Mercury is the smallest and closest planet to the sun in the Solar System. Its orbit around the Sun takes 87.97 Earth days, which is the shortest of all the planets in the solar system. Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet.Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin–orbit resonance,[16] meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[a][17] 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 Mercurian years.

Facts about Mercury

Mercury has been known to humanity since ancient times and although its discovery date is unknown, the first mentions of the planet are believed to be around 3000 BC by the Sumerians.

A year in Mercury is 88 days, yet a Mercury day is 176 Earth days. Mercury is nearly tidally locked to the Sun – also known as a gravitational lock – and over time this has slowed the rotation of the planet to almost match its orbit around the Sun.

Mercury orbits so quickly around the Sun that early civilizations believed it was actually two different stars – one which appeared in the morning and another which appeared in the evening.

Mercury is the smallest planet in the solar system with a diameter of 4,879 km and is one of five planets that is visible to the naked eye.

After the Earth, Mercury is the second densest planet. Despite its small size, Mercury is very dense because it is composed mainly of heavy metals and rock – the main characteristic of terrestrial planets.

Mercury is named after the messenger of the Roman gods, who is also known as Hermes in Greek mythology. This is because of the speed in which Mercury orbits the Sun and the speed with which Mercury the Roman deity was able to deliver messages.

Astronomers didn’t realize that Mercury was a planet until 1543 when Copernicus published his Sun-centered model of the Solar System – putting the Sun as the centre of the solar system rather than the previously believed centre, the Earth.

The planet has just 38% of the gravity on Earth. This means that Mercury isn’t able to hold the atmosphere it has and it instead gets blown away by solar winds. However those same solar winds are also bringing in new gases, radioactive decay and dust from micrometeorites – replenishing the atmosphere.

Mercury has no moons or rings because of its low gravity and lack of atmosphere.

It was once believed that a planet called Vulcan existed between the orbit of Mercury and the Sun – however the existence of such a planet was never found. The orbit of Mercury is an ellipse rather than circular. It has the most eccentric orbit in the solar system and the least circular of all of the planets, according to scientists and astronomers.

Mercury is only the second hottest planet. Venus, though farther from the Sun than Mercury, actually experiences higher temperatures. This is because Mercury has no atmosphere to regulate temperature and results in the most extreme temperature change of all the planets – ranging from -170°C (-280°F) during the night to 430°C (800°F) during the day.

Mercury does not experience any seasons. The axis of Mercury has the smallest tilt of all other planets, and this results in a lack of seasons on its surface.

Mercury is the only planet which doesn’t rotate exactly once every year – instead rotating three times for every two orbits of the Sun. This is because it is nearly tidally locked to the Sun.

The orbit of Mercury was important in proving Albert Einstein’s theory of General Relativity.

Mercury has a large iron core that is around 40% of its volume (compared to a core volume of 17% for Earth) in its centre whose radius is 1800 to 1900 kilometers (1100 to 1180 miles).

Scientists believe the core of Mercury is probably molten.

The outer shell of Mercury is only 500 to 600 kilometers (310 to 375 miles) thick. Earth’s outer shell (the mantle and crust) is 2930 kilometers (1819 miles) thick.

Mercury has a very thin atmosphere, which is made up of atoms from the surface of the planet that have been blown away by solar winds. As Mercury is so hot, these atoms quickly escape into space and so its atmosphere is constantly being replenished.

Mercury has a weak magnetic field whose strength is about 1% of the magnetic field on Earth.

Only two spacecraft have ever visited Mercury. It is difficult to reach the planet due to its proximity to the Sun and any spacecraft visiting would need to travel 91 million kilometers into the Sun’s gravitational potential well. The Mariner 10 visited during 1974-75, flying by Mercury three times and mapping half its surface. On March 24, 1975 it ran out of fuel and is still believed to be orbiting the Sun. The MESSENGER probe was launched in 2004 to explore Mercury’s high density, its geological history, the nature of its magnetic field and more. Another mission, BepiColombo, is to be launched in 2015 by the European Space Agency and Japan is expected to reach Mercury in 2019.

Mercury has more craters and impact marks that any other planet. The surface is similar to that of the Moon, as unlike most planets, Mercury isn’t geologically active and cannot “self heal” from impacts with asteroids and comets. Most of the Mercurian craters are named after famous writers and artists. If a crater is larger than 250 km in diameter, it is known as a Basin. The largest Basin on Mercury, the Caloris Basin, is around 1,550 km in diameter and was discovered by the Mariner 10.