HEOs

HEOs, in spaceflight terminology, most often stands for Highly Elliptical Orbit or High Earth Orbit, referring to a class of Earth-centered orbits with notably stretched shapes. These orbits are characterized by an eccentric trajectory that has a low-altitude perigee and a far higher apogee, placing the satellite well above typical low Earth orbit levels for portions of its path.

Key features include a wide range of eccentricities, commonly from about 0.2 to well over 0.5, and perigee distances that keep the spacecraft relatively close to Earth while allowing extended dwell times near apogee. Orbital periods can vary from several hours to roughly a day or more, depending on the apogee height. Because the apogee often lies above the mid to high latitudes, HEOs are especially valued for communications and observation in northern regions where coverage from geostationary orbit is limited.

Molniya-type orbits are a well-known subset of HEOs, typically with an inclination near 63.4 degrees and a 12-hour period, designed to maximize time over high latitudes. Other HEO configurations include 24-hour or longer-period orbits (sometimes termed Tundra-like) to sustain extended ground footprint over specific areas.

Applications of HEOs include communications networks serving high-latitude regions, reconnaissance, and weather or Earth-observation missions that benefit from prolonged apogee passes. Limitations involve complex ground-track planning, higher radiation exposure, and more demanding propulsion requirements compared with shorter, more regular orbits.