Mercury is the closest planet to the Sun. It has no satellites and only trace amounts of hydrogen and helium in its atmosphere (temporarily captured from the Solar wind). Its surface appears very similar to Earth's Moon, but the planet has a much larger iron core and is therefore much denser. Its most distinctive known surface feature is Caloris Basin, a colossal impact crater approximately 1350 km in diameter.
Mercury has been known of since at least the time of the Sumerians (3rd millennium BC). It was given two names by the ancient Greeks, Apollo when visible in the morning sky and Hermes when visible in the evening, but Greek astronomers knew that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbited the Sun, not the Earth.
The orbit of Mercury is highly eccentric, ranging from 46 million kilometers to 70 million kilometers in diameter. The slow precession of this orbit around the sun could not be completely explained by Newtonian Classical Mechanics, and for some time it was thought that another planet might be present in an orbit even closer to the sun (sometimes referred to as Vulcan) to account for this perturbation. Einstein's General Theory of Relativity provided the explanation for this small discrepancy instead, however.
Mercury is one of the least-studied of the solar system's planets. It was visited by only one spacecraft, Mariner 10, which flew past it on three occasions in 1974 and 1975. Only 45% of the planet's surface was mapped, and it is located too near the sun for existing telescopes to conduct further mapping from Earth. Until radar observations in 1965 proved otherwise it was thought that Mercury was tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the sun at all times. Instead, Mercury is in a 3:2 resonance, rotating three times for every two revolutions around the sun. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in it's 3:2 resonance, so showing the same face, which would be also the case if it was totally locked.
Recent radar observations have suggested that water ice may be present in small quantities at Mercury's north pole. Such water, if it exists, is likely located at the permanently shaded bottoms of craters where it is deposited by comet impacts. A new mission to Mercury has been approved by Nasa, named Messenger, and will launch in 2004 to orbit Mercury starting in 2009.
- Mass: 3.3×1023 kg (Approximately 5.5% of the Earth's mass)
- Diameter: 4878 km
- Surface Area: 75 million km2
- Length of year: 87.97 Earth days
- Rotation period: 58.65 Earth days
- Mean surface temperature: 452 K (Kelvin), ranging between 90 to 700 K.
- Average distance from Sun: 0.387 AU
- Perihelion: 45 million km
- Aphelion: 69.7 million km