General Information

Mercury is the closest planet to the Sun and the smallest of the Inner Planets. About 4.6 billion years ago when Mercury formed, the heat of the nearby Sun prevented gases from becoming part of the Protoplanet. Mercury is rocky and barren. When the planet is visible on Earth's horizon just after sunset or before dawn, it is obscured by the haze and dust in our atmosphere. Only radar telescopes gave any hint of Mercury's surface conditions prior to the voyage of Mariner 10. The photographs Mariner 10 radioed back to Earth revealed an ancient, heavily cratered surface, closely resembling our own Moon.

Mariner 10, launched on November 3, 1973, from Kennedy Space Center in Florida obtained the first close-up views of Mercury. After a journey of nearly five months, which included a flyby of Venus, the spacecraft passed within 703 kilometres (437 miles) of the solar system's innermost planet on March 29, 1974. Until Mariner 10, little was known about Mercury. Even the best telescopic views from Earth showed Mercury as an indistinct object lacking any surface detail. The planet is so close to the Sun that it is usually lost in solar glare. Instruments on Mariner 10 discovered that Mercury has a weak magnetic field and a trace of atmosphere — a trillionth the density of Earth's atmosphere and composed chiefly of argon, neon and helium. When the planet's orbit takes it closest to the Sun, surface temperatures range from 467°C (872°F) on Mercury's sunlit side to -183°C (-298°F) on the dark side. This range in surface temperature — 650°C (1170°F) — is the largest for a single body in the solar system. Mercury literally bakes and freezes at the same time. After the initial Mercury encounter, Mariner 10 made two additional flybys — on September 21, 1974, and March 16, 1975 — before control gas used to orient the spacecraft was exhausted and the mission was concluded. Each flyby took place at the same local Mercury time when the identical half of the planet was illuminated; as a result, we still have not seen one-half of the planet's surface.

Mercury is sometimes visible with binoculars or even the naked eye, but it is always very near the Sun and difficult to see in the twilight sky.