Low Earth orbit

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A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never more than about one-third of the radius of Earth.

The term LEO region is also used for the area of space below an altitude of 2,000 km (1,200 mi) (about one-third of Earth's radius). Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital, are carefully tracked since they present a collision risk to the many LEO satellites.

All crewed space stations to date have been within LEO. From 1968 to 1972, the Apollo program's lunar missions sent humans beyond LEO. Since the end of the Apollo program, no human spaceflights have been beyond LEO.

Defining characteristics

A wide variety of sources define LEO in terms of altitude. The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as 30 km (19 mi) (especially for polar orbits) due to the oblateness of Earth's spheroid figure and local topography. While definitions based on altitude are inherently ambiguous, most of them fall within the range specified by an orbit period of 128 minutes because, according to Kepler's third law, this corresponds to a semi-major axis of 8,413 km (5,228 mi). For circular orbits, this in turn corresponds to an altitude of 2,042 km (1,269 mi) above the mean radius of Earth, which is consistent with some of the upper altitude limits in some LEO definitions.

The LEO region is defined by some sources as a region in space that LEO orbits occupy. Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee) but are not in an LEO orbit because their highest altitude (or apogee) exceeds 2,000 km (1,200 mi). Sub-orbital objects can also reach the LEO region but are not in an LEO orbit because they re-enter the atmosphere. The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits.

Orbital characteristics

The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 kilometres per second (17,000 mph), but reduces for higher orbits. Calculated for a circular orbit of 200 kilometres (120 mi) it is 7.79 km/s, and for 1,500 kilometres (930 mi) it is 7.12 km/s. The launch vehicle's delta-v needed to achieve low Earth orbit starts around 9.4 km/s.

The pull of gravity in LEO is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is much less than the Earth's radius. However, an object in orbit is in a permanent free fall around Earth, because in orbit both, the gravitational force and the centrifugal force balance out each other. As a result, spacecrafts in orbit continue to stay in orbit, and people inside or outside such crafts continuously experience weightlessness.

Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–600 km above the surface) or exosphere (approximately 600 km or 400 mi and higher), depending on orbit height. Orbits of satellites that reach altitudes below 300 km (190 mi) decay fast due to atmospheric drag. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt.

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times of low-latitude places on Earth and have the lowest delta-v requirement (i.e., fuel spent) of any orbit, provided they have the direct (not retrograde) orientation with respect to the Earth's rotation. Orbits with a very high inclination angle to the equator are usually called polar orbits or Sun-synchronous orbits.

Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation.

In 2017, "very low Earth orbits" (VLEO) began to be seen in regulatory filings. These orbits, below about 450 km (280 mi), require the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.

Use

Roughly half an orbit of the International Space Station.

A low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communication latency. Satellites and space stations in LEO are more accessible for crew and servicing.

Since it requires less energy to place a satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as the Iridium phone system. Some communication satellites use much higher geostationary orbits and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

Disadvantages

Unlike geosynchronous satellite, satellites in LEO have a small field of view and so can observe and communicate with only a fraction of the Earth at a time. That means that a network (or "constellation") of satellites is required to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay and require either periodic re-boosting to maintain a stable orbit or launching replacement satellites when old ones re-enter.

Examples

• The International Space Station is in a LEO about 400 km (250 mi) to 420 km (260 mi) above Earth's surface, and needs re-boosting a few times a year due to orbital decay.
• The Iridium telecom satellites orbit at about 780 km (480 mi).
• Earth observation satellites, also known as remote sensing satellites, including spy satellites and other Earth imaging satellites, use LEO as they are able to see the surface of the Earth more clearly by being closer to it. A majority of artificial satellites are placed in LEO. Satellites can also take advantage of consistent lighting of the surface below via Sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. Envisat (2002–2012) is one example.
• The Hubble Space Telescope orbits at about 540 km (340 mi) above Earth.
• The Chinese Tiangong space station was launched in April of 2021 and currently orbits between about 340 kilometres (210 mi) and 450 kilometres (280 mi).

In fiction

• In the film 2001: A Space Odyssey, Earth's transit station ("Space Station V") "orbited 300 km above Earth."

Former

• The Chinese Tiangong-1 station was in orbit at about 355 kilometres (221 mi), until its de-orbiting in 2018.
• The Chinese Tiangong-2 station was in orbit at about 370 km (230 mi), until its de-orbiting in 2019.
• Gravimetry missions such as GOCE orbited at about 255 km (158 mi) to measure Earth's gravity field at highest sensitivity. The mission lifetime was limited because of atmospheric drag. GRACE and GRACE-FO were orbiting at about 500 km (310 mi).

Space debris

The LEO environment is becoming congested with space debris because of the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can be dangerous or deadly. Collisions can produce additional space debris, creating a domino effect known as Kessler syndrome. The Combined Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), tracks around 8,500 objects larger than 10 cm in LEO. According to an Arecibo Observatory study, there may be one million dangerous objects larger than 2 millimeters in orbit, which are too small to be visible from Earth-based observatories.

See also

• Comparison of orbital launch systems
• Geostationary orbit (GEO)
• Heavy lift launch vehicle
• High Earth orbit (HEO)
• Highly elliptical orbit (HEO)
• List of orbits
• Medium Earth orbit (MEO)
• Medium-lift launch vehicle
• Specific orbital energy examples
• Suborbital spaceflight
• Ukrainian Optical Facilities for Near-Earth Space Surveillance Network
• Van Allen radiation belt