M-4S R/B

M-4S R/B is a spent rocket body associated with Japan, launched on 1971-02-16 from Uchinoura Space Center, Japan on the MS-T1 launch. After more than 55 years in orbit, it is one of the longest-surviving objects in the space catalogue. It orbits in Low Earth Orbit (LEO) at altitudes between 968 km and 986 km with an inclination of 29.7°. It travels at approximately 26,515 km/h (7.37 km/s), completing one full orbit every 105 minutes — that’s roughly 13.76 orbits per day. At its current altitude, the estimated orbital lifetime before atmospheric re-entry is ~100–500 years. Spent rocket bodies like M-4S R/B are among the largest pieces of uncontrolled space debris and are priority targets for collision avoidance manoeuvres and future active debris removal efforts.

M-4S R/B orbits at an average altitude of 977 km in the upper LEO band, where atmospheric drag is negligible and objects can persist for centuries to millennia. This altitude is used by broadband constellations like OneWeb and by scientific missions requiring stable orbits far from the densest debris bands. Within ±50 km of M-4S R/B’s average altitude, there are currently 370 active payloads and 1,174 tracked debris or rocket body fragments. With an inclination of 29.7°, M-4S R/B passes over latitudes between 29.7°N and 29.7°S, covering the tropical and temperate zones where most of the world’s population resides. Low-to-mid inclination orbits are efficient to reach from equatorial and mid-latitude launch sites. Japan operates approximately 208 active satellites in total, of which 3 share a similar altitude band with M-4S R/B.