Mercury

Mercury is the innermost major planet in the Solar System. It is smaller than Earth and travels around the Sun on a compact orbit that takes far less time to complete than Earth's orbit.

Its cratered rocky surface can look somewhat Moon-like, but Mercury is a planet with its own unusual environment, orbital behavior, and history of spacecraft exploration.

Mercury travels around the Sun faster than any other major planet. Its year lasts about 88 Earth days.

Mercury has almost no substantial atmosphere to redistribute heat across its surface. Areas in direct sunlight can become extremely hot while nighttime regions become extremely cold.

Despite being closest to the Sun, permanently shadowed craters near Mercury's poles can remain cold enough for water ice to exist.

Traveling to Mercury requires careful spacecraft navigation because a mission approaching the inner Solar System gains speed as it falls deeper into the Sun's gravity.

NASA's Mariner 10 and MESSENGER missions transformed scientists' understanding of Mercury, and the BepiColombo mission continues the exploration of the innermost planet.

That comparison turns orbital period from an abstract number into visible motion.

These linked mission pages explain how spacecraft have studied mercury: their objectives, paths, instruments, and results. Each page carries its own mission record rather than repeating the planet description.