U.S. cargo ship departs from space station

A cargo ship operated by U.S. space firm Orbital ATK departed from the International Space Station on Tuesday after a two-and-a-half-month stay.

Cygnus, which arrived at the orbiting lab with 7,500 pounds (3,400 kilograms) of cargo on March 26, was released by U.S. astronaut Tim Kopra using the station's Canadarm2 robotic arm at 9:30 a.m. EDT (1330 GMT).

The U.S. space agency NASA said that it will light a fire in an experiment called Saffire-I on board the Cygnus spacecraft five hours after its departure from the station.

"Saffire-I provides a new way to study a realistic fire on a spacecraft," NASA said in a statement, noting "this hasn't been possible in the past because the risks for performing such studies on crewed spacecraft are too high."

The oxygen consumption and flame will be measured by instruments on the returning cargo ship.

NASA hoped the results would determine microgravity flammability limits for spacecraft materials, which would facilitate the agency's material selection, and help scientists understand how the microgravity and limited oxygen affect flame size.

On Wednesday, Cygnus will also release five microsatellites called CubeSats from an external deployer, part of a remote sensing satellite constellation that provides global ship tracking and weather monitoring.

The cargo ship will continue to orbit the Earth for up to eight days as it transmits hi-resolution images and data from the Saffire-I experiment.

After that, the spacecraft's engines will fire twice, pushing it into Earth's atmosphere where it will burn up on June 22.