Mercury

Mercury is the closest planet to the Sun. Its orbit is said to be "inferior", as it lies between the Sun and the Earth, and, consequentialy, it always appears in the sky comparatively near to the Sun, being visible a little before sunrise or a little after sunset. It takes 87 deg9 days to complete its orbit.

The planet has no appreciable atmosphere, and the temperature on the surface facing the Sun is as high as 430 degC. On its night side, it falls to below -170 degC since it has no atmosphere. In many ways, Mercury's surface resembles that of the Moon. Key differences are fewer mid-size craters; no mountain ranges; many shallow, scalloped cliffs, called scarps; fewer basins and large lava flows; and more relatively uncratered plains. The scarps vary in length from 20 to 500 km and have heights from a few hundred metres up to 2 km. They result from surface shrinkage in radius of about 2 km, from cooling of either its core or its crust billions of years ago.

The planet rotates once every 58.6 Earth days, and all of its surface sees the Sun as a result. Interestingly, its rotation is tidally coupled to the Sun, rotating 1.5 times every orbit, in much the same way that the Moon always keeps one face towards the Earth. Simple maths show that its solar day is just twice the length of its year; 176 days.

Probing the inner limits Much of the information on Mercury was found by the NASA 1973 to 1975 Mariner 10 mission launched from Cape Canaveral. Cameras on board scanned 50% of the surface to increase our resolution of its detail by 5000 times. Shortly after the year 2000 a new probe will enter orbit around Mercury.