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Mercury: the First Rock from the Sun

Mercury is the closest planet to the Sun.  It is a rocky, or terrestrial, planet like Venus, Earth and Mars.

Mercury has the second highest density of all the planets.  Only the Earth is denser.  Part of the high density of the Earth is caused by gravitational contraction.  The actual materials of Mercury include a high proportion of dense materials like Iron.  It is believed that both Mercury and the Earth have molten Iron cores. Both planets have significant magnetic fields although Mercury’s magnetic field is not as strong as the Earth's.

Mercury does not have much atmosphere.  It has a very thin one consisting mainly of Helium.

Mercury in Antiquity

Because it is so close to the Sun there are some difficulties in seeing this planet.  However, it can be seen with the naked eye at some times of the year, either in the early morning or in the evening. 

Humans must have seen Mercury in the sky from the time that they first started to look up and wonder about the lights in the sky.  The ancient Greeks were aware of Mercury, but did not realise that the object that appeared in the sky in the morning was the same as the one they sometimes saw in the evening.

Rotation and Revolution

Mercury revolves round the Sun every 88 Earth days.  For every two orbits of the Sun, Mercury rotates on its axis three times.  That is, Mercury’s year is one and a half of Mercury’s days.

Mercury’s orbit is more elliptical than any other planet.   Pluto’s orbit is more elliptical, but Pluto is considered a dwarf planet in most places now.  Mercury’s orbit is also about 7 degrees out of the planetary plane.


Mercury has only been visited twice by Earth spacecraft. 

In 1975, Mariner 10 succeeded in doing flybys of the planet and mapped 45 percent of its surface.

Launched in 2004, the MESSENGER space probe has already done flybys of Mercury, as well as the Earth and Venus, and is due to go into orbit round Mercury in 2011.

The Future

So far, not much attention has been directed to Mercury.  There are obvious difficulties in using a planet so close to the Sun. However, this does not mean this planet will never be used. 

There have been several science fiction stories including the idea that Mercury will be settled and the apparently abundant minerals used, as well as making use of the high intensity solar energy.
  These stories include Rendezvous with Rama by Arthur C Clarke, and the Red Mars, Green Mars, Blue Mars trilogy by Kim Stanley Robinson.  These are both very good books, not only having excellent stories, but also they both have real ideas about the future.


Image processing by R. Nunes.
Original data from NASA.
Mercury from 125,000 miles away
This Picture is made up of images sent back by NASA's probe, Mariner 10 in the 1970's.