Learning objectives
By the end of this lesson, you should be able to;
- Define orbit.
- Describe motion of objects in their orbits.
- Describe launch to orbit.
- Explain types of orbits.
An orbit refers to a curved trajectory that an object follows. For example, the trajectory followed by the earth around the sun, and the trajectory followed by a planet around a star. Natural or man-made satellites also follow an orbit. Normally, an orbit is a regularly repeating trajectory. However, orbit can also refer to non-repeating trajectory.
The motion of objects following an orbit is influenced by the force of gravity and can be approximated using Newtonian mechanics.
Orbits can be understood in the following common ways;
- A force, like gravity pulls an object through a curved path as the object attempts to fly in a straight line.
- As an object is pulled towards a massive body, the object falls toward the body. However, if the object has enough tangential velocity, it will continue to follow the trajectory, and not fall into the body. The object is referred to as orbiting the body.
Objects in space that have mass are attracted to one another due to gravity. When these objects are brought together, with enough momentum, they orbit each other.
Objects that have the same mass orbit each other with nothing at the center. Small objects in space orbit around larger objects. For example, in the solar system the moon orbits the earth, and the earth orbits the sun. However, some larger objects do not remain completely still. Due to gravity, the earth is slightly pulled from its center by the moon. This causes tides in our oceans. The earth is also slightly pulled from its center by the earth as well as other planets.
During the creation of the solar system, dust, ice and gas traveled in space with both momentum and speed, and surrounding the sun as a cloud. Since the sun is larger than these objects, they were attracted by gravity towards the sun, forming a ring around it.
Over time, these particles started to clump together and grew larger until they formed planets, asteroids, and moons. This is the reason why planets have orbits around the sun, and they orbit in the same direction as the particles, and in roughly the same plane.
When rockets launch satellites, they position them into the orbit in space. The satellite is maintained at the orbit by the force of gravity. Similarly, the moon is kept in the orbit of the earth by gravity.
Note that in space, there is no air. Therefore, there is no air friction to hinder the movement of an object in space. Gravity makes satellites to orbit around the earth without any further resistance. Sending satellites to the orbit of the earth enables us to apply technology in different fields like, telecommunication, weather forecast, navigation, and astronomy observations.
Launch to orbit
The launch of satellites to orbit is done using rockets. The choice of launch vehicle is dependent primarily on the mass of the satellite, and the distance from earth that the satellite needs to travel. A high altitude orbit or a heavy payload requires more power to overcome the gravity of the earth.
Types of orbits
Once a satellite or spacecraft is launched, it is placed in one of the following orbits;
- Geostationary orbit. The image above is an illustration of a geostationary orbit. Satellites in this orbit circle the earth from west to east, above the equator, and following the rotation of the earth. They travel at the same rate as the earth, and it takes 23 hours, 56 minutes, and 4 seconds to complete a rotation. This makes the satellites in this orbit to appear stationary in a fixed position. In order to match the rotation of the earth perfectly, the speed of satellites in this orbit is approximately 3 kilometers per second, and an altitude of 35,786 kilometers.
- Low earth orbit. The image above is an illustration of a low earth orbit. This orbit is relatively close to the orbit of the earth. It is located at an altitude below 1000 kilometers, and can be located as low as 160 kilometers above the surface of the earth. Satellites in this orbit do not have to follow a specific path around the earth. There is more than one available route in this orbit. This makes it the most commonly used orbit. This is the orbit used for the international space station. Due to its close proximity to the earth, it is used for satellite imagery and produces images of high resolution.
- Medium earth orbit. The image above is an illustration of a medium earth orbit. This is made up of a wide range of orbits. Satellites in this orbit are not required to take specific paths. It is most commonly used by navigation satellites.
- Polar orbit. The image above is an illustration of a polar orbit. Satellites in this orbit travel from north to south over the poles of the earth. Satellites in this orbit do not necessarily pass by the poles, as they can deviate within 20 to 30 degrees. Polar orbits are found at low altitudes, 200 to 1000 kilometers above the earth. Sun-synchronous orbit is a type of polar orbit that run over the Polar Regions, and are synchronous to the sun. This means that satellites in this orbit are synchronized to be in the same position relative to the sun.
- Transfer orbit.The image above is an illustration of a transfer orbit. These orbits are used to get from one orbit to the other. When satellites are in this orbit, it is easy to move them to another orbit. This allows satellites to reach a high-altitude orbit without needing a launch vehicle to carry it all the way.
Summary
We've learned that;
- An orbit refers to a curved trajectory that an object follows.
- The motion of objects following an orbit is influenced by the force of gravity.
- Objects that have the same mass orbit each other with nothing at the center but small objects in space orbit around larger objects.
- We use rockets to launch satellites into space orbits.
