuttle Columbia could seriously jeopardize the continued operation of the outpost.
With no permanent crew aboard, the space station can operate in a ``dormant'' mode as long as occasional maintenance is performed by visiting astronauts. In fact, NASA had already been considering a "demanning'' contingency for 2003 before Saturday's events.
But the longer the station goes unoccupied, the greater the chances that it would deteriorate to an uninhabitable state. A dormant period would also cause a significant interruption in the station's continuing assembly and scientific research program.
Expedition Six, as the current crew is called, arrived at the station in November and consists of NASA astronauts Ken Bowersox and Don Pettit and Russian Soyuz commander Nikolai Budarin. The three men could remain where they are until June without a visit from the space shuttle, said space shuttle program manager Ron Dittemore.
An unmanned supply vessel was to be launched Sunday from Russia's Baikonur Cosmodrome. It was scheduled to arrive at the orbiting station Tuesday.
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