Orbits and Their Descriptions
- 19th Feb 2024
- Author: Ed Turner
Here at the National Space Centre, we love talking about satellites that have been sent up into space, but the definitions of these orbits can start to get a little confusing. Let's dive into these descriptions and try to work out what an NRHO is, and why MEO sounds like we’ve sent a satellite made of cats into space.
Most Common Orbits
With over 8000 satellites in orbit around Earth, many of them occupy these common orbits.
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![Low Earth Orbit]()
ESA Low Earth Orbit - LEO
Low Earth orbit is defined as a region of space below around 1000 kilometres, or around 620 miles. These orbits are the easiest to launch to, and have additional benefits of being more accessible for rendezvous and allowing satellites to observe the Earth in higher detail.
For these reasons, LEO is chosen for satellites such as Earth observation satellites, the International Space Station, and the Hubble Space Telescope, the latter of which was designed to be serviceable by the Space Shuttle. Additionally, both SpaceX and Amazon have targeted LEO for their communication satellite constellations.
There are drawbacks to launching satellites into this orbit. Firstly, there is non-negligible drag effect on the satellites caused by the atmosphere, which causes their orbits to decay and eventually reenter the atmosphere. Being closer to the Earth also means satellites have a smaller field of view.
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![Galileo Constellation]()
ESA Medium Earth Orbit - MEO
MEO is essentially the space above LEO and below GEO, and provides an excellent location for navigation satellites, such as the European Space Agency's Galileo satellite constellation or the USA's Global Positioning System (GPS) network.
MEO requires a higher launch energy than LEO, however satellites at this altitude have a larger field of view of Earth. Additionally, MEO satellites are in a lower orbit than GEO satellites, meaning transmission delays are reduced.
A large issue that satellites in this orbit encounter are that they are subjected to the radiation environment of the Van Allen belts, which can effect electronics and other sensitive systems.
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![Geostationary Orbit]()
ESA Geostationary Orbit - GEO
While other orbits have a range of heights above Earth's surface, geostationary satellites must orbit at 35,786 kilometres. At this height, a satellite has an orbital period equal to the Earth's rotational period, and so it seems to hover at the same point in the sky at all times of the day.
This orbit is especially useful for weather satellites such as Meteosat, as they are able to see weather patterns changing over time in a specific region, and are able to act as early warning systems for disasters such as hurricanes and typhoons.
There is a similar orbit known as a geosynchronous orbit. Satellites here will orbit at the same height as geostationary satellites, but rather than being located above the equator, these satellites have their orbits slightly inclined, so they seem to wobble up and down in the sky.
Specialist Orbits
Sun-Synchronous Orbit
A Sun-synchronous orbit, or SSO, is a low polar orbit that grants a satellite a constant view of Earth's surface in sunlight. This type of orbit utilises the precession caused by the fact that the Earth isn't perfectly spherical, but rather is slightly oblate. Rather than being perfectly polar with an inclination of 90°, a Sun-synchronous satellite has an orbital inclination of 98°.
An additional advantage of being in constant sunlight is that a satellite is always charging its battery using solar panels. Unfortunately, constantly being in sunlight means a satellite will be constantly affected by the heating effects, so must have a way of cooling itself.
Near-Rectilinear Halo Orbit
An NRHO hangs a bit like a necklace around a celestial object, and this type of orbit has been chosen by NASA for their Lunar Gateway space station. The benefits of an NRHO for Gateway are that it provides a fuel-efficient orbit for spacecraft due to the stability compared to a low lunar orbit, while also providing good access to the lunar surface.
On 28 June 2022, NASA launched their CAPSTONE mission aboard Rocket Lab’s Electron rocket to test the stability and feasibility of this orbit, which it has since succeeded in doing, paving the way for future lunar exploration. NASA expect to launch the first segment of Gateway no earlier than November 2025.
Molniya and Tundra Orbit
At high latitudes, a geostationary satellite isn’t much use, as it requires high broadcasting power for any signal to pass through the atmosphere. To combat this issue, a series of Soviet communications satellites called Molniya satellites (hence the orbit name) utilised a highly elliptical orbit that makes them seemingly loiter over the northern hemisphere.
The drawbacks with using this orbit are that at least three satellites are needed to provide constant communications, and the satellites regularly plunge through the Van Allen belts, which damage sensitive equipment onboard. A Tundra orbit negates these issues by orbiting at a higher altitude and requiring less satellites in a single constellation. Despite this, Molniya orbits are used more frequently due to the lower launch energy required, hence reducing the cost.
Suborbital spaceflight
When launched, not all spacecraft make it into orbit. Those that travel towards space but don't make it around the Earth and instead return following a sort of parabola-shaped trajectory, are known as suborbital flights. When launched from Earth, a spacecraft reaches outer space but isn't travelling at a high enough velocity to go into Earth orbit, so it returns to the planet.
Suborbital flights are often used to test spacecraft and rockets that are planned to make orbital flights at a later date, and also to investigate the Earth's atmosphere.
Lagrange Points
Confusingly named after the second person to discover them, Lagrange Points refer to regions of space linked to an orbiting body where gravitational influences are equal, such as between the Earth and the Moon. This makes them useful destinations for satellites that you want to stay in one place relative to the orbiting body. In total there are five Lagrange Points for every large celestial object.
The Sun-Earth L1 point is an excellent location for space-based solar telescopes, such as ESA’s SOHO spacecraft, due to the uninterrupted view of the Sun while staying in close contact with Earth.
The Sun-Earth L2 point is an incredible spot for space telescopes to block the light and heat from the Sun, Earth and Moon in one go, very useful for infrared observatories like NASA's James Webb Space Telescope and ESA's Euclid spacecraft.
We have discovered asteroids located at the L4 and L5 points of all the planets except Mercury. These are known as planetary trojans. The Trojan asteroids of Jupiter have names based on the Trojan War, with L4 being the Greek camp, and L5 being the Trojan camp. Funnily enough, this naming convention has resulted in a Greek and Trojan 'spy' in opposite camps, as the asteroids Patroclus and Hector were named before the Greek/Trojan rule was created.
Hopefully this blog has made the orbital dance of satellites a little clearer, and you can now brag to your friends about the difference between a Halo and an LEO.
Full Credits / References
(Banner image) Satellites with their orbit trails shown. Credit: ESA
(1a) An illustration of low Earth orbit. Credit: ESA
(1b) ESA's Galileo satellite constellation in medium Earth orbit. Credit: ESA
(1c) An illustration of geostationary orbit. Credit: ESA
(2a) An animation of the near-rectiliear halo orbit Gateway will use. Credit: NASA
(2b) An illustration of a molniya orbit. Credit: NASA
(3) Video following a sounding (research) rocket on a suborbital spaceflight. Credit: NASA/Goddard Space Flight Center https://www.youtube.com/watch?v=TTfgOYb1Fn8
(4) The five Lagrange points linked to the Sun-Earth system. A satellite can be seen orbiting at L2. Credit: NASA/WMAP Science Team
