Space Physics: The Solar System and Orbits

Space Physics: The Solar System and Orbits

The Solar System

  • The solar system consists of the Sun, planets, dwarf planets, moons, asteroids, and comets.

  • The Sun is located at the centre of the solar system and is a star.

  • Planets, dwarf planets and their moons, as well as many asteroids and comets, orbit around the Sun.

  • The eight recognised planets in our solar system are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

  • Dwarf planets, such as Pluto, are smaller bodies that have not cleared their orbital path of other debris.

  • Comets are icy bodies that develop tails as they approach the Sun.


  • The gravitational pull of the Sun is what keeps planets and other bodies in their orbits.

  • In the solar system, all planets move in elliptical (oval-shaped) orbits around the Sun following Kepler’s first law of planetary motion.

  • The closer a planet is to the Sun, the stronger the Sun’s gravitational pull and the faster the planet moves.

  • Orbital speed varies depending on the distance a planet or object is from the Sun: objects further away from the Sun move more slowly than those closer to it.

  • Satellites, both natural (like moons) and artificial, also follow elliptical orbits around the planets they orbit.

Gravitational Field Strength

  • The force of gravity decreases with distance. It is stronger closer to a planet and weaker the further away you are.

  • This change in gravitational field strength explains why objects weigh less the further they are from the centre of the Earth.

Centripetal Force

  • When bodies move in circular paths, they constantly change direction, which means that they are accelerating.

  • This acceleration is caused by a resultant force, known as a centripetal force, which always acts towards the centre of the circle.

  • In the case of orbits, the centripetal force is provided by gravity.

** Satellites and their uses**

  • Artificial satellites are human-made objects that orbit Earth or other celestial bodies.

  • There are two types of artificial satellites: geostationary satellites, which stay above the same point on the Earth’s equator, and low Earth orbit satellites, which orbit closer to the Earth and travel around it very quickly.

  • Uses for these satellites include communications, weather forecasting, observing the Earth’s climate, and scientific research.

  • The time periods and paths of satellites depend on their height above the Earth. The higher the satellite is above the Earth, the longer its orbital period.

Space Travel

  • Missions such as those conducted by NASA’s Apollo programme have allowed astronauts to land on the moon and return safely to Earth.

  • Modern space exploration includes unmanned missions to Mars, the use of space telescopes and the international space station.

  • The challenges of such exploration include surviving in a vacuum and dealing with extreme temperatures, managing power supplies, taking off and landing, and the long durations of space travel.