An experimental satellite launched in 1974 disappeared from ground-based sensors in the 1990s and was found again this week. Some defunct satellites or debris can often be lost for years, posing a hazard in an increasingly crowded Earth orbit. But how exactly do objects in space disappear?
The Infrared calibration balloon The satellite (S73-7) was part of the US Air Force’s space test program. After launch on April 10, 1974, a large reconnaissance satellite called KH-9 Hexagon ejected the 26-inch-wide (66-centimeter-wide) satellite and placed it into a 500-mile (800-kilometer) circular orbit.
The tiny satellite was intended to inflate into orbit and serve as a calibration target for remote sensing devices. However, its mission failed and it became another piece of space junk. Reviewing the satellite’s archival data, Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, discovered that it had disappeared before. Radars tracked him in the 1970s before he disappeared, and then again in the 1990s before he disappeared again.
The satellite was rediscovered earlier this week after not being tracked for the past 25 years, according to Space Force tracking data 18th Space Defense Squadron. “The problem is that it may have a very low radar cross-section,” McDowell told Gizmodo by phone. “And maybe the thing they’re tracking is a donor or a part of the balloon that didn’t deploy properly, so it’s not metal and it’s not clearly visible on radar.”
Ground-based radar and optical sensors track more than 20,000 objects in orbit, and that can get pretty difficult. There is a global network of sensors that relay information to a current catalog of satellites, but most objects do not communicate their identities. Instead, the sensors rely on identifying the orbit of a moving object and matching it with the intended orbit of a satellite.
“It’s basically like air traffic control,” McDowell said. “All this stuff is floating around and when you try to fly through it, you want to know what the dangers are.”
After a satellite is launched, engineers on the ground know approximately where it is and at what altitude it will drift. If an object is found in the specified area, they can rewind that orbit and see if it matches the orbit where the satellite was last seen.
“If you have a current orbital data set and there aren’t too many things that have a similar orbit, the match is probably easy,” McDowell said. “But if it’s a very crowded parameter space and you haven’t seen it for a long time, it’s not so easy to assign it.”
Tracking satellites in geostationary orbit – a circular orbit directly above the equator – can be challenging because there are no radars to accurately monitor objects at the equator. “There is actually a gap in tracking,” McDowell said. “Hugging the equator allows you to hide from persecution.”
If a satellite also performs an unexpected maneuver, engineers are forced to search for it in Earth orbit. “If you don’t know exactly where the maneuver occurred, it can be difficult to locate,” McDowell said. “If I rewind the orbit of one object and fast forward for the missing object, do they meet, and is the point where they meet the place where the maneuver occurred?”
Most things lost in space are either broken satellites or broken fragments of debris. The Department of Defense’s Global Space Surveillance Network is currently operational persecution More than 27,000 objects in orbit, most of which are spent rocket engines and operational and dead satellites.
As Earth’s orbit becomes more crowded with a growing number of satellite constellations and rocket launches, keeping track of all of these objects becomes increasingly important.
“If you miss one or two objects, it’s not a big risk,” McDowell said. “But you want to do your job as well as possible.”
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