M-4S R/B

M-4S R/B is a spent rocket body associated with Japan, launched on 1971-09-28 from Uchinoura Space Center, Japan on the MS-F2 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 867 km and 1,748 km with an inclination of 32.0°. It travels at approximately 25,938 km/h (7.20 km/s), completing one full orbit every 112 minutes — that’s roughly 12.89 orbits per day. At its current altitude, the estimated orbital lifetime before atmospheric re-entry is thousands of 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 1,308 km in the uppermost reaches of Low Earth Orbit. At this altitude, orbital decay is effectively zero without active deorbiting, and coverage footprints are significantly larger than lower LEO, though at the cost of higher latency. Within ±50 km of M-4S R/B’s average altitude, there are currently 20 active payloads and 381 tracked debris or rocket body fragments. This is a relatively sparse altitude band, containing less than 1% of all active satellites. With an inclination of 32.0°, M-4S R/B passes over latitudes between 32.0°N and 32.0°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 2 share a similar altitude band with M-4S R/B.