The European Space Agency’s Envisat Earth-observation satellite had a close encounter Jan. 21 with a discarded Chinese upper-stage rocket body, according to HeinerKlinkrad, head of the European Space Agency’s (ESA) Space Debris Office in Darmstadt, Germany.

A head-on collision between the 8-ton Envisat and nearly 4-ton Chinese rocket stage would have produced a major orbital debris issue for other operational satellites, Klinkrad said Feb. 6 during the 33rd annual Guidance and Control Conference  in Breckenridge, Colo.

Klinkrad told Space News that ESA received advance warning of the near miss from the U.S. Joint Space Operations Center,  which  uses the Space Surveillance Network of radar and optical telescopes to track objects in space.

An independent assessment of the Chinese upper stage’s orbit, Klinkrad said, was done for ESA by operators of the German Tracking and Imaging Radar system, located at the Research Establishment for Applied Science near Bonn, Germany.

“That then gave us a flyby distance of 48 meters total, in the radial direction,” Klinkrad said, a result confirmed by the U.S. Joint Space Operations Center. The collision risk was more than 1 in 70, “which is the highest for such a big object that we’ve had so far.”

Klinkrad said ground operators fired Envisat’s thrusters twice in an avoidance maneuver that increased the flyby distance to about 200 meters.

If the two objects had hit, there would have been a high chance of doubling the catalog of human-made objects orbiting Earth, Klinkrad said, which includes active and inactive satellites, spent rocket bodies, debris and fragments. “And that would be at an altitude where we have big problems already,” he said.