EXPLORER 25 (INJUN-4)

EXPLORER 25 (INJUN-4) is an active satellite operated by NASA LaRC (United States), launched on 1964-11-21 from Vandenberg Space Force Base, United States on the AD-B launch. After more than 62 years in orbit, it is one of the longest-surviving objects in the space catalogue. It orbits in Medium Earth Orbit (MEO) at altitudes between 524 km and 2,261 km with an inclination of 81.3°. It travels at approximately 25,795 km/h (7.17 km/s), completing one full orbit every 114 minutes — that’s roughly 12.67 orbits per day. Its orbital eccentricity of 0.1119 gives it a noticeably elliptical path, with significant altitude variation between perigee and apogee. At its current altitude, the estimated orbital lifetime before atmospheric re-entry is thousands of years. Orbital Radar tracks EXPLORER 25 (INJUN-4) in real time using the latest two-line element set (TLE) data, providing live position, altitude, speed and orbital path updated continuously.

EXPLORER 25 (INJUN-4) orbits at an average altitude of 1,393 km in Medium Earth Orbit, the region between LEO and GEO (2,000–35,786 km). MEO’s higher altitude gives each satellite a much larger ground footprint than LEO, meaning fewer spacecraft are needed for global coverage — but signal latency is higher and radiation from the Van Allen belts is a significant design constraint. Within ±50 km of EXPLORER 25 (INJUN-4)’s average altitude, there are currently 284 active payloads and 260 tracked debris or rocket body fragments. With an inclination of 81.3°, EXPLORER 25 (INJUN-4) passes over latitudes between 81.3°N and 81.3°S, providing near-global coverage including the polar regions. Polar and near-polar orbits are used for reconnaissance, weather monitoring and Earth-observation missions that need to image every part of the planet. United States operates approximately 13,276 active satellites in total, of which 13 share a similar altitude band with EXPLORER 25 (INJUN-4).