After drifting undetected in space for 25 years, an experimental satellite launched in 1974 was found using U.S. Space Force tracking data.

The Infrared Calibration Balloon (S73-7) satellite began its journey into the great unknown after its launch on April 10, 1974, as part of the US Air Force Space Test Program. It was originally located in the so-called “Hexagon System”, in which a smaller satellite, S73-7, was deployed from the larger KH-9 Hexagon in space. S73-7 was 26 inches (66 centimeters) wide and began life heading into a 500-mile (800-kilometer) circular orbit.

While in orbit, S73-7 was intended to be inflated and serve as a calibration target for remote sensing equipment. Failing to do so during deployment, the satellite disappeared into the abyss and joined the unwanted space junk until it was rediscovered in April.

Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, shared in an interview with Gizmodo that he reviewed data archives and found that radars had gone off grid not once but twice before the latest discovery. In the 1970s and then again in the 1990s.

“The problem is it can have a very low radar cross-section,” McDowell told Gizmodo in a phone interview. “And maybe what they’re tracking is a dispenser or a piece of a balloon that’s not opening properly, so it’s not metallic and doesn’t show up very well on radar.”

Knowing the location and description of every object in orbit is no easy task, as there are currently more than 20,000 of them. Space debris can be tracked with the help of optical sensors as well as ground-based radars. required, is entered into a satellite catalogue, but exactly what each item is is a complexity. Sensors can detect an object in orbit, but only if it is paired with a satellite that is also on the same path.

“If you have recently acquired orbital data and there aren’t many things that look like orbits, it’s probably an easy match,” McDowell said. “But if there’s a very crowded parameter space and you haven’t seen it in a while, then it’s not that easy to find a match.”

After launch, ground engineers have a good idea of ​​where the satellite is going and how high it needs to drift. With this information in the log, they can look back at their progress and compare it to where the satellite was last reported. But if there are any changes to the original maneuvering plans or if the satellite drifts in orbit, engineers will need to do more work to find it again.

“If you don’t know exactly where the maneuver is, you may have difficulty identifying it,” McDowell said. “If I rewind the object’s trajectory and fast forward for the missing object, will they meet and will the maneuver occur at the point where they meet?”

The discovery is therefore a victory for the men and women trying to find the tens of thousands of lost moons and other debris orbiting our planet. But as more and more satellites go into space, the challenge of knowing exactly what’s out there and what threats it might pose will become even greater.

“If you’re missing one or two objects, it’s not a big risk,” McDowell told Gizmodo. “But you want to do the best job possible.”