Personnel of the U.S. Space Surveillance Network (SSN) have identified two new cases of accidental collisions between cataloged objects from different missions. One collision is recent, having occurred in January 2005, while the other is a much older event which occurred in late 1991 but has just now been recognized.
The subjects of the recent collision were a 31- year-old U.S. rocket body (1974-015B, U.S. Satellite Number 07219) and a fragment (1999-057CV, U.S. Satellite Number 26207) from the third stage of a Chinese CZ-4 launch vehicle, which exploded in March 2000. The event occurred on 17 January 2005 at an altitude of 885 km above the south polar region (Figure 1), a regime in low Earth orbit (LEO) with an above-average satellite population density. Both objects were in similar retrograde orbits at the time of the collision.
Analysis indicates that the orbits of both objects were slightly perturbed at the same time that three debris (subsequently cataloged as U.S. Satellite Numbers 28591-28593) were released from the U.S. rocket body. The rocket body was the relatively small (1 m2) upper portion of a Thor Burner 2A final stage (Figure 2). The fragment of the Chinese rocket body possessed a radar cross-section of only 0.06 m2.
The recently recognized collision of late December 1991 involved a Russian non-functional navigation satellite, Cosmos 1934 (1988-023A, U.S. Satellite Number 18985) and a piece of debris from a sister spacecraft, Cosmos 926. Both objects were in similar orbits with a mean altitude of 980 km and an inclination of 83°. Two pieces of debris (1988- 023C, U.S. Satellite Number 21912 and 1988-023D, U.S. Satellite Number 22919) from Cosmos 1934 were discovered by the SSN within a few weeks of the event, although they were not cataloged until later.
In this case the smaller impacting object apparently broke-up into much smaller debris and was no longer trackable by the SSN, i.e., it could not be found after the collision. The event was only recognized recently when SSN specialists were examining historical tracking data.
The first recognized collision between cataloged objects from different missions involved an operational spacecraft and a fragment from a launch vehicle upper stage which had suffered a postmission breakup. In that event, which occurred on 24 July 1996, the French CERISE spacecraft (1995-033B, U.S. Satellite Number 23606) collided with a fragment (1986-019RF, U.S. Satellite Number 18208) from the third stage of an Ariane 1 launch vehicle, which had exploded ten years earlier.
As the number of objects in Earth orbit increases, the likelihood of accidental collisions will also increase. Currently, hundreds of close approaches (i.e., passes within less than one kilometer) 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 (Orbital Debris Quarterly News, 5-1, pp. 1-3).
