By about 350 BC, the ancient Greeks had realized the two stars were one. The ancients knew Mercury by different names depending on whether it was an evening star or a morning star. The BepiColombo spacecraft is planned to arrive at Mercury in 2025. Two spacecraft have visited Mercury: Mariner 10 flew by in 19 and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015. The planet has no known natural satellites. The polar regions are constantly below 180 K (−93 ☌ −136 ☏). Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 ☌ −280 ☏) at night to 700 K (427 ☌ 800 ☏) during the day across the equatorial regions. Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Its orbital eccentricity is the largest of all known planets in the Solar System at perihelion, Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄ 30 degree). An observer on Mercury would therefore see only one day every two Mercurian years. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. It is tidally locked with the Sun in a 3:2 spin–orbit resonance, meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. Mercury rotates in a way that is unique in the Solar System. From Earth, the planet telescopically displays the complete range of phases, similar to Venus and the Moon, which recurs over its synodic period of approximately 116 days. At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus.

This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight. Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, and its apparent distance from the Sun as viewed from Earth never exceeds 28°. It is named after the Roman god Mercurius ( Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (Ἑρμῆς). Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. Mercury is the smallest planet in the Solar System and the closest to the Sun.

Mercury color