Pluto is the odd man out among the planets -- indeed, many astronomers doubt whether it deserves to be called a planet at all. It is by far the smallest planet. Pluto's diameter of under 2400 km is not only less than that of our own Moon but smaller even than Neptune's largest satellite, Triton; in fact, the icy surface of Triton, photographed by Voyager 2, probably looks much like that of Pluto. Triton and Pluto may both have wandered the outer reaches of the Solar System before Triton was captured by Neptune; the strange orbits of Triton and Nereid around Neptune could therefore be the consequences of that disruptive event. Certainly, Pluto has the oddest orbit of any planet: it crosses the orbit of Neptune, so that at times Neptune temporarily becomes the outermost planet of the Solar System, as was the case for 20 years between February 1979 and February 1999. Pluto's average distance from the Sun is 5900 million km. Pluto is not alone in its 248-year-long orbit around the Sun. In 1978 astronomers discovered that it has a moon, since named Charon, half Pluto's diameter. Charon orbits its parent every 6.4 days, the same time that the planet takes to spin on its axis. Charon therefore hangs over one spot on the surface of Pluto, visible permanently from one hemisphere of the planet but invisible from the other. Being so faint and insignificant, Pluto was not discovered until 1930. For decades prior to that, various astronomers had tried to predict where a planet beyond Neptune might lie, but without success. In the end, Pluto was found by Clyde Tombaugh at the Lowell Observatory in Arizona as the result of a deliberate round-the-sky photographic search for new planets. Tombaugh's search failed to detect any sign of a tenth planet lurking beyond Pluto, as have all subsequent searches. But what has been found is a swarm of icy bodies, like large comet nuclei, called the Kuiper Belt (sometimes, the Edgeworth-Kuiper Belt), which is an inner extension of the larger Oort Cloud of comets (see Comets & Meteors). Pluto can be thought of as the largest member of the Kuiper Belt.