Pluto has been an intriguing object ever since its discovery in 1930, lying in relative darkness nearly 31 astronomical units (at present) from the Sun. It is difficult to study because of its great distance from the Earth and the Sun; however, observational efforts since the 1970s have revealed it to be an interesting place. In 1978 Pluto was discovered to have a satellite, Charon, about half its diameter, which orbits Pluto with a period of 6.37 days—the same as Pluto's rotation period. Spectral measurements reveal Pluto to have surface ices of methane (CH₄), carbon monoxide (CO), and nitrogen (N₂), and the gaseous phase of these molecules forms a tenuous atmosphere that stretches at least 100 km (60 mi) above the planet's surface.