Chapter 10: Space physics

The chapter problem: the orbiting telescope

A telescope orbits the Earth. It circles the planet once every 95 minutes, about 540 km above the ground. From above the air, it takes clear pictures of the sky.

These are the facts to use all the way through the chapter.

• The telescope is an artificial satellite. It follows a nearly circular orbit about 540 km above the Earth's surface.
• One orbit takes about 95 minutes.
• Radio signals from the telescope travel at 3.0 × 10⁸ m/s.
• It photographs clouds of gas and dust where new stars are forming. The Sun formed from a cloud like these.
• It also photographs very distant galaxies. Their light is red-shifted: the wavelength is longer than the same light measured in a laboratory.
• The further away a galaxy is, the faster it is moving away from us.

By the end of the chapter you will explain what keeps the telescope in orbit, and describe the pull and push inside a star like the Sun. You will explain why the light from distant galaxies is red-shifted, and what that tells us about the universe. You will also work out how long a radio signal takes to reach the ground, and how long light has travelled from a distant galaxy.

First contact: no working yet. Predict. Red light has a longer wavelength than blue light. The light from a distant galaxy has a longer wavelength than the same light made in a laboratory. Is the galaxy moving towards the telescope or away from it? Write your prediction down, with a reason.

By the end of this chapter

You will be able to:

• describe the main features of the solar system, including the planets, the dwarf planets, the moons and the artificial satellites;
• compare the planets, their moons and artificial satellites, and describe what they have in common and how they differ;
• explain how gravity keeps a planet or a satellite in a circular orbit;
• explain how the Sun formed from a cloud of dust and gas, and why fusion reactions keep it stable;
• describe the life cycle of a star the size of the Sun and of a much more massive star;
• explain why the light from distant galaxies is red-shifted;
• explain how red-shift is evidence that the universe is expanding, and how it supports the Big Bang model;
• calculate how long light takes to travel a given distance, using $\text{time} = \dfrac{\text{distance}}{\text{speed}}$.

One explanation is for Higher tier only: how the force of gravity changes the velocity of a satellite without changing its speed, and why the radius of a stable orbit must change if the speed changes.

Where it is used

Space physics explains the night sky and the story of the universe. Satellites send television pictures and navigation signals, watch the weather, and photograph the ground after a flood. Telescopes on the ground and in space collect the light from distant galaxies. The same ideas also tell us where the atoms in our bodies were made.

Your turn to connect it

Where have you seen or used something to do with space? It might be a satellite navigation app, a weather forecast, a photograph from a space telescope, or a news story about the universe. Write one example, and say what it depends on.

Roadmap

1. 10.1 The solar system and orbits
2. 10.2 Red-shift and the Big Bang
3. Review