A rocket lifts astronauts and their spacecraft off the ground by burning fuel and pushing hot gas out of its engine. When the rocket pushes its exhaust out, the exhaust pushes the rocket the other way. Rocket scientists call the law behind liftoff Newton's third law. For every action there is an equal and opposite reaction. A rocket engine carries everything it needs to burn its fuel. So it can work in space. There is no air there.

The International Space Station is a home and laboratory. Astronauts live and work there while it orbits Earth. Astronauts and everything in the station float, because the station is weightless inside. They float because they and the station are falling around Earth together. It is not because there is no gravity. Earth's gravity still pulls on the station. At that height it is only about 10 percent weaker than on the ground. The station circles Earth at about 28,800 kilometers an hour, 400 kilometers up. One circuit takes about 92 minutes. The astronauts see 16 sunrises and 16 sunsets every day.

In space, food floats, and astronauts need no plate to eat. The station cleans and recycles water from sweat and breath, and astronauts drink it. Astronauts grow plants, in a small garden called Veggie, on the space station. Astronauts exercise to keep their muscles and bones strong, because space makes them weaker.

Astronauts wear spacesuits for spacewalks, outside the station. Space is airless and dangerous without one. A spacesuit backpack holds oxygen for astronauts to breathe. It removes the carbon dioxide they breathe out. Spacesuits protect astronauts from radiation and from bits of space dust. In the near vacuum of space there is no air to carry heat away. So suits cool astronauts with water in tubes. Astronauts practice spacewalks in a huge pool of water. It is the closest you can get to microgravity on Earth.

Apollo 11 carried Neil Armstrong, Michael Collins and Buzz Aldrin. It launched on July 16, 1969. On July 20, 1969, Neil Armstrong became the first person to walk on the Moon. Buzz Aldrin joined him. While they were on the Moon, Michael Collins stayed in orbit in the command module Columbia. Armstrong and Aldrin spent about 21 hours on the Moon's surface. About two and a half hours of it was outside.

On April 12, 1961, Yuri Gagarin became the first person to orbit Earth, in Vostok 1. His flight around Earth lasted 108 minutes. Valentina Tereshkova became the first woman in space on June 16, 1963. Sally Ride became the first American woman in space on June 18, 1983.

Words to know

• orbits: circles around
• exhaust: hot gas out
• weightless: floating, no weight
• spacewalks: trips outside
• microgravity: almost no weight