The Solar System

Mercury

The smallest planet and the closest to the Sun: a grey, cratered rock, baked by day and frozen by night.

Mercury
The innermost planet, mapped by NASA’s MESSENGER spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, Public domain (NASA).
Size
4,879 km across. About 38% as wide as Earth.
One year
88 Earth days
One day
176 Earth days
Moons
None
Spacecraft
Mariner 10 and MESSENGER mapped it. BepiColombo, from Europe and Japan, arrives in orbit in late 2026.

Sizes and periods from NASA’s planetary fact sheets; moon counts from NASA Science.