Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit

From GPS to the venerable Hubble Space Telescope, low Earth orbit has proved to be a very useful piece of space real estate indeed.
Christopher McFadden
Satellite is orbiting around the Earth.
Satellite is orbiting around the Earth.iLexx/iStock
  • Often taken for granted today, telecommunication services and more are possible thanks to our ability to place bits of technology in orbit around the Earth.
  • A low Earth orbit (LEO) is an orbit in which an object revolves around the Earth at an altitude that is toward the lower end of the range of possible orbits.
  • LEO is where the International Space Station (ISS) and most satellites are located.

Technologically speaking, we are truly blessed today. You can call someone anywhere on the planet (so long as you have their number) or use any number of GPS-enabled apps to find your way around.

Often taken for granted today, these services and more are partly or wholly possible thanks to our ability to place bits of technology in orbit around the Earth. There are various heights and orbits around the Earth that can be selected, but one of the most congested is something called low Earth orbit.

But what is it? Let's find out.

What is Low Earth Orbit?

To put it simply, a low Earth orbit (LEO) is an orbit in which an object revolves around the Earth at an altitude that is toward the lower end of the range of possible orbits. This equates to approximately 1,200 miles (2,000 kilometers) altitude or less and can be as low as 100 miles (160 km) above the Earth.

LEO is where the International Space Station (ISS) and most satellites are located.

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
A view from low Earth orbit.
Bogdan Khmelnytskyi/iStock

That might not sound very "low," but remember that the closer you get to a large mass, like the Earth, the greater the influence of gravity on a body. If you get too close and do not have enough momentum, such bodies will quickly begin to be pulled towards the Earth's center of mass. 

That is a journey that would not end well without some descent arresting equipment like a parachute or retro thrusters. 

For this reason, a satellite must move at a speed of about 17,500 miles per hour (7.8 kilometers per second) to stay in this orbit. At this speed, a complete orbit of the planet takes about 90 minutes, give or take.

The same force that pulls us to the planet's surface and makes orbits possible is gravity. A satellite would fly off in a different direction if gravity weren't present to keep it orbiting the Earth, just as humans would float out into space if it didn't exist.

A spacecraft that is moving exceedingly quickly, say faster than the Earth's escape velocity of 25,000 mph (11.2 km/s), can actually experience this. However, if an object is moving considerably more slowly, it will fall back to Earth just as inevitably as you do when you leap into the air.

The speed at which the force of gravity prevents an object from flying off on a tangent is 17,500 mph (7.8 km/s). An object traveling at this speed will simply continue to circle the Earth. This speed is horizontal and parallel to the planet's surface.

If you've ever seen a rocket go into space, you might find this odd because they appear to shoot straight up vertically. But that's because, in order to avoid drag forces, they must immediately ascend above the atmosphere (or at least the majority of it). However, once they reach above the atmosphere, they actually begin moving horizontally.

You can sort of see this during rocket launches on a clear day when the vapor trail appears to curve or arc with height. 

A satellite is considered to be in orbit once it reaches this orbital velocity of at least 17.500 mph (7.8km/s) for LEO.

However, the speed necessary to maintain a satellite in orbit changes at greater altitudes. The required speed, it turns out, actually decreases with the increase in altitude.

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Skylab satellite.
Tokarsky/iStock

This does not, however, imply that a rocket must use less energy to launch a satellite into a higher orbit. This is because it takes a great amount of energy just to reach that higher height. Along with other factors, including the higher resolution images, Earth-observing satellites can obtain from closer proximity, and that there are more available routes for satellites in LEO, this additional work required to reach higher altitudes is one of the reasons the majority of satellites are deployed in LEO.

However, there is one specific high-altitude orbit that is worth the extra work to get there; geostationary or geosynchronous equatorial orbit (GEO).

Satellites in LEO orbit the Earth around 16 times a day/per one full rotation of the Earth. However, satellites in GEO circle Earth above the equator from west to east, following Earth’s rotation and traveling at the same rate as the Earth – taking 23 hours, 56 minutes, and 4 seconds. This means that satellites in GEO appear to be ‘stationary’ over a fixed position. GEO is at an altitude of around 22,000 miles (36,000 km), at which point the orbital speed has slowed, so a single orbit corresponds to precisely one rotation of the Earth.

This makes satellites at that height particularly suitable for satellite TV and other communications systems since they virtually hover over a single area of the Earth's surface.

In contrast, orbiting satellites usually follow an oval-type path called an ellipse. The length and width of which are known as the major and minor axes.

