WHAT IS SPACE?

Space is a boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of the boundless four-dimensional field known as space-time. In mathematics spaces with different numbers of dimensions and with different underlying structures can be examined. The concept of space is measured to be of fundamental importance to an understanding of the universe although disagreement continues between philosopher over whether it is itself an entity, a relationship between entities, or part of a conceptual structure.

Many of the philosophical questions arose in the 17th century, during the early development of classical mechanics. In Isaac Newton's view, space was unconditional - in the sense that it existed permanently and independently of whether there were any matter in the space. Other natural philosophers, notably Gottfried Leibniz, thought instead that space was a collection of relationships between objects, given by their distance and direction from one another. In the 18th century, Immanuel Kant described space and time as elements of a systematic framework which humans use to structure their experience. In the 19th and 20th centuries mathematicians began to examine non-Euclidean geometries, in which space can be said to be curved, rather than flat. According to Albert Einstein's theory of general relativity, space around gravitational fields deviates from Euclidean space. Experimental tests of general relativity have established that non-Euclidean space provides a better model for explaining the existing laws of mechanics and optics.

Outer space comprises the relatively unfilled regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations. There is no apparent boundary between Earth's atmosphere and space as the density of the atmosphere gradually decreases as the altitude increases. Nevertheless, the Federation Aéronautique International has established the Kerman line at an altitude of 100 kilometers (62 mi)) as a working classification for the boundary between aeronautics and astronautics. This is used because above an altitude of roughly 100 kilometers (62 mi), as Theodore von Kerman calculated, a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the environment to support itself. The United States designates people who travel above an altitude of 50 miles (80 km)) as astronauts. During re-entry, roughly 120 kilometers (75 mi) marks the boundary where atmospheric drag becomes noticeable, depending on the ballistic coefficient of the vehicle.

Contrary to popular sympathetic, outer space is not completely empty (i.e. a perfect vacuum,) but contains a low density of particles, principally hydrogen plasma, as well as electromagnetic radiation. Supposedly, it also contains dark matter and dark energy.

The term outer space was first recorded by H. G. Wells in his novel First Men in the Moon in 1901. The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.