Mercury is the nearest planet to the Sun, but unlike most of the planets whose orbits are nearly circular, it has an elliptical orbit. At its closest to the Sun it is a mere 46 million kilometres away increasing to 70 million kilometres at its furthest. If you were to stand on the surface you would see a noticeable change in the size of the Sun over a period of 88 days which is the time the planet takes to go around the Sun once. Mercury rotates on its axis once in 59 days. Its day is 2/3 the length of it's year. This is no coincidence, because it is so close to the Sun the gravity of the Sun forces the planet to rotate in this way. A similar effect occurs between the Earth and the Moon where the Moon is forced by the Earth's gravity to rotate once on it's axis every time that the Moon completes on orbit of the Earth.

Mercury is smaller than Ganymede, a large moon of Jupiter and Titan, the large moon of Saturn. It has a large iron core making up 50% of the planet. By comparison Earth's is 16% and the Moon's 4%. It has a weak magnetic field about 1% as strong as the Earth's.

The surface is very inhospitable, if you were to stand on the surface you would see a huge Sun nine times larger than it looks from Earth but because of the elliptical orbit this would change during the 88 days Mercury took to orbit the Sun. The average daytime temperature is 460 ºC although it goes up to 700 °C in places, hot enough to melt lead. At night the temperature falls to -180 °C. The temperature variation between day and night is the greatest in the Solar System.

The atmosphere of Mercury is very thin and is made up of particles of the solar wind captured by the planet. Mercury's small gravity and high temperatures cause the captured particles to be quickly driven away from the planet so its atmosphere needs to be continuously replenished by solar wind particles.

Much remains to be learned about Mercury as only one spacecraft has visited the planet, Mariner 10 in 1973-4.

Mercury is too close to the Sun for the Hubble Space Telescope to study it as the telescope could be damaged if direct sunlight were to enter the telescope.

A new mission to Mercury called Messenger was launched on 3rd August 2004. It will travel in a complicated trajectory flying by Venus and Mercury to eventually go into Mercury orbit in March 2011

To find out more about the Messenger mission click here >>>

Vital statistics for Mercury click here >>>

© Matthew Wallace 2006