In astronomy and common usage, space is the part of the universe lying outside of the limits of the Earth's atmosphere; the apparently empty parts of the Universe, between planets and stars; the expanse in which the Solar System, stars, and galaxies exist.

    Space is not really empty, but the material in space is so dilute that it is hard to detect it. It contains magnetic fields, electromagnetic radiation, gases, dust and other particles. It is sometimes called "outer space" to distinguish it from airspace and terrestrial locations.

As Earth's atmosphere has no abrupt cut-off, but rather thins gradually with increasing altitude, there is no definite boundary between the atmosphere and space. In the United States, persons who travel above an altitude of 50 miles (80 kilometers) are designated as astronauts. 400,000 feet (75 miles or 120 kilometers) marks the boundary where atmospheric effects become noticeable during re-entry. The altitude of 100 kilometers or 62 miles is also frequently used as the boundary between atmosphere and space.