Beginning in the 1950s with the Sputnik, Vostok and Mercury programs, human beings began to “slip the surly bonds of Earth”. And for a time, all of our missions were what is known as Low-Earth Orbit (LEO). Over time, with the Apollo missions and deep space missions involving robotic spacecraft (like the Voyager missions), we began to venture beyond, reaching the Moon and other planets of the Solar System.
But by and large, the vast majority of missions to space over the years – be they crewed or uncrewed – have been to Low-Earth Orbit. It is here that the Earth’s vast array of communications, navigation and military satellites reside. And it is here that the International Space Station (ISS) conducts its operations, which is also where the majority of crewed missions today go. So just what is LEO and why are we so intent on sending things there?
Technically, objects in low-Earth orbit are at an altitude of between 160 to 2,000 km (99 to 1200 mi) above the Earth’s surface. Any object below this altitude will being to suffer from orbital decay and will rapidly descend into the atmosphere, either burning up or crashing on the surface. Objects at this altitude also have an orbital period (i.e. the time it will take them to orbit the Earth once) of between 88 and 127 minutes.
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Objects that are in a low-Earth orbit are subject to atmospheric drag since they are still within the upper layers of Earth’s atmosphere – specifically the thermosphere (80 – 500 km; 50 – 310 mi), theremopause (500–1000 km; 310–620 mi), and the exosphere (1000 km; 620 mi, and beyond). The higher the object’s orbit, the lower the 1atmospheric density and drag.
However, beyond 1000 km (620 mi), objects will be subject to Earth’s Van Allen Radiation Belts – a zone of charged particles that extends to a distance of 60,000 km from the Earth’s surface. In these belts, solar wind and cosmic rays have been trapped by Earth’s magnetic field, leading to varying levels of radiation. Hence why missions to LEO aim for attitudes between 160 to 1000 km (99 to 620 mi).
Within the thermosphere, thermopause and exosphere, atmospheric conditions vary. For instance, the lower part of the thermosphere (from 80 to 550 kilometers; 50 to 342 mi) contains the ionosphere, which is so-named because it is here in the atmosphere that particles are ionized by solar radiation. As a result, any spacecraft orbiting within this part of the atmosphere must be able to withstand the levels of UV and hard ion radiation.
Temperatures in this region also increase with height, which is due to the extremely low density of its molecules. So while temperatures in the thermosphere can rise as high as 1500 °C (2700 °F), the spacing of the gas molecules means that it would not feel hot to a human who was in direct contact with the air. It is also at this altitude that the phenomena known as Aurora Borealis and Aurara Australis are known to take place.
The Exosphere, which is outermost layer of the Earth’s atmosphere, extends from the exobase and merges with the emptiness of outer space, where there is no atmosphere. This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide (which are closer to the exobase).
In order to maintain a Low-Earth Orbit, an object must have a sufficient orbital velocity. For objects at an altitude of 150 km and above, an orbital velocity of 7.8 km (4.84 mi) per second (28,130 km/h; 17,480 mph) must be maintained. This is slightly less than the escape velocity needed to get into orbit, which is 11.3 kilometers (7 miles) per second (40,680 km/h; 25277 mph).
Despite the fact that the pull of gravity in LEO is not significantly less than on the surface of Earth (approximately 90%), people and objects in orbit are in a constant state of freefall, which creates the feeling of weightlessness.
Uses of LEO:
In this history of space exploration, the vast majority of human missions have been to Low Earth Orbit. The International Space Station also orbits in LEO, between an altitude of 320 and 380 km (200 and 240 mi). And LEO is where the majority of artificial satellites are deployed and maintained. The reasons for this are quite simple.
For one, the deployment of rockets and space shuttles to altitudes above 1000 km (610 mi) would require significantly more fuel. And within LEO, communications and navigation satellites, as well as space missions, experience high bandwidth and low communication time lag (aka. latency).
For Earth observation and spy satellites, LEO is still low enough to get a good look at the surface of Earth and resolve large objects and weather patterns on the surface. The altitude also allows for rapid orbital periods (a little over one hour to two hours long), which allows them to be able to view the same region on the surface multiple times in a single day.
And of course, at altitudes between 160 and 1000 km from the Earth’s surface, objects are not subject to the intense radiation of the Van Allen Belts. In short, LEO is the simplest, cheapest and safest location for the deployment of satellites, space stations, and crewed space missions.
Issues with Space Debris:
Because of its popularity as a destinations for satellites and space missions, and with increases in space launches over the past few decades, LEO is also becoming increasingly congested with space debris. This takes the form of discarded rocket stages, non-functioning satellites, and debris created by collisions between large pieces of debris.
The existence of this debris field in LEO has led to growing concern in recent years, since collisions at high-velocities can be catastrophic for space missions. And with every collision, additional debris is created, creating a destructive cycle known as the Kessler Effect – which is named after NASA scientist Donald J. Kessler, who first proposed it in 1978.
In 2013, NASA estimated that there may be as much as 21,000 bits of junk bigger than 10 cm, 500,000 particles between 1 and 10 cm, and more than 100 million smaller than 1 cm. As a result, in recent decades, numerous measures have been taken to monitor, prevent, and mitigate space debris and collisions.
For instance, in 1995, NASA became the first space agency in the world to issue a set of comprehensive guidelines on how to mitigate orbital debris. In 1997, the U.S. Government responded by developing the Orbital Debris Mitigation Standard Practices, based on the NASA guidelines.
NASA has also established the Orbital Debris Program Office, which coordinates with other federal departments to monitor space debris and deal with disruptions caused by collisions. In addition, the US Space Surveillance Network currently monitors some 8,000 orbiting objects that are considered collision hazards, and provides a continuous flow of orbit data to various agencies.
The European Space Agency’s (ESA) Space Debris Office also maintains the Database and Information System Characterizing Objects in Space (DISCOS), which provides information on launch details, orbital histories, physical properties and mission descriptions for all objects currently being tracked by the ESA. This database is internationally recognized and is used by almost 40 agencies, organizations and companies worldwide.
For over 70 years, Low-Earth Orbit has been the playground of human space capability. On occasion, we have ventured beyond the playground and farther out into the Solar System (and even beyond). In the coming decades, a great deal more activity is expected to take place in LEO, which includes the deployment of more satellites, cubesats, continued operations aboard the ISS, and even aerospace tourism.
Needless to say, this increase in activity will require that we do something about all the junk permeating the space lanes. With more space agencies, private aerospace companies, and other participants looking to take advantage of LEO, some serious cleanup will need to take place. And some additional protocols will surely need to be developed to make sure it stays clean.
We have written many interesting articles about orbiting the Earth here at Universe Today. Here’s What is the Orbit of Earth?, How High is Space?, How Many Satellites are in Space?, The Northern and Southern Lights – What is an Aurora? and What is the International Space Station?
We’ve also recorded an entire episode of Astronomy Cast all about Getting Around the Solar System. Listen here, Episode 84: Getting Around the Solar System.