Good question! It seems funny that astronauts need to wear a helmet out in space when there is nothing to bump their heads on! Astronauts wear special suites (and helmets) in space because the pressure is so low. The suits that astronauts wear are pressurized to simulate the pressure that humans live in on Earth. The Earth’s atmosphere is composed of gas (mostly nitrogen and oxygen), and the gas basically weighs down on the surface of the Earth. The weight of the atmosphere increases the pressure of the air that we live in. The pressure at the surface of the Earth is just right for humans and other animals and plants to live. Out in space there is very little gas, so the pressure is very low– way too low for people to survive in. We need the pressure to be just right. Likewise, people could not live at the bottom of the ocean because the pressure from the weight of all of the water would be way too high.