The Infra-Red Calibration Balloon (S73-7), launched on April 10, 1974 by the US Air Force Space Test Program, is a satellite that has been missing from radar since the 1990s. Originally intended to be a calibration target for remote sensing equipment, the mission failed and the satellite became space junk until it was rediscovered recently.
According to astrophysicist Jonathan McDowell, S73-7 disappeared from radar twice, first in the 1970s and then in the 1990s. Tracking objects in orbit is challenging due to the large number of objects, over 20,000, currently in orbit. Ground-based radar and optical sensors help track space junk, but identifying each object remains difficult.
Engineers can monitor the progress of a satellite based on its original path and altitude, but deviations from the plan or drifts can make finding it again a challenge. The rediscovery of S73-7 is a success in tracking lost satellites and space junk but with more satellites being launched, understanding what is in orbit and potential threats will become increasingly difficult.
As McDowell explains, missing a few objects may not pose a significant risk, but the goal is to do the best possible job of tracking objects in space. The discovery of lost satellites like S73-7 is a step towards improving our knowledge of what is in orbit around Earth.