The Solar System and orbits

In a nutshell

In our solar system, the Sun is orbited by eight planets and a number of other smaller objects. These include dwarf planets, asteroids and comets. Artificial satellites are built by humans and natural satellites are not man-made. An orbit is a curved path of a satellite around a star, planet or moon.

The Solar System

In our solar system, the Sun is in the centre. The Sun is orbited by eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (in order of increasing distance from the Sun).

Physics; Space physics; KS4 Year 10; The Solar System and orbits

1.	The Sun
2.	Mercury
3.	Venus
4.	The Earth (and Moon)
5.	Mars
6.	Jupiter
7.	Saturn
8.	Uranus
9.	Neptune

The Sun is also orbited by a number of different smaller objects, including dwarf planets, asteroids and comets. Dwarf planets are planet-like bodies that are too small to be considered a main planet. Asteroids are small rocky bodies and comets are small icy bodies.

Example

An example of a dwarf planet is Pluto which was classified as a dwarf planet in $2006$ .
An example of an asteroid is Ceres located in the asteroid belt between Mars and Jupiter.
An example of a comet is Halley's comet which will next be visible to the naked eye from Earth in mid- $2061$ .

Satellites

Satellites are celestial (in space) bodies that orbit another celestial body. There are two types of satellites: natural and artificial.

Note: In this context, a body is defined as the same as an object!

An artificial satellite is one that humans have built. These satellites tend to orbit the Earth but there are some artificial satellites that orbit other planets and collect data, such as photographs and climate measurements.

A natural satellite is one that is not man-made. These make up most of the satellites in the Solar System.

Examples of satellites

Artificial	Natural
The Hubble Space telescope	The Moon
The International Space Station (ISS)	Io (moon of Jupiter)
GPS satellites	Asteroids in the asteroid belt

Orbits

Each of the planets in the Solar system has its own orbit. An orbit refers to the curved path of a body around a star, planet or moon. Most orbits are elliptical, which means they are not in a perfect circle. However, the orbits of the planets are almost circular.

Example

The Earth completes one full orbit of the Sun in $365.25\ days$ . It is closest to the Sun around the $4th\ January$ each year.

To visualise how orbits work, consider a small celestial body in a circular orbit around a large celestial body.

An orbit is caused by the gravitational force of the larger body on the smaller body.

The body is already travelling in its orbital path so the force instead causes it to change its direction. This occurs continuously so the body is constantly changing direction. This means that its constantly changing velocity (but not speed).

A constant change in velocity requires an acceleration on the smaller body. This comes from the gravitational force acting on it. The body keeps accelerating towards the large body but the velocity of the smaller body is always directed at a right angle to this. The result is a circular orbit around the large body.

The gravitational force on an object increases as it gets closer to the large body it orbits. This means it needs to be moving faster around the body to prevent it from falling into it.

This works the other way as well. If an object in a stable orbit changes its speed, the radius of its orbit will also change. For example, if it slows down, the radius of its orbit will increase as the distance it is from the large body increases.

Satellite orbits

Before humans launch artificial satellites, they can work out the path its orbit should be and direct it to follow this path.

Physics; Space physics; KS4 Year 10; The Solar System and orbits

1.	Polar orbit
2.	Low Earth orbit
3.	Geostationary orbit
4.	Highly elliptical orbit

Types of orbits

Orbit	Path
Polar	Satellite will orbit around the poles of the Earth. Given enough time it will pass over all areas of the Earth.
Highly elliptical	Satellite will orbit so that they are closer to locations nearer the poles. This is used for communication with the poles.
Geostationary	Satellite will orbit so that it is at a fixed position above the Earth. They must match the Earth's angular velocity and travel at a speed of $3070\,m/s$ .
Low Earth	Satellite orbits at a close distance to the Earth. This requires a smaller amount of fuel and they orbit at a speed of $7500\,m/s$ .