BOULDER, Colo . — In the latest reported instance of a space collision that created more debris in low Earth orbit, pieces of a U.S. and a Chinese rocket 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.

According to the April issue of Orbital Debris Quarterly News, the collision occurred Jan. 17 at an altitude about 885 kilometers (550 miles) above the Earth’s surface. That area of low Earth orbit has an above-average satellite population density.

The U.S. Surveillance Network detected the collision, according to the Quarterly, which is a publication of the NASA Orbital Debris Program Office at the Johnson Space Center in Houston, Texas.

The collision involved a discarded U.S. Thor Burner 2A upper stage that was used to launch 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 that exploded in March 2000.

When the objects smacked into each other, analysis indicates that the orbits of both were slightly altered 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 reported the discovery of 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 collision between cataloged objects from different missions involved an operational spacecraft and a fragment from a launch vehicle upper stage that had suffered a post-mission breakup.

In that event — which happened July 24, 1996 — the French CERISE spacecraft collided with a fragment from the third stage of an Ariane 1 booster, which had exploded 10 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 [low Earth orbit] 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.”