Pressure on the International Space Station's cargo delivery schedule has left NASA with no way to get an international antimatter experiment to the station.

The US Congress has directed NASA to find a way to transport the Alpha Magnetic Spectrometer (AMS) detector to the ISS despite the fact that it is not scheduled to fly on any of the 10 remaining ISS missions to be carried out by the Space Shuttle fleet before its retirement.

But NASA has rejected as unfeasible suggestions that cargo delivery be rescheduled, including by use of a European Space Agency Automated Transfer Vehicle (ATV) to take onboard equipment designated for the Space Shuttle payload bay.

The AMS experiment, designed to be attached to the ISS to detect unusual types of matter including antimatter and dark matter, has the involvement of US, Chinese, Korean, Russian, Taiwanese and western European organisations.

The AMS will take up a quarter of the volume of the Shuttle's payload bay. In its support structure for Shuttle transportation AMS has a mass of 4,200kg (9,200lb). It would operate at ISS for up to five years, located at the external payload site on the station's starboard truss.

To make room for AMS in the orbiter's payload bay the ISS cargo rescheduling idea was to move equipment from the bay's interior into a Multi Purpose Logistics Module (MPLM), which is transported by Shuttle, or ESA's ATV.

For displaced cargo the MPLM can deliver 10,000kg while the ATV, which docked with the ISS for the first time on 3 April, has a 7,600kg cargo capacity.

However, NASA rejected that approach and says: "[The agency] has studied off-loading critical space from the Shuttle to an MPLM and ATV and determined that it wasn't feasible.

"The MPLM shuttle flights are fully loaded. More importantly, the large-scale external-mounted space station critical spares will not readily adapt physically to the MPLM in terms of volume and interfaces."

The agency rejected the use of the ATV as it deemed the resupply craft was "principally designed as a tanker" and because its dry cargo had to fit through the vehicle's 800mm (31.4in)-diameter hatch.

NASA points out that none of the large-scale station critical spares cargo can fit through that hatch or be attached to the vehicle's exterior.