The ASRB would have produced additional force in order to increase shuttle payload, so that it could carry modules and construction components to the ISS.

The official added that the policy's effects would not be evident for a few years because shuttle payloads were booked so long in advance.

The Chandra observatory and its rocket engine, weighing more than 20 tons and measuring 57 feet in length, are the largest and heaviest shuttle payload ever to fly.

"This investment," he recently wrote, "would pay for itself in less than a year by launching not only shuttle payloads but also many payloads that are currently launched on U.S. and foreign 'throwaway' launchers."

Also responsible for all shuttle payloads, from Spacehab to the Hubble Space Telescope to deployable satellites.

The usual practice is for the primary shuttle payload to be released the first day of a flight, just in case the mission is suddenly cut short because of malfunctions.

Oh, perhaps the military shuttle payloads are classified.

Another competing approach was maintaining the Saturn V production line and using its large payload capacity to launch a space station in a few payloads rather than many smaller shuttle payloads.

Already, past delays have cut the flight rate of the shuttles to unusually low levels, and NASA officials are scrambling to find new shuttle payloads to compensate for looming gaps in outpost construction.

This allowed shuttle payloads to be lifted with the station's engines during the routine firings, rather than have the shuttle lift itself and the payload together to a higher orbit.