Highly Elliptical Orbit (HEO)

A Highly Elliptical Orbit (HEO) is a stretched-out, oval path around Earth that lets a satellite hang for hours over one part of the planet, then race past on the far side. It is the clever trick that brought TV, radio, and weather data to the far north, where ordinary satellites struggle.

Quick facts

• Eccentricity (how stretched the oval is, from 0 for a circle to nearly 1 for a long ellipse): typically 0.5 to 0.9.
• Molniya orbit (the classic HEO): perigee, the low point, about 600 km up; apogee, the high point, about 39,700 to 40,000 km up; period about 12 hours; inclination 63.4 degrees.
• Tundra orbit (a 24-hour cousin): period about 24 hours; inclination about 63.4 degrees; eccentricity about 0.2 to 0.3.
• Speed varies enormously: fastest at perigee (several km/s) and slowest at apogee (a bit under 4 km/s for a Molniya orbit), which is why the satellite lingers up high.
• Dwell time: a Molniya satellite stays visible from one high-latitude ground station for 8-plus hours of its 12-hour orbit; a Tundra satellite dwells 16 to 18 hours over its target.

How it works

An HEO traces a long oval rather than a near-perfect circle. A 17th-century rule called Kepler’s second law says a satellite sweeps out equal areas in equal times, which simply means it speeds up when it is close to Earth and slows down when it is far away. So the spacecraft spends most of each orbit loitering high above one region near apogee, then whips quickly through the low perigee pass on the opposite side of the planet. Think of a stone on a string swung in a lopsided loop: it crawls at the far end and snaps through at the near end.

The 63.4-degree tilt, or inclination, is not arbitrary. Earth is slightly bulged at the equator, and that bulge normally drags the orbit’s high point slowly around the planet. At exactly 63.4 degrees a key term in the math (4 minus 5 times the square of the sine of the inclination) becomes zero, so the high point stops drifting. This is called a frozen orbit, and it keeps the apogee parked over the chosen region without burning fuel to hold it there. Aiming that apogee over the Northern Hemisphere makes the satellite hover over high northern latitudes hour after hour.

Why it’s used

Geostationary satellites sit over the equator, so from the Arctic, Russia, Canada, northern Europe, or Alaska they appear low on the horizon or vanish entirely. An HEO solves this by giving long, high-elevation coverage of exactly those high-latitude places. Science missions love HEO too: a long, uninterrupted stretch at high altitude is ideal for astronomy or for studying Earth’s magnetic environment and the solar wind, while the low perigee makes data downlink easy and the launch cheaper. HEO is also easier to reach than geostationary orbit from far-north launch sites. Because no single satellite covers a region the whole time, a typical communications setup uses three Molniya satellites, timed so at least one is always near apogee over the service area.

Notable missions

• Molniya 1/2/3 series (Soviet/Russian, from 1965): the namesake satellites that established the orbit for high-latitude communications.
• Sirius Satellite Radio (about 2000-2016): three satellites in Tundra orbits delivering digital radio across the continental US and Canada.
• Arktika-M (Russian, from 2021): Molniya-type Arctic weather and climate monitoring; apogee about 39,750 km, perigee about 1,043 km, eccentricity about 0.7.
• Chandra X-ray Observatory (NASA, 1999): apogee about 140,000 km and perigee about 10,000 km, giving long observing runs above Earth’s radiation belts.
• TESS (NASA, 2018): an exoplanet hunter in a high lunar-resonant orbit, apogee about 373,000 km, perigee about 108,000 km, eccentricity about 0.54.
• ESA/CAS SMILE (launched May 2026): a mission studying how the solar wind interacts with Earth’s magnetosphere from an HEO with an apogee near 120,000 km.

The trade-off is complexity: HEO demands a small fleet rather than one satellite, ground antennas that steer to track the moving target, and electronics tough enough to survive repeated passes through the radiation belts. In exchange, it delivers something a single equatorial satellite never can: hours of steady, high-up coverage over the top of the world.