An unmanned Russian spacecraft is tumbling toward Earth after a failed mission to take supplies to the international space station.

Former NASA astronaut Leroy Chiao told Newstalk ZB's Rachel Smalley it's unclear what happened, but controllers lost contact with the ship.

"They're tracking some debris, so the speculation is there could've been some kind of explosion at separation that caused this problem."

Chiao says the spacecraft is a total loss, as it's designed to burn up when it re-enters Earth's atmosphere.

He says cargo ships are unloaded at the space station, then packed full of garbage the astronauts want thrown away, and sent off.