The environments of Mars and Venus are relatively similar to that of Earth’s, even though they are hostile to human life. We might be able to change them one day by terraforming: adapting the atmosphere of terrestrial planets to make them breathable, with a habitable humidity and temperature. NASA is currently testing equipment designed to produce oxygen from Mars’ atmosphere. It works by taking in carbon dioxide, which makes up 96 per cent of the planet’s gas, and splitting it electrochemically to release oxygen. It’s a tentative step towards a colony on Mars. BB