Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from the Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after the Roman deity Mercury, the fast-flying messenger to the gods. Since it has almost no atmosphere to retain heat, Mercury's surface experiences the greatest temperature variation of all the planets, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day.
Mercury was first visited by the Mariner 10 probe, in January 1974, the probe was designated to study both Venus and Mercury. The United States decided to use the probe as a pathfinder and a technology demonstrator for future manned missions to both these worlds in the early years of the Space Race.
Another Mariner-Venus-Mercury probe launched in 1978 performing multiple flybys in its two year spaceflight career. Unlike the previous mission (which relied on early expendable rockets) this mission utilized the fully reusable Space Shuttle and Chemical Space Tug.
In 1984 NASA dispatched the Mercury Orbiter Mission to conduct a Planetary Wide in-depth mapping. This mission also used the Space Shuttle and Chemical Space Tug but in addition also utilized a newer form of Space Propulsion, Solar Electric Propulsion (SEP) to reach Mercury and slow itself into orbit. There it deployed Penetrator probes to land on the surface and sample the immediate subsurface (down to 1.4 meters deep).