Popular Orbits 101

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.

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.

With the exception of the Apollo missions, all human space activity has occurred in low Earth orbit. The International Space Station, which currently hosts six astronauts, has an average altitude of about 350 kilometers.

 

The Iridium satellite constellation is a 66-satellite full-coverage constellation orbiting at about 780 kilometers. Iridium provides personal communication services through satellite phones.

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.

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.

The Global Positioning System (GPS) is a 24-satellite constellation, with each satellite in circular MEO with an altitude of about 20,000 km. The constellation is oriented such that at any given moment, every point on Earth has access to four GPS satellites.

One example of a highly elliptical orbit is a Molniya orbit. Molniya orbits have an inclination of 63.4 degrees, an apogee of about 40,000 km and a perigee of about 1,000 km. For the majority of their periods, satellites in Molniya orbit are primarily observing the northern hemisphere of the Earth. Since both the United States and Russia are in the northern hemisphere, Molniya orbits were ideal for reconnaissance in the Cold War Era.

Most satellites in GEO have no inclination, meaning they orbit directly above Earth’s equator. Satellites with these properties are called geostationary, since from any point on Earth they appear in precisely the same point in the sky at any given moment. Geostationary satellites are popular for communications and broadcasting.

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.

Some examples of satellites in GEO include the Intelsat communications satellites and the DISH Network direct broadcasting satellites.

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 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.