Motion in the Universe

Nothing out in space can sit still - planets, moons, asteroids, comets and satellites are constantly moving in loops called orbitals around one another and a held in place by the force of gravity. Different celestial objects travel at different speeds and in different shaped orbitals.

 
 

Astronomy keywords

  • Universe: a collection of billions of galaxies

  • Galaxy: a collection of billions of stars

  • Milky Way: the name of the galaxy which contains our solar system

  • Satellite: anything in space which travels in an orbit. Satellites can be natural, such as the moon and Earth or they can be artificial, such as the International Space Station

  • Asteroid: lumps of rock and metal that orbit the Sun

  • Comet: lumps of dust and ice that orbit the Sun in really long orbits

Gravitational field strength, g

Gravity is a force which prevents us from floating about on Earth and also keeps moons and planets in orbit. All objects with mass produce a gravitational field and the heavier the object, the larger the gravitational field. This is why the force of gravity is much stronger on the Earth than the moon, since the Earth has a larger mass. Gravitational field strength also depends on distance - the closer you are to Earth the stronger the gravitational field strength (so as an astronaut travels away from Earth, the force of gravity decreases).

Gravitational forces keep things in orbit, such as:

  • the moons around planets

  • planets and comets around the Sun

  • artificial satellites around the Earth

Orbits

Planets, moons and comets travel in orbits of different shapes. The orbits of planets around stars, such as the Sun and the orbits of moons around planets are an (almost) circular shape, whereas the orbits of comets are an elliptical shape. The further the planets are to the Sun, the longer it takes them to complete a full orbital. The dwarf planet Pluto takes an astonishing 248 years to complete one lap around the Sun, which means that it has not even travelled half-way since it was discovered in 1930. Comets are basically a lump of ice which orbit the Sun. As they pass close to the Sun, the ice vaporises and can be seen as a ‘tail’ as the comet travels through space. Comets take much longer to complete one orbit compared to moons and planets since they travel to the outer edges of the solar system. As they pass close to the Sun, their speed increases due to the pull of gravity exerted by the Sun.

Orbital Speed

The speed of an object that is travelling in an orbit can be calculated using the following equation:

 
 

You may be asked to calculate the orbital radius, as in the worked example below. To do this you’ll need to rearrange the equation. If you’re not given the speed but you know the distance and time taken, remember that speed = distance / time.

Worked example: calculating orbital radius

Calculate the orbital radius of a moon which orbits a planet in 45 hours with a speed of 6000 m/s.

  • First rearrange the equation to give orbital radius = speed x time period / 2π

  • Convert time period to seconds. 45 hours x 60 x 60 = 162,000 seconds

  • Radius = 6000 x 162 000 / 2π

  • Radius = 1526814029.64 meters or 1526814 km