Outer space
Exploring space by direct travel is difficult, and more or less impossible beyond the near planets. Space contains no air beyond the Earth. It takes three days of travelling to reach the Moon and, depending on speed, it would take a very long time to reach the closest star (Proxima Centauri). Manned spacecraft are designed to keep air inside them and to protect astronauts from extreme temperatures. It is widely thought that people will not be able to travel to nearby stars. A serious problem for humans is the great times taken for interstellar flights, and the danger of radiation to human life.