Syllabus Section: Science and Technology
An orbit is the curved path that an object in space (such as a star, planet, moon, asteroid or spacecraft) takes around another object due to gravity. Gravity causes objects in space that have mass to be attracted to other nearby objects. If this attraction brings them together with enough momentum, they can sometimes begin to orbit each other.
When rockets launch our satellites, they put them into orbit in space. There, gravity keeps the satellite on its required orbit – in the same way that gravity keeps the Moon in orbit around Earth.
This happens in a way that is similar to throwing a ball out of the window of a tall tower – to get the ball going, you need to first give it a ‘push’ by throwing it, making the ball fall towards the ground on a curved path. Whilst it is your throw that gives the ball its initial speed, it is gravity alone that keeps the ball moving towards the ground once you let go.
Types of orbits:
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.
- 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)
Geostationary orbit (GEO)
- 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 meter away compared with if you were a centimeter from it. So, to see all of Earth at once from GEO far fewer satellites are needed than at a lower altitude.
Low Earth orbit (LEO)
- 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 aero planes 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.
Medium Earth orbit (MEO)
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.
- The more correct term would be near polar orbits.
- These orbits have an inclination near 90 degrees. This allows the satellite to see virtually every part of the Earth as the Earth rotates underneath it.
- It takes approximately 90 minutes for the satellite to complete one orbit.
- These satellites have many uses such as measuring ozone concentrations in the stratosphere or measuring temperatures in the atmosphere.
- Also known as geostationary orbits, satellites in these orbits circle the Earth at the same rate as the Earth spins.
- The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, this would put the satellite at approximately 35,790 km above the Earth.
- The satellites are located near the equator since at this latitude, there is a constant force of gravity from all directions. At other latitudes, the bulge at the center of the Earth would pull on the satellite.
- Geosynchronous orbits allow the satellite to observe almost a full hemisphere of the Earth. These satellites are used to study large scale phenomenon such as hurricanes, or cyclones. These orbits are also used for communication satellites.
- The disadvantage of this type of orbit is that since these satellites are very far away, they have poor resolution.
- The other disadvantage is that these satellites have trouble monitoring activities near the poles.