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Photorealistic 3d illustration of a satellite orbiting the Earth.
imaginima/iStock

The orbits of the majority of satellites are nearly circular, but occasionally the elliptical can be significantly larger, with the major axis being much longer than the minor axis.

For instance, the Molniya satellite orbit, which is utilized for communications in high northern latitudes, has a high point of around 25,000 miles (40,000 km) and a low point of about 308 miles (495 km).

LEO is by far the most common type of orbit, but not the only one. Apart from GEO, there are also Medium Earth Orbit (MEO), High Earth Orbit (HEO), Sun-synchronous orbit (SSO), geostationary transfer orbit (GTO), and Lagrange points (L-points).

How many satellites are there in low Earth orbit?

In short, a lot. 

As of September 1, 2021, there were roughly 4,550 satellites orbiting the Earth orbiting in LEO, MEO, HEO, and GEO. These satellites are operated by a variety of private and public organizations, including the likes of SpaceX, the U.S. Air Force, ESA, the Chinese Ministry of National Defense, etc. 

Unsurprisingly, the United States has the most satellites orbiting Earth, with somewhere around 2,804 satellites that are owned or operated by an entity from the U.S.

This figure alone makes up more than half of the total number of space satellites that are currently in orbit. All others are operated by about 75 different countries that have at least one satellite orbiting Earth.

More than 3,500 satellites were in low Earth orbit (LEO) as of 2021, which is where you will find satellites typically employed for communications and remote sensing satellite systems.

The ISS, the Hubble Space Telescope, and the SpaceX Starlink satellites are all located here too.

Something like 565 satellites, the second highest number, were in a geosynchronous orbit (GSO) / geostationary orbit (GEO), which is used for communications and Earth observation.

The orbital speeds of satellites in GSO and GEO match the rate of rotation of the Earth; GEO objects orbit the equator, giving them the impression of being in a fixed location.

Somewhere around 139 satellites are in medium Earth orbit (MEO), which is where navigation systems like GPS are located.

Finally, there are about 56 satellites in a high elliptical orbit (HEO), which are utilized for remote sensing, satellite radio, and other purposes. The oblong shape of this kind of orbit sets it apart from the others because one end is substantially closer to Earth than the other.

As satellites are continuously launched into orbit by SpaceX and other businesses and organizations, the overall number of satellites orbiting the Earth will keep rising inexorably over time. In fact, SpaceX plans to launch 42,000 satellites into orbit over the next two decades alone!

What is a Low Earth Orbit satellite used for?

As we previously stated, a low Earth orbit (LEO) is an orbit that is quite close to the surface of the Earth. It is often less than 621 miles (1000 km) above Earth, although it can be as low as 100 miles (160 km), which is low relative to other orbits but still very high above the planet's surface.

Consider that most commercial aircraft do not fly at altitudes much higher than between 31,000 feet (9.5 km) and 38,000 feet (11.5 km).

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Space internet network.
BlackJack3D/iStock

LEO satellites do not always have to follow a specific course around Earth in the same way, since their plane can be inclined, unlike satellites in GEO that must always circle near the equator. As a result, there are more options for satellite paths in LEO, which is one of the reasons it is such a popular orbit.

However, since they have a smaller field of view than geosynchronous satellites, LEO satellites can only "see" and communicate with a limited portion of the Earth at any one time. In order to offer continuous coverage, a network (or "constellation") of satellites is necessary. Lower LEO satellites have more rapid orbital decay as well, necessitating periodic re-boosts to keep them in a stable orbit or the launch of new satellites as the old ones deorbit and re-enter the atmosphere.

LEO is useful for a number of reasons because of its close proximity to Earth. For example, it is the orbit that satellites most frequently used for imaging since being close to the surface enables them to capture photos with a higher resolution.

It is also the orbit in which the International Space Station (ISS) is located since astronauts can more easily and more quickly fly to and from it.

Because they move so quickly across the sky and can be difficult for ground stations to detect, individual LEO satellites are usually less valuable for operations like telephony.

Instead, in order to provide continual coverage, LEO communications satellites frequently operate as a huge combination or constellation of several spacecraft. These constellations, which include multiples of the same or similar satellites, are occasionally launched together to form a "net" encircling Earth in order to maximize coverage. This enables them to simultaneously cover a large portion of the Earth collectively.

One primary example is SpaceX's Starlink network. 

Traditional satellite internet service has been provided by geosynchronous satellites, which as a consequence of their orbital altitude, have high latency. Additionally, capacity is constrained, and services have data caps due to the low number of satellites (HughesNet, for instance, only has five total satellites).

