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MERCURY is the smallest planet in the solar system. It is only slightly smaller than distant Pluto. It is the closest planet to the sun and is hence very difficult to see, only becoming visible shortly before sunrise and sunset.

Mercury is a barren, rocky planet, similar in its outward appearance to the Earth's own moon. The temperatures on Mercury, however, are extremely diverse. The planet has four poles, two hot ones and two warm ones, and all are located on the equator. Temperatures at one of the hot poles may range from 400C at noon to -170C at nighttime.

The impossibility of seeing many details on the planet's surface because of the nearby glaring sun, meant that exploration via machine was essential for greater knowledge. In 1973, the Mariner 10 mission to the planet began - still one of the most successful space exploration programmes undertaken. The spacecraft used the gravitational field of Venus to allow it to swing on a trajectory to pass Mercury three times. The third time, when the Mariner spacecraft was only 200 miles from the surface of this tiny planet, remarkable pictures were obtained.

The surface of Mercury is more heavily cratered than that of our moon. This leads astronomers to presume that Mercury is over 4,000 million years old. The largest crater on Mercury is the Caloris Basin which reaches 1,300km (810 miles) in diameter. Mercury has experienced little volcanic activity. There are no basalt 'seas' on the surface as there are on the Earth's moon.

The density of Mercury is not much less than the Earth's density. The planet has a large, metallic core which may be about the size of our moon. Apart from the core, the planet is probably composed of silicate materials, as is the Earth's mantle. However, underneath the surface, some molten activity must be occuring because Mercury, like the Earth, has a strong magnetic field, caused by such activity.