Highly elliptical orbit

A highly elliptical orbit (HEO) or highly eccentric orbit is an orbit of one body about another with high eccentricity, usually referring to one around Earth.

Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, and Tundra orbits. Many US satellites also have used these orbits, satellites such as the Trumpet electronics intelligence satellites.

The acronym HEO normally is expanded to Highly Eccentric Orbit by orbital analysts since all orbits around planets, etc are ellipses - the term "highly elliptical" is not very clear as to what is exaggerated. It would be more proper to call these orbits "elongated" than "highly elliptical".

Highly eccentric orbits have two main uses - as transfer orbits and as good orbits for communication with or surveillance of the Polar regions. The transfer orbit was proposed by the German scientist Walter Hohmann in 1925, it connects two circular orbits, a lower one and a higher one, with an eccentric orbit. There are many HEO orbits where a launch vehicle initially injects a spacecraft into a LEO orbit and then an upper stage injects the payload, intended to go to geosynchronous orbit, into a HEO transfer orbit and then a final stage or engine circularizes the payload in the intended geosynchronous orbit.

When chosen for communications or surveillance, these extremely elongated orbits also can provide long dwell times at a point in the sky during the approach to, and descent from, apogee. Bodies moving through the long apogee dwell appear to move slowly, and remain at high altitude over Polar latitude ground sites for long periods of time.