The innermost planet in the Solar System is a dense, heavily cratered world that takes about 59 Earth days to fully rotate on its own axis as it travels on its 88-day journey around the Sun.
Find out more about the other planets in the Solar System
Photo: Mercury taken by the Messenger probe (NASA/JHU Applied Physics Laboratory/Carnegie Institution)
The closest planet to the Sun is hard to spot.
Sir Patrick Moore talks to Dr David Rothery about Messenger's Mercury images.
The Messenger probe is looking at the side of Mercury that Mariner 10 couldn't photograph and is studying the planet's magnetic field and dense core. Having now completed a series of flybys, in 2011 it will enter a yearlong orbit of the planet.
BBC News's Christine McGourty reports on Messenger's mission.
In a 14 January 2008 BBC News report, science correspondent Christine McGourty reports on Messenger's mission at Mercury as the spacecraft prepares for its first flyby of the innermost planet in the Solar System. She speaks with Alison Boyle from London's Science Museum.
Sir Patrick Moore has doubts about Lowell's powers of observation.
Sir Patrick Moore describes how Percival Lowell mapped Mercury. Though he recognises that Lowell made many contributions to astronomy, Sir Patrick notes that his maps of other planets were not accurate.
Tortured Mercury suffers the biggest temperature swings of all the planets.
Professor Brian Cox looks at Mercury, the smallest planet in the Solar System, and explains why it lost its atmosphere.
Learn how to remember all the planets of the Solar System in order.
Here’s an animated guide to remembering all the planets of the Solar System in order, using an easy mnemonic trick.
Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System. It is named after the Roman deity Mercury, the messenger to the gods.
Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, so it can only be seen visually in the morning or the evening sky, and never exceeds 28° away from the Sun. Also, like Venus and the Moon, the planet displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, the so-called synodic period. Although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus.
Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance, and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[a] 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. An observer on Mercury would therefore see only one day every two years.
Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 degree), and its orbital eccentricity is the largest of all known planets in the Solar System.[b] At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, surface temperatures varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no known natural satellites.
Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; 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.