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Popular Orbits 101 by AS

Discover the air and space domains the endless void of what is home to countless satellites, yet most of these human-engineered objects are confined to just three orbital pathways: low Earth orbit, medium Earth orbit, and geosynchronous orbit.

Low Earth Orbit (LEO)

Just a stone's throw away from Earth, cosmically speaking, lies low Earth orbit (LEO). This bustling region of space houses the largest population of satellites, all orbiting between 160 and 2,000 kilometers above our planet. LEO satellites are speedy travelers, completing one lap around Earth in as little as 90 minutes. Thanks to their prime viewing spot, these satellites excel at capturing images of our planet and gathering crucial data. In fact, a whopping fifty-five percent of all active satellites call LEO their cosmic address

Polar Orbits

Some LEO satellites orbit such that they pass over (or nearly pass over) both of the Earth’s poles during orbit. This highly-inclined, low-altitude orientation is called a polar orbit. Due to the rotation of the Earth, satellites in polar orbit pass over a different vertical swath of the planet’s surface on each revolution. Using a polar orbital regime, a single satellite could observe every point on Earth twice in one 24-hour day.

Satellite constellations

Although a single satellite in polar orbit can eventually observe every point on Earth’s surface, it is not able to create a snapshot of the Earth; that is, a composite image of every inch of the planet’s surface at a single moment. Such a capability can only be completed in low Earth orbit by a large collection of satellites, or a satellite constellation. Satellite constellations can be utilized for personal communications, early missile-warning, and space-based weapons systems.

Medium Earth Orbit (MEO)

Although over 90 percent of all satellites are situated in LEO (below 2,000 kilometers) and GEO (near 36,000 kilometers), the space between the two most popular orbital regimes can be an ideal environment for a smaller subset of satellite systems. Satellites in this middle-of-the-road region, appropriately named medium Earth orbit, have larger footprints than LEO satellites (meaning they can see more of the Earth’s surface at a time) and lower transmission times time than GEO satellites (meaning they have a shorter signal delay because they aren’t as far away).

Van Allen Radiation Belt

One reason there are fewer satellites in MEO than LEO or GEO is the presence of the Van Allen belts. The Van Allen belts are two doughnut shaped regions surrounding the Earth, centered on its polar axis, where the Earth’s magnetic field traps charged particles from solar winds and cosmic rays, which can damage satellites’ onboard electronic systems. High radiation environments in general can also damage solar arrays, which convert energy to electricity to power satellites after they have achieved their desired orbit.

The inner belt extends from roughly 500 km to 5,500 km at the equator and the outer belt extends from 12,000 km to 22,000 km. Satellites in these regions can be outfitted with shielding to lower the risk of damage during their operational lifetime.

Highly Elliptical Orbits (HEO)

Although many of the orbits previously discussed assume a circular or nearly circular path around the Earth, some satellites are situated such that they orbit the Earth in an oblong elliptical path, called highly elliptical orbit, or HEO. While, a satellite in an inclined circular orbit spends an equal amount of time in the northern and southern hemispheres, satellite in inclined HEO spends a significantly greater portion of its orbit over one hemisphere than the other, due to Kepler’s second law of planetary motion.

MEO and HEO. Medium Earth Orbit (MEO), is shown in red, and Highly Elliptical Orbit (HEO), in green.

Geosynchronous Earth Orbit (GEO)

The period of a satellite, or how long it takes to orbit the Earth one time, is dependent on its orbital altitude. Satellites in LEO, like the International Space Station, take about 90 minutes to orbit the Earth. Satellites in MEO take about 12 hours to do the same.

Satellites orbiting at 35,786 km have a period precisely equal to one day. Satellites in this orbit, known as geosynchronous Earth orbit, or GEO, observe the Earth as if it were not rotating. Because of this property, satellites in GEO are constantly in the field of view for approximately one-third the planet’s surface.

While about 55 percent of all operational satellites are in LEO, another 35 percent are in GEO, making it the second most popular orbital regime.

Geostationary Orbit

Geosynchronous orbit is significantly farther away from Earth than LEO or MEO. For satellites located in LEO, the time it takes for a signal to be transmitted from the ground to a satellite and back is approximately 0.003 seconds. For a satellite in GEO, that delay increases to 0.25 seconds, requiring echo control and time delay considerations that are more negligible for lower altitude orbits.

Other Orbits

Some orbits have special properties that make them ideal for specific satellite missions. Only a small fraction of operational satellites fall into this category.

Sun-Synchronous Orbit

Because of irregularities in Earth’s gravitational field, satellite orbits around the Earth precess over time, meaning the orbital plane slowly rotates about one of planet’s axes. When a satellite orbit’s precession aligns with the rotation of the Earth about the Sun, the satellite is in sun-synchronous orbit, or SSO. When satellites in SSO pass over a given point on Earth, they observe it at the same local time on each orbit. Such conditions are ideal for Earth imaging systems. Some SSOs are oriented such that the satellite’s solar arrays are constantly facing the sun, lessening their dependence on onboard batteries. Satellites in SSO may also be in LEO or MEO.

Lagrange Points

Lagrange points are special locations in the Earth-Sun orbital plane in which satellites orbit the Sun while maintaining a fixed position relative to Earth’s center of gravity. Lagrange points are caused by the balance between the gravitational fields of two large bodies; equilibria between two pulling forces. Of the five Lagrange points in the Earth-Sun system, three are unstable (L1, L2, and L3), meaning they require periodic orbital adjustments to maintain their position, and two are stable (L4 and L5), meaning no adjustments are required.

Lagrange points, especially L2, are particularly well situated for scientific studies of the universe, including NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and European Space Agency’s Planck mission.

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