WASHINGTON — Two decades-old defunct spacecraft are in danger of colliding Jan. 29, an event experts argue is more evidence of the need to clean up low Earth orbit.

LeoLabs, a California company that operates a network of ground-based radars that track objects in orbit, announced Jan. 27 that it had identified a potential conjunction, or close approach, between the Infrared Astronomical Satellite (IRAS) and the Gravity Gradient Stabilization Experiment (GGSE) 4 satellite in LEO. The company said there was an approximately 1-in-100 chance that the two satellites would collide at 6:39 p.m. Eastern Jan. 29 an altitude of about 900 kilometers, almost directly above the city of Pittsburgh.

LeoLabs, in an update Jan. 28, revised the probability of a collision downward, to about 1 in 1,000, estimating that the two spacecraft will pass between 13 and 87 meters of each other. Other sources have estimated similar probabilities of a collision between the two objects using other data, such as that from the catalog maintained by the U.S. Air Force.

Neither IRAS, launched in 1983, nor GGSE-4, launched in 1967, are operational today and have the ability to maneuver. While close approaches between debris are not uncommon, the circumstances of this event make it unusual and, to some orbital debris experts, worrying.

“This is a little bit unusual,” said Dan Oltrogge, director of the Center for Space Standards and Innovation at Analytical Graphics, Inc., in a Jan. 28 interview. The two spacecraft are in “counterrotating” orbits, meaning a collision would effectively be head-on, at an estimated relative velocity of 14.9 kilometers per second. That would maximize the energy of any collision.

Moreover, IRAS is a large satellite, with a mass of more than 1,000 kilograms. GGSE-4 — also known as POPPY-5B, a signals intelligence satellite — is much smaller, at 85 kilograms. However, it has a boom 18 meters long that will be perpendicular to the direction of motion. “In this case, that tends to maximize the collision potential,” Oltrogge said.

It may not be clear for hours after closest approach if the two satellites avoided a collision, depending on what assets are available to track them. Even if they miss, though, he said the conjunction should serve as a reminder of the hazards that other large objects, both satellites and upper stages, pose in Earth orbit, and the need to remove them.

“Even if these don’t hit, there will be others that will,” he said. “I think this can serve as a wakeup call for us to look at not only avoiding collisions with active satellites, but also remediating, figuring out how to remove debris in orbit.”

A paper that Oltrogge and others presented at the International Astronautical Congress in October 2019 discussed the risks of such collisions. They modeled the collision of two upper stages in orbit at 981 kilometers, concluding it could create between 3,375 and 12,860 objects at least 5 to 10 centimeters in size, as well as more than 200,000 additional debris objects at least 1 centimeter across dubbed “lethal nontrackable” because they are large enough to damage or destroy a satellite but too small to be tracked.

Another co-author of that paper was Darren McKnight of Centauri, who made similar arguments in a presentation at the Advanced Maui Optical and Space Surveillance Technologies conference in September. He noted that, in May, two rocket bodies that are part of a “cluster” of such objects at an altitude of 850 kilometers passed within 87 meters of each other. “They’re big yellow school buses with no driver,” he said. “If they collide, it would have doubled the catalog population in one event.”

“I would hope that we could take this, and other conjunction events and close approaches, to try and get another look at active debris removal and other remediation techniques,” Oltrogge said of this potential conjunction. “But time will tell.”