The Solar System, Orbital Motion and Satellites
The Solar System's structure and the physics of orbital motion, examined only in separate GCSE Physics and not in Combined Science: Trilogy, cover why gravity keeps planets, moons and satellites moving on a stable, roughly circular path, and how an object's orbital speed is linked to the radius of its orbit. It also covers the difference between natural satellites, such as the Moon, and artificial satellites, human-made objects launched into orbit for uses such as communication and monitoring the weather.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the layout of the Solar System: the Sun at the centre, the eight planets in order of increasing distance (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) orbiting it, dwarf planets, moons orbiting some planets, and asteroids and comets also orbiting the Sun.
- For 'why does an orbiting object not fly off in a straight line' questions, start from Newton's first law: an object continues at a constant velocity unless a resultant force acts on it; a satellite in orbit does have a resultant force acting on it, so its velocity is not constant.
- Identify that force as gravity, acting towards the centre of the orbit. This force continuously changes the object's direction, and therefore its velocity, even though it does not change the object's speed, because the force acts at right angles to the object's motion at every point on a circular orbit.
- Learn the link between orbital radius and orbital speed for a stable circular orbit: a larger orbital radius requires a lower orbital speed, and a smaller orbital radius requires a higher orbital speed.
- Use this link to explain what happens if an orbiting object's speed changes: if its speed increases above the value needed for its current radius, the object moves into a larger orbit; if its speed decreases below that value, it moves into a smaller orbit.
- Apply the same radius-speed link to compare planets: a planet with a large orbital radius, further from the Sun, such as Neptune, has a lower orbital speed and takes far longer to complete one orbit than a planet with a small orbital radius, closer to the Sun, such as Mercury.
- Distinguish satellite types clearly: natural satellites, such as the Moon, are not human-made; artificial satellites are human-made objects placed deliberately into orbit around the Earth, or another body, for a purpose, such as satellite television and telephone signals, GPS navigation, or monitoring weather and climate from above.
Worked example
A satellite orbits the Earth in a stable, circular orbit. Engineers fire the satellite's thrusters briefly, increasing its orbital speed. Describe and explain what happens to the satellite's orbit as a result.
- Before the thrusters fire, the satellite is in a stable orbit, meaning its speed exactly matches the speed needed for its current orbital radius.
- Firing the thrusters increases the satellite's speed above the value needed for a stable orbit at its current radius.
- Because orbital speed and orbital radius are linked, a faster stable orbit corresponds to a smaller radius, and a slower stable orbit corresponds to a larger radius, so a speed that is now too high for the original radius means the satellite can no longer stay on that original circular path.
- The satellite therefore moves outwards into a new orbit with a larger radius.
- Final answer: once it settles into this new, larger orbit, the satellite's speed will have decreased again, back down to the lower speed needed for a stable orbit at that larger radius; the overall effect of the thruster firing is to raise the satellite into a higher orbit.
Practice questions
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Q1State the name of the force that keeps a satellite in orbit around the Earth.Show answer
Answer: Gravity (the Earth's gravitational force/attraction on the satellite).
Q2Put the following in order of increasing distance from the Sun: Mars, Venus, Saturn, Earth.Show answer
Answer: Venus, Earth, Mars, Saturn.
Q3Explain why a satellite moving in a stable circular orbit has a changing velocity even though its speed stays constant.Show answer
Answer: Velocity has both a size (speed) and a direction; as the satellite moves around a circular orbit its direction is constantly changing, so its velocity is constantly changing, even while its speed remains the same.
Q4State what provides the centripetal force that keeps a planet in orbit around the Sun.Show answer
Answer: The Sun's gravitational force (gravity) acting on the planet.
Q5State two uses of artificial satellites.Show answer
Answer: Any two of: satellite television or telephone communication; GPS/satellite navigation; monitoring weather; monitoring climate; scientific research/observing space.
Q6A satellite in a stable, small-radius, close orbit around the Earth is compared with a satellite in a stable, large-radius, distant orbit. State which of the two satellites has the greater orbital speed.Show answer
Answer: The satellite in the smaller, closer orbit has the greater orbital speed.
Q7Explain, in terms of forces, why a satellite in a stable orbit does not fall towards the Earth or fly off into space.Show answer
Answer: Gravity continuously pulls the satellite towards the Earth, providing exactly the centripetal force needed to keep changing its direction and hold it on a circular path; because this balances the satellite's tendency, by Newton's first law, to keep moving in a straight line, the combined effect is a stable curved orbit rather than the satellite falling in or flying away.
Q8State one difference between a moon and an artificial satellite.Show answer
Answer: A moon is a natural satellite that formed naturally and orbits a planet; an artificial satellite is human-made and deliberately launched into orbit for a purpose.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A satellite's engines fire briefly and reduce its orbital speed. Describe and explain what happens to the radius of its orbit as a result.
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A student compares two planets orbiting the same star: Planet A orbits with a small orbital radius, and Planet B orbits with a much larger orbital radius. (a) State which planet has the greater orbital speed. (b) State which planet takes longer to complete one orbit of the star. (c) Explain your answer to part (a) in terms of the relationship between orbital radius and orbital speed.
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Explain, using ideas about force and motion, why a satellite orbiting the Earth in a stable circular orbit keeps moving on a circular path rather than moving off in a straight line or spiralling in towards the Earth.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the the solar system, orbital motion and satellites worksheet pack - 13 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 24 of GCSE Physics Workbook, the whole course as one free printable PDF.
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