Rebecca McNaught, a pupil at Queens High School, asks :-

What do we know about the planet Mercury?

Frank Andrews, an astronomer-educator at the Carter Observatory, Wellington, responded.

Mercury has been visited by only one spacecraft, Mariner 10. This was launched from Earth on November 3rd 1973 and made a total of three flybys of Mercury during 1974 and 1975. (Bill Pickering, New Zealand born and educated Director of NASA's Jet Propulsion Lab, directed this event and so appeared on the front cover of Time magazine.)

Mercury is a surprisingly difficult planet to reach from the Earth because it travels very fast in its orbit around the Sun (average orbital velocity of 47.87km/sec). Mariner 10 used a flyby of Venus to accelerate it fast enough to catch up with Mercury.

Before the Mariner 10 flybys virtually nothing was known about Mercury. Because it is never more than 28 angular degrees from the Sun as viewed from the Earth, telescopic observations must be made during daytime or at twilight through a long path length of the earth?s turbulent atmosphere. As a result telescopic observations are poor compared to those of most other planets.

With a diameter of 4,878km Mercury is between our Moon and Mars in size. Measurements of Mercury's size and mass show it is basically a large Iron ball-bearing covered by a thin layer of rock. Its small size and high temperatures means all gas has escaped from its surface so it has no atmosphere. Mercury has the greatest temperature range of any planet in our Solar System. Daytime temperatures reach 427 degrees Celsius while at night the temperature falls to minus 173 degrees Celsius.

Mariner 10 found Mercury to have a surface rather like the Moon with heavily cratered areas and comparatively smooth planes. One important difference is the presence of what are called "Rupes". These are ridges that can extend for over 2000 kilometers and frequently cross craters. They seem to be places where the crust has cracked and one part ridden over the other. (Geologists call this "overthrusting"). These ridges are thought to be the result of shrinkage of the large iron core as it cooled forcing the relatively thin crust to adapt to the reduced volume of the core.

You will be interested to know that two new space probes are being planned to visit and orbit Mercury. These are "BepiColombo" due for launch by the European Space Agency in 2009 and "Messenger" which NASA intends to launch about the same time. BepiColombo will land a small probe near one of Mercury's poles to examine the nature of the planet's soil. The lander will also have a camera, a seismometer and temperature measuring instruments. It is also intended to measure the density and hardness of the soil.