WASHINGTON - US space agency NASA on Monday ignited a fire inside the private Cygnus cargo spaceship in order to learn more about how flames spread in microgravity.

Cygnus, operated by US space firm Orbital ATK, departed from the International Space Station early in the day, completing a 29-day stay at the orbiting laboratory.

Under normal circumstances, the resupply craft, packed with tons of trash from the space station, will soon re-enter the Earth's atmosphere for a fiery destruction over the Pacific Ocean.

But this time, ground engineers set nine materials inside the spacecraft on fire through remote control.

"We've received confirmation that the first Saffire-II test sample has been ignited," NASA said in an update at 7:14 pm EST (0014 GMT).

"We're lighting a fire in space ... on purpose! We want to learn how fire burns in space in order to prevent them," the space agency added via Twitter.

The materials involved included "flame retardant fabrics used for astronaut clothing, station Plexiglas window samples with edge variations and structures used for storage containers and silicone composites," said NASA.

Each sample is two by 11 inches (five by 28 centimeters), the size sample NASA uses to screen materials on Earth before they are used on a spacecraft, the space agency said.

This is the second time Cygnus has been used as a platform for fire experiments in space by NASA and Orbital ATK. NASA has planned altogether three Saffire experiments, deemed critical for the safety of astronauts who live and work in space.

Besides the fire experiment, Cygnus will release four small satellites called CubeSats from an external deployer on Friday, sending them to join a remote sensing satellite constellation that provides global ship tracking and weather monitoring.

The spacecraft will remain in orbit until Nov 27, when it will reenter Earth's atmosphere and burn up over the Pacific Ocean.

Cygnus was launched on Oct 17 on an Antares rocket from NASA's Wallops Flight Facility in Virginia, for Orbital ATK's sixth NASA-contracted commercial resupply mission.