According to Starlink, these problems can be resolved by using a large number of low Earth orbit satellites, which have many more satellites at a very low orbital height resulting in low latency and higher capacity.

To fully operate, Starlink plans to launch 12,000 satellites before it is considered fully functional. This looks like an ambitious number when compared to the total of 2,298 satellites that were in orbit in 2019.

However, Starlink already has several thousand satellites in orbit, and new ones are being launched every week.

What are some examples of spacecraft in Low Earth orbit?

So, we hope you now have a good grasp of what low earth orbit is. Armed with that knowledge, let's now look at some of the most famous and important spacecraft placed in LEO from the early days of the "Space Age" up to the modern day,

1. Sputnik 1 was the very first LEO satellite

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Replica of Sputnik 1
Source: NASA/Wikipedia

Sputnik 1, was the first artificial satellite to successfully enter orbit around the Earth. The word "Sputnik" in Russian means "companion" ("satellite" in the astronomical sense).

It was the first of three planned satellites that were intended as part of the former Soviet Union's Sputnik program as part of the International Geophysical Year (1957-1958). As 'Sputnik' was the Russian term for "satellite", the word Sputnik also appeared in the names of spacecraft in other Soviet space programs and was used in the west for any Soviet satellite whose original Soviet name was not known at the time.

Sputnik 1 was a 22.8-inch (58.0 cm)-diameter aluminum sphere with four long whip-like antennas. The antennae resembled long "whiskers" that were pointed in one direction.

The spacecraft collected information on radio signal propagation in the ionosphere and the density of the upper layers of the atmosphere. Transmitters operating at 20.005 and 40.002 MHz (about 15 and 7.5 m in wavelength) were among the equipment and power supplies enclosed in a sealed capsule. Emissions occurred in alternating groups of 0.3 s duration. Temperature information about the sphere's interior and outside was included in the downlink telemetry.

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Exploded view of Sputnik 1
Source: Музей Космонавтики/Wikimedia Commons

After completing roughly 1,440 orbits of the Earth and covering a total distance of 70 million kilometers, the orbit of the then dormant satellite was subsequently shown optically decaying around 92 days after launch (January 4, 1958).

The Sputnik 1 rocket booster also made it into Earth orbit and was visible at night from the ground as a first magnitude object, but it was more challenging to optically track the small but highly polished sphere, which was barely visible at sixth magnitude. There are several Sputnik 1 satellite replicas on display in Russian museums, and there is also one in Washington, D.C. at the Smithsonian National Air and Space Museum.

2. The Hubble Space Telescope lies in low earth orbit

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
The Hubble Space Telescope orbits with Earth in the background.
NASA

Probably the best-known satellite, other than the ISS, spacecraft parked in LEO is the Hubble Space Telescope. 

At a height of roughly 340 miles (547 km), the Hubble Space Telescope completes a single orbit of Earth every 95 minutes at a speed of 17,500 mph (27,300 kph).

Hubble gets clear images because it’s above Earth’s atmosphere, not because it travels or flies closer to cosmic objects.

This is because the light is distorted and blurred, and some electromagnetic spectrum wavelengths are absorbed by Earth's atmosphere. The atmosphere not only hinders the ability of ground-based telescopes to obtain sharp views of cosmic objects but also helps shield us from hazardous radiation.

4. The Spot family of satellites is also in LEO

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Ayers Rock as imaged from LEO by Spot 6.
ESA

A lesser-known example of spacecraft currently orbiting close to the Earth is the Spot family of satellites.

Deployed starting in 1986 with SPOT 1, the SPOT family of satellites, whose name is derived from the French "Satellite pour l'Observation de la Terre" (satellite for Earth observation), has provided high-resolution, wide-area optical imagery ever since. Seven satellites developed by the French National Centre for Space Studies (CNES) were launched between 1986 and 2014, exposing the Earth's surface in great detail. This enabled new mapping, vegetation monitoring, land use and land cover, and natural catastrophe effect applications.

Data continuity is guaranteed through 2024 thanks to the most recent satellites in the series, SPOT 6 and SPOT 7, which are owned by Airbus Defence and Space.

All of the SPOT satellites provide imagery in panchromatic and multispectral bands with a swath of 37 miles (60 km).

The SPOT series is a component of the European Space Agency's (ESA) "Third Party Missions Programme", under which ESA and Airbus Defence and Space have a contract for the distribution of mission data products.

