Clouds of choking smog; beaches coated in oil; rivers turned red by chemicals. All of these are obvious examples of pollution.
However, not all types of pollution are easily seen. For example, invisible plumes of gas may drift over our towns and cities. They can cause asthma and chest problems. They can also kill trees.
Another problem is finding out the size of the area affected. Winds can blow pollutants across thousands of kilometres, from one side of an ocean to the other. Chemicals flowing down large rivers can pollute many countries on their way to the sea.
Then there is the problem of finding out who is to blame.
Satellites are key weapons in this never-ending fight to keep the planet clean. Space-based instruments can detect harmful gases and chemical waste released by industry, power plants and traffic.
Satellites can pinpoint the sources of pollution, watch it move through the air and see where it ends up. They can also study ocean colour and detect oil spills at sea, both day and night.
From hundreds of kilometres above the Earth, these “spies in the sky” help to identify major exporters and importers of pollution across the world.