EXPLORER 31 (ISIS-X)

EXPLORER 31 (ISIS-X) is an active satellite operated by GSFC (United States), launched on 1965-11-29 from Vandenberg Space Force Base, United States on the Alouette 2 launch. After more than 61 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 503 km and 2,784 km with an inclination of 79.8°. It travels at approximately 25,388 km/h (7.05 km/s), completing one full orbit every 119 minutes — that’s roughly 12.08 orbits per day. Its orbital eccentricity of 0.1423 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 31 (ISIS-X) in real time using the latest two-line element set (TLE) data, providing live position, altitude, speed and orbital path updated continuously.

EXPLORER 31 (ISIS-X) orbits at an average altitude of 1,644 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 31 (ISIS-X)’s average altitude, there are currently 22 active payloads and 163 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 79.8°, EXPLORER 31 (ISIS-X) passes over latitudes between 79.8°N and 79.8°S, covering most populated land masses in both hemispheres. This mid-inclination band balances global coverage with efficient launch energy requirements. United States operates approximately 13,279 active satellites in total, of which 3 share a similar altitude band with EXPLORER 31 (ISIS-X).