Earth's atmosphere — the thin blanket of air that surrounds the planet — is the boundary between our world and outer space. While there is no definitive border that separates the atmosphere from space, anyone traveling above an altitude of 80 kilometers (50 miles) is designated an astronaut — a space traveler — by the United States. In comparison, commercial jetliners typically fly at an altitude of about 10.5 kilometers (6.5 miles).

Gases in the atmosphere act as a barrier, protecting Earth from harmful radiation such as gamma rays, X rays, and ultraviolet light. The atmosphere absorbs these wavelengths, along with infrared and microwave wavelengths. In addition, wind and turbulence in the air cause incoming light to bend, which results in the twinkling of stars and blurring of images. Since astronomical research relies on studying light from celestial objects, it is best to place observatories above as much of the atmosphere as possible. From high atop mountains or out in space, telescopes are able to capture light before the atmosphere has distorted or absorbed it.

Since 2000, many astronauts have had a chance to work aboard the International Space Station (ISS) — an orbiting laboratory that is contributing to research in many areas of science. At an altitude of about 400 kilometers (250 miles), ISS crew members have a unique opportunity to look both down onto Earth and out into space from above the atmosphere.

From space, stars do not appear to twinkle but instead shine steadily. In fact, without interference from the atmosphere, astronauts are able to see stars in all directions and even to distinguish their different colors. They can also see the Moon more clearly — its features appear more distinct and its color more true. Astronauts see other satellites orbiting above and below them, and they can sometimes see airplanes in flight. They can also see changes on Earth such as seasons. At night they can see city lights, and during the day — with the help of binoculars — they may see other human-made objects such as roads and the Egyptian pyramids.

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