Upon launch, a satellite or spacecraft is most often placed in one of several particular orbits around Earth – or it might be sent on an interplanetary journey, meaning that it does not orbit Earth anymore, but instead orbits the Sun until its arrival at its final destination, like Mars or Jupiter.
There are many factors that decide which orbit would be best for a satellite to use, depending on what the satellite is designed to achieve.
- Geostationary orbit (GEO)
- Low Earth orbit (LEO)
- Medium Earth orbit (MEO)
- Polar orbit and Sun-synchronous orbit (SSO)
- Transfer orbits and geostationary transfer orbit (GTO)
- Lagrange points (L-points)
Satellites in geostationary orbit (GEO) circle Earth above the equator from west to east following Earth’s rotation – taking 23 hours 56 minutes and 4 seconds – by travelling at exactly the same rate as Earth. This makes satellites in GEO appear to be ‘stationary’ over a fixed position. In order to perfectly match Earth’s rotation, the speed of GEO satellites should be about 3 km per second at an altitude of 35 786 km. This is much farther from Earth’s surface compared to many satellites.
GEO is used by satellites that need to stay constantly above one particular place over Earth, such as telecommunication satellites. This way, an antenna on Earth can be fixed to always stay pointed towards that satellite without moving. It can also be used by weather monitoring satellites, because they can continually observe specific areas to see how weather trends emerge there.
Satellites in GEO cover a large range of Earth so as few as three equally-spaced satellites can provide near global coverage. This is because when a satellite is this far from Earth, it can cover large sections at once. This is akin to being able to see more of a map from a metre away compared with if you were a centimetre from it. So to see all of Earth at once from GEO far fewer satellites are needed than at a lower altitude.
ESA’s European Data Relay System (EDRS) programme has placed satellites in GEO, where they relay information to and from non-GEO satellites and other stations that are otherwise unable to permanently transmit or receive data. This means Europe can always stay connected and online.
A low Earth orbit (LEO) is, as the name suggests, an orbit that is relatively close to Earth’s surface. It is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth – which is low compared to other orbits, but still very far above Earth’s surface.
By comparison, most commercial aeroplanes do not fly at altitudes much greater than approximately 14 km, so even the lowest LEO is more than ten times higher than that.
Unlike satellites in GEO that must always orbit along Earth’s equator, LEO satellites do not always have to follow a particular path around Earth in the same way – their plane can be tilted. This means there are more available routes for satellites in LEO, which is one of the reasons why LEO is a very commonly used orbit.
LEO’s close proximity to Earth makes it useful for several reasons. It is the orbit most commonly used for satellite imaging, as being near the surface allows it to take images of higher resolution. It is also the orbit used for the International Space Station (ISS), as it is easier for astronauts to travel to and from it at a shorter distance. Satellites in this orbit travel at a speed of around 7.8 km per second; at this speed, a satellite takes approximately 90 minutes to circle Earth, meaning the ISS travels around Earth about 16 times a day. However, individual LEO satellites are less useful for tasks such as telecommunication, because they move so fast across the sky and therefore require a lot of effort to track from ground stations. Instead, communications satellites in LEO often work as part of a large combination or constellation of multiple satellites to give constant coverage. In order to increase coverage, sometimes constellations like this, consisting of several of the same or similar satellites, are launched together to create a ‘net’ around Earth. This lets them cover large areas of Earth simultaneously by working together.
L
Medium Earth orbit comprises a wide range of orbits anywhere between LEO and GEO. It is similar to LEO in that it also does not need to take specific paths around Earth, and it is used by a variety of satellites with many different applications.
It is very commonly used by navigation satellites, like the European Galileo system (pictured). Galileo powers navigation communications across Europe, and is used for many types of navigation, from tracking large jumbo jets to getting directions to your smartphone. Galileo uses a constellation of multiple satellites to provide coverage across large parts of the world all at once.
Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth’s poles.
Satellites in a polar orbit do not have to pass the North and South Pole precisely; even a deviation within 20 to 30 degrees is still classed as a polar orbit. Polar orbits are a type of low Earth orbit, as they are at low altitudes between 200 to 1000 km.
Sun-synchronous orbit (SSO) is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun. This means they are synchronized to always be in the same ‘fixed’ position relative to the Sun. This means that the satellite always visits the same spot at the same local time – for example, passing the city of Paris every day at noon exactly.
This means that the satellite will always observe a point on the Earth as if constantly at the same time of the day, which serves a number of applications; for example, it means that scientists and those who use the satellite images can compare how somewhere changes over time.
This is because, if you want to monitor an area by taking a series of images of a certain place across many days, weeks, months, or even years, then it would not be very helpful to compare somewhere at midnight and then at midday – you need to take each picture as similarly as the previous picture as possible. Therefore, scientists use image series like these to investigate how weather patterns emerge, to help predict weather or storms; when monitoring emergencies like forest fires or flooding; or to accumulate data on long-term problems like deforestation or rising sea levels.
Often, satellites in SSO are synchronized so that they are in constant dawn or dusk – this is because by constantly riding a sunset or sunrise, they will never have the Sun at an angle where the Earth shadows them. A satellite in a Sun-synchronous orbit would usually be at an altitude of between 600 to 800 km. At 800 km, it will be travelling at a speed of approximately 7.5 km per second.
Transfer orbits are a special kind of orbit used to get from one orbit to another. When satellites are launched from Earth and carried to space with launch vehicles such as Ariane 5, the satellites are not always placed directly on their final orbit. Often, the satellites are instead placed on a transfer orbit: an orbit where, by using relatively little energy from built-in motors, the satellite or spacecraft can move from one orbit to another.
This allows a satellite to reach, for example, a high-altitude orbit like GEO without actually needing the launch vehicle to go all the way to this altitude, which would require more effort – this is like taking a shortcut. Reaching GEO in this way is an example of one of the most common transfer orbits, called the geostationary transfer orbit (GTO).
Orbits have different eccentricities – a measure of how circular (round) or elliptical (squashed) an orbit is. In a perfectly round orbit, the satellite is always at the same distance from the Earth’s surface – but on a highly eccentric orbit, the path looks like an ellipse.
On a highly eccentric orbit like this, the satellite can quickly go from being very far to very near Earth’s surface depending on where the satellite is on the orbit. In transfer orbits, the payload uses engines to go from an orbit of one eccentricity to another, which puts it on track to higher or lower orbits.
After liftoff, a launch vehicle makes its way to space following a path shown by the yellow line, in the figure. At the target destination, the rocket releases the payload which sets it off on an elliptical orbit, following the blue line which sends the payload farther away from Earth. The point farthest away from the Earth on the blue elliptical orbit is called the apogee and the point closest is called the perigee.
When the payload reaches the apogee at the GEO altitude of 35 786 km, it fires its engines in such a way that it enters onto the circular GEO orbit and stays there, shown by the red line in the diagram. So, specifically, the GTO is the blue path from the yellow orbit to the red orbit.
