Our solar system has one star, the Sun, eight planets, dwarf planets such as Pluto, and natural satellites (moons) that orbit planets. Artificial satellites orbit the Earth for communication, weather forecasting, navigation and observing space. The solar system formed about 4.6 billion years ago from a cloud of dust and gas pulled together by gravity.
Gravity provides the force that keeps planets and satellites in nearly circular orbits. In a circular orbit the speed stays constant but the direction keeps changing, so the velocity changes and the object is accelerating towards the centre. For a stable orbit, an object closer to the body it orbits must move faster. Geostationary satellites orbit once every 24 hours above the equator, so they stay over the same point on Earth.
Worked example
Why must a satellite in a lower orbit travel faster?
- Gravity is stronger closer to the Earth.
- A stronger pull needs a faster speed to stay in a circle at that radius.
Answer:If it went slower, it would fall to a lower orbit.
Key words
- natural satellite
- a moon orbiting a planet
- artificial satellite
- a human-made object in orbit
- geostationary orbit
- 24-hour orbit above the equator
- centripetal force
- the force towards the centre of a circle
New questions every time.
NGSS-referenced: HS-ESS1-4