5. The Iridium satellite constellation was launched in the 1990s

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Graphic showing Iridium satellite constellation
Source: Iridum

Thanks to the Iridium satellite constellation of LEO satellites, phones, pagers, and integrated transceivers may access L band speech and data information across the entire surface of the Earth. The constellation is owned and run by Iridium Communications, which also offers equipment and access to its services for sale. It was developed by Iridium and financed by Motorola. It was initially developed by Bary Bertiger, Raymond J. Leopold, and Ken Peterson in late 1987 (protected by patents Motorola filed in their names in 1988). The satellites were deployed between 1997 and 2002.

There are 66 operational satellites in the constellation, which are necessary for providing global coverage, as well as additional backup satellites. (Early calculations indicated that 77 satellites would be needed to provide global coverage, so the project was given the name Iridium — the element with atomic number 77).

With an inclination of 86.4° and a height of roughly 485 miles (781 km), the satellites sit within low Earth orbit. Regardless of the location of ground stations and gateways, the almost polar orbit and communication between satellites through Ka-band inter-satellite links offer global service availability (covering both poles, oceans, and airways). The constellation has six orbital planes spaced 30° apart, with 11 satellites in each plane (not including backups).

The first-generation Iridium satellites incidentally focussed sunlight on a tiny area of the Earth's surface due to the geometry of their reflective antennae. This led to a phenomenon known as Iridium flares, in which the satellite briefly stood out among the brightest stars, and they were occasionally visible throughout the day. The more recent Iridium satellites don't emit flares.

5. The Chinese Tiangong space station sits comfortably in LEO

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Rendering of Tiangong Space Station
Source: Shujianyang/Wikimedia Commons

Officially known as the Tiangong space station (Chinese for "Palace in the Sky" or "Heavenly Palace"), this is a space station that China is currently building in low Earth orbit. Orbiting Earth between 210 miles (340km) and 280 miles (450 km), it comprises the so-called "Third Step" of China's Space Program and will be China's first permanent space station. When finished, Tiangong will weigh between 80 and 100 tonnes, making it around one-fifth the mass of the International Space Station and roughly the same size as the Russian Mir space station (which is no longer in use).

The station is being built using the knowledge obtained from its predecessors, Tiangong-1, and Tiangong-2.

Several crewed and uncrewed missions, as well as the launch of two additional modules in 2022, came after the Tianhe ("Harmony of the Heavens") core module, which was launched on April 29, 2021.

Chinese officials have stated the hope that the research conducted on the station will increase researchers' capacity to conduct scientific experiments in space beyond the time and duration provided by China's current space facilities.

6. Envisat is now largely redundant

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Model of Envisat in original size.
Source: abrev /Wikimedia Commons

Envisat (short for "Environmental Satellite), is a large, now inactive Earth-observing spacecraft still in orbit that is now regarded as space junk. It was the biggest civilian Earth observation satellite in the world and was run by the ESA.

It was put into a Sun-synchronous polar orbit at a height of about 491 miles (790 km) on March 1, 2002, using an Ariane 5 rocket from the Guyana Space Centre in Kourou, French Guiana. The now-dead satellite completes one orbit of the Earth in around 101 minutes. Envisat's mission was officially ended on May 9, 2012, by the ESA, after contact with the satellite was unexpectedly lost in April of 2012.

The development and deployment of Envisat cost roughly 2.3 billion euros (including 300 million euros for five years of operations). The Sentinel family of satellites has taken over the mission, the first of which Sentinel 1 took over the radar duties of Envisat since its launch in April 2014.

7. There is also a lot of junk in LEO

Low Earth orbit: Seven significant spacecraft that lie in low Earth orbit
Space debris in LEO is a growing problem
EvgeniyShkolenko/iStock

While not technically speaking a spacecraft, per se, LEO is rapidly becoming very congested with leftover bits of spacecraft over time. There is so much stuff there, and it travels so fast that this issue is growing into a very serious navigational hazard as time goes by.

If the problem gets even worse, many have postulated that it could lead to a domino effect of collisions called the "Kessler syndrome."

At present, somewhere in the order of 27,000 objects, most of which are more than 10 cm in size, are tracked in LEO by the Department of Defense’s global Space Surveillance Network.

For smaller objects, the issue could be much, much worse. For example. the Arecibo Observatory suggests that there may be a million hazardous objects in orbit that are larger than 2 millimeters but too small to be seen by Earth-based observatories. According to NASA, there are somewhere around 100 million pieces of debris around one millimeter and larger in orbit.

And that, LEO fans, is your lot for today.

Placing things in LEO has proved invaluable to our species over the last few decades, with many more plans to place stuff up there over the coming years. Whether we'll ever find a solution to clean it up or whether it will become a series of problems for future spaceflights is yet to be seen.

But, plans are afoot to clean things up, up there.

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