A privately owned U.S. communications satellite "Iridium 33" collided with defunct Russian satellite "Cosmos 2251" in outer space on Tuesday, shooting out two massive debris clouds and posing a slight risk to the international space station, the U.S. NASA said Wednesday.

It was the first time that two satellites collided in full in space, NASA said.

Prior to that, three major collisions were confirmed between spacecraft and space junk in history.

In December 1991, defunct Russian satellite "Cosmos 1934" hit a big debris released by another Russian satellite, "Cosmos 926," breaking the satellite in half.

In July 1996, French telecommunications satellite "Helios 1A" was hit by a debris produced during the process of an "Ariane" rocket entering orbit years before, damaging one of its observation device, and pushing the satellite out of its normal orbit.

On Jan. 17, 2005, wreckage of the Chinese "Long March 4" rocket which was launched six years before, collided with the castoff of an American rocket "Thor" at a speed of 5.73 km per second, lowering the rocket wreckage's perigee orbit by 14 km while breaking the castoff into four pieces.

Since the former Soviet Union sent the first man-made satellite into space in 1957, human space activities have created hundreds of millions of pieces of space junk. If hit with spacecraft, they will cause various degrees of damage to the ships.

Source:Xinhua