Space is a term that can refer to various phenomena in science, mathematics, and communications. In astronomy and cosmology, space is the vast 3-dimensional region that begins where the earth's atmosphere ends. Space is usually thought to begin at the lowest altitude at which satellites can maintain orbits for a reasonable time without falling into the atmosphere. This is approximately 160 kilometers (100 miles) above the surface. Astronomers may speak of interplanetary space (the space between planets in our solar system), interstellar space (the space between stars in our galaxy), or intergalactic space (the space between galaxies in the universe). Some scientists believe that space extends infinitely far in all directions, while others believe that space is finite but unbounded, just as the 2-space surface of the earth has finite area yet no beginning nor end. In mathematics, space is an unbounded continuum (unbroken set of points) in which exactly three numerical coordinates are necessary to uniquely define the location of any particular point. It is sometimes called 3-space because it contains three distance dimensions. If a continuum requires fewer or more than three coordinates (dimensions) to uniquely define the location of a point, that continuum is sometimes called n-space or n-dimensional space, where n is the number of dimensions. Thus, for example, a line constitutes 1-space and a plane constitutes 2-space. When time is considered as a dimension along with the usual three in conventional space, the result is sometimes called 4-space, 4-dimensional space, time-space, or space-time. In digital communications, the term space refers to an interval during which no signal is transmitted, or during which the signal represents logic 0. The term space may also be used in reference to the time interval separating two characters, bytes, octets, or words in a digital signal