Low Earth orbit (LEO)

Low Earth orbit (LEO) refers to orbits around Earth that range from about 2,000 kilometers (1,200 miles) down to about 160 kilometers (99 miles) above the planet’s surface. Low Earth orbit is most commonly used for commercial and government satellites. It provides many advantages over other orbits, avoiding the radiation damage possible at higher altitudes and allowing satellites to rapidly circle the planet. Additionally, placing objects in higher orbits is significantly more costly than placing them in lower orbits. However, objects placed in low Earth orbit will suffer from atmospheric drag, which may cause them to gradually slow until they crash or incinerate in the atmosphere. For this reason, satellites that seek to maintain a stable orbit must use propulsion systems to periodically adjust their course.

Because most satellites are launched into low Earth orbit, the region has become crowded with space debris. Much of this space junk is dead satellites, pieces of satellites, or discarded rocket boosters. Scientists believe the debris may make safely launching satellites into low Earth orbit more difficult in the future. Debris falling from a low Earth orbit is also a concern for those on the ground.

Background

An orbit is a curved path an object in space takes around another massive object. Orbits are produced by gravity, the force that causes objects with mass to attract one another. If two objects of similar mass encounter each other in space, they may enter into an orbit with neither object at the center. If a smaller object encounters a larger object, the smaller object can enter orbit around the larger one. For example, Earth and all the planets orbit around the sun, while the moon and numerous artificial satellites orbit around Earth.

A smaller object that orbits a larger object is called a satellite. Natural satellites are celestial bodies that orbit other celestial bodies. Moons are the most common natural satellites in the solar system. Every planet except for Mercury and Venus has at least one natural satellite. Saturn has the most moons in the solar system with eighty-two, while Jupiter has seventy-nine. Orbits are elliptical, which means they have oval-like shapes. Some are nearly circular, while others are more elongated, like a squished circle.

An orbit occurs when two objects in space reach a balance between an object’s gravitational pull and forward momentum. Objects in space move very rapidly. When two objects move near one another, gravity pulls them together. However, the force of the objects’ forward momentum stops them from colliding with one another. Instead, the object with the weaker gravitational pull begins to circle the other. For an orbit to form, inertia and gravity must be balanced. If the gravitational pull is too strong, the two objects will collide. If the inertia is too strong, the two objects will speed away from one another. Most orbits will eventually end similarly. For example, Earth’s Moon is gradually spinning away from the planet and will eventually end its orbit by escaping Earth’s gravitational pull.

Overview

Low Earth orbits are commonly defined as orbits ranging from about 160 kilometers to 2,000 kilometers above Earth’s surface. Although 160 kilometers is about ten times higher than commercial airliners fly, that point is still the lowest altitude at which a stable orbit can be formed. Objects orbiting below the altitude suffer from orbital decay. Orbital decay means that the orbit will gradually become unstable, leading to the object breaking away from the larger object or smashing into it. Smaller orbiting objects are more likely to burn up in Earth’s atmosphere than crash to its surface.

Objects in low-earth orbit also suffer from atmospheric drag. This means they experience friction, a force moving against the forward motion of the object, as they pass through Earth’s atmosphere. Over time, atmospheric drag pulls objects back towards Earth. For this reason, in order to maintain a stable orbit, artificial satellites must periodically take corrective measures to alter their orbital paths. These corrective measures typically involve the use of onboard propulsion systems.

Similarly, objects that travel higher than 1,000 kilometers (620 miles) above Earth suffer problems with the Van Allen radiation belts. These belts are regions of charged particles from solar winds and cosmic rays trapped in the upper atmosphere. For this reason, most commercial and government satellites are sent into orbits where they will not have to encounter this radiation.

Most human space missions involve traveling or sending equipment to low Earth orbit. For example, the International Space Station (ISS) orbits Earth at an average height of 400 kilometers (250 miles) above the surface. Most commercial and government satellites do not need to travel farther than low Earth orbit to accomplish their goals. Low Earth orbit provides enough altitude for observational or spy satellites to be difficult to spot from the surface, while still providing a clear view of the planet. As an additional benefit for observational satellites, or any satellite that must periodically drift over different portions of the Earth, a low Earth orbit allows satellites to orbit the entire planet in a period of one to two hours. Higher orbits are larger, and orbit at a slower rate.

Finally, though there may be some advantages to placing satellites at higher orbits, the process is significantly more expensive. Launching an object farther into space requires more fuel, and is often much more difficult. Many governments and private space companies have come to the conclusion that low Earth orbit is the most economically feasible.

However, due to the popularity of low Earth orbits, the area has become congested with space debris. Most of this debris is made up of functioning satellites, the remains of decommissioned satellites, and discarded rocket boosters from space missions. As this debris continues to accumulate, it may make launching new missions into low orbit increasingly difficult. Additionally, large debris chunks falling from low Earth orbit may pose risks to people on Earth. In the future, it may be necessary to remove space junk from Earth’s atmosphere.

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