In a unique case of space bumper cars, two pieces of rocket hardware have collided high above Earth. The orbital run-in involved a 31-year-old U.S. rocket body and a fragment from a more recently launched Chinese rocket stage.

The collision occurred on January 17 of this year, with the incident happening some 550 miles (885 kilometers) above Earth. That area of low Earth orbit (LEO) has an above-average satellite population density. The American and Chinese space hardware cruised through space in similar orbits at the time of the rear-ender.

The U.S. Surveillance Network of space-watching gear detected the collision, with the episode reported in the April issue of The Orbital Debris Quarterly News, a publication of the NASA Orbital Debris Program Office at the Johnson Space Center in Houston, Texas.

Involved in the collision was a discarded U.S. Thor Burner 2A upper stage used to toss a satellite payload into Earth orbit back in 1974.

The other piece was a fragment of the third stage of a Chinese CZ-4 launch vehicle exploded in March 2000.

When the objects smacked into each other, analysis indicates that the orbits of both were slightly perturbed at the same time that three more chunks of debris - large enough to be detected and cataloged - were released from the U.S. rocket body.

The Orbital Debris Quarterly News also reports another accidental collision.

This one took place in late December 1991. In this case, a Russian non-functional navigation satellite, Cosmos 1934, had a run-in with a piece of junk from a sister spacecraft, Cosmos 926. The event was only recognized recently when U.S. Space Surveillance Network specialists were examining historical tracking data. Debris resulted from this collision too, but the fragments were too small to be tracked.

Sobering note

As noted by the NASA newsletter on orbital debris, the first recognized fender-bender between cataloged objects from different missions involved an operational spacecraft and a fragment from a launch vehicle upper stage which had suffered a post-mission breakup.

In that event -- which happened on July 24, 1996 -- the French CERISE spacecraft collided with a fragment from the third stage of an Ariane 1 booster, which had exploded ten years earlier.

Looking into the future, the Orbital Debris Quarterly News adds this sobering note:

"As the number of objects in Earth orbit increases, the likelihood of accidental collisions will also increase. Currently, hundreds of close approaches...between cataloged objects occur on a daily basis. If future spacecraft and rocket bodies are not removed from LEO within a moderate amount of time after the end of mission, e.g., within 25 years, the rate of accidental collisions will increase markedly later in this century."