Because the interim control module also uses a docking mechanism different from Zvezda, astronauts would have to reconfigure the dock in the Zarya module now in orbit.
While the smaller interim control module contains no living quarters or laboratory space like Zvezda, it can stabilize the space station and reboost it to counteract orbital decay.
The interim control module, a contingency NASA proposed in 1997, was developed by the Naval Research Laboratory using components it designed to maneuver spy satellites for the Pentagon.
If Zvezda is successfully launched on time, the interim control module will still become part of the station, but in a slightly different role.
This "interim control module" will have no living quarters, but it could do the job of keeping the station in orbit while it is put together.
NASA insisted this spring that Russia correct the rocket problems and launch Zvezda by this summer, or the agency would launch an American interim control module and continue space station construction without waiting for the Russians.
To guard against further delays and restart building of the orbiting station, Mr. Goldin said, he has ordered completion of a so-called interim control module that would be launched on a space shuttle in December.
In February, NASA's administrator, Daniel S. Goldin, announced that if Russia did not launch Zvezda by early August, NASA would instead send up its interim control module at the end of the year.
Mr. Herring said that setting Zvezda's launching date would not change plans to ready the substitute American unit, called the interim control module, which Mr. Goldin said could fly as early as December if the Russian component was not launched on time.
