📘 Definition
A Highly Elliptical Orbit (HEO) has a large difference between its perigee (closest point to Earth) and apogee (farthest point). The classic example is the Molniya orbit: 500 km perigee, 40,000 km apogee, 63.4° inclination, 12 hour period. A satellite in a Molniya orbit spends most of its time near apogee over the northern hemisphere, providing quasi-stationary coverage for high-latitude regions where GEO satellites sit low on the horizon. Used by Russian early warning and communications systems.
500 km (Molniya)
Perigee
40,000 km (Molniya)
Apogee
12 hours (Molniya)
Period
63.4° (critical)
Inclination
Understanding HEO
The Molniya Orbit
Russia's Molniya orbit was designed to provide communications coverage over Arctic and Siberian regions where GEO satellites appear too low above the horizon for reliable reception. At 63.4° inclination (the "critical inclination"), the argument of perigee doesn't precess — keeping apogee locked over the northern hemisphere.
Tundra Orbit
A variation of the Molniya is the Tundra orbit: 24 hour period (like GEO) but with high eccentricity and 63.4° inclination. The UK's proposed Space-Based Early Warning System (SBEWS) would use a Tundra orbit.