WAILEA, Hawaii — Despite the ongoing debate about the orbital debris risks posed by proposed satellite megaconstellations, one expert believes that an even greater risk comes from clusters of objects already in orbit.

In a concluding presentation at the Advanced Maui Optical and Space Surveillance Technologies, or AMOS, conference here Sept. 20, Darren McKnight of Centauri said that hundreds of rocket upper stages left behind in several “clusters” in low Earth orbit, primarily by Russia between 1980 and 2000, are a significant risk because of their size and lack of maneuverability.

“Constellations are not bad,” he said. “Clusters are worse.”

Each of those upper stages, he noted, is large, with masses of 1,000 to 8,000 kilograms, and being dead objects, cannot maneuver. “What you get is amounts of mass in these clusters that greatly surpass the constellation of OneWeb,” he said. “Yet, no ability to avoid themselves.”

McKnight identified three distinct clusters. One, at an altitude of 775 kilometers, affects the most operational satellites, and also includes the European Space Agency’s Envisat satellite, the largest defunct satellite in orbit. The second, at 850 kilometers, contains some of the largest objects and thus could create the largest amount of debris in a collision. The third, at 975 kilometers, is the largest and has the highest probability of a collision.

The threat is not theoretical. In May, two of the rocket bodies in the cluster at 850 kilometers passed within 87 meters of each other, with a relative velocity of 14 kilometers per second. “They’re big yellow school buses with no driver,” he said. “If they collide, it would have doubled the catalog population in one event.”

That population of objects, McKnight argued, makes the case for debris remediation, or the active removal of debris. With all the attention on space situational awareness and space traffic management, he said there needs to also be a focus on “space environment management” that includes not just reducing of creating new debris but also removing the debris already in space, like those rocket bodies.

That issue, he said, is being overlooked for now given the attention on space traffic management. “Nobody’s in charge of it,” he said. “What a surprise we’re not doing anything about it.”

In his talk, he outlined several methods for debris remediation, which goes beyond active debris removal to measures to prevent rocket stages from colliding in orbit. One example is a cubesat “nano-tug” that could attach to a rocket body and both detumble the stage and nudge its orbit.

Other solutions could include relasing a cloud of powder in front of a stage for a just-in-time adjustment of its orbit to avoid a collision, and even space-based lasers to nudge objects, an option he acknowledged would have political challenges. “It may be politically difficult, but if we have a problem, we shouldn’t say there’s no solution, we just have solutions that require some international political dynamics,” he said.

“If we don’t do debris mitigation and remediation, we’re going to make it really hard for SSA and space traffic management,” he said, saying that those activities can’t wait until SSA and space traffic management are improved. “This is all together.”