Mercury

The planet Mercury is very difficult to study from the Earth because it is always so close to the Sun. It is the second smallest planet (it was believed to be the smallest until the discovery that Pluto is actually much smaller than originally thought), and also the fastest in its orbit since it is the innermost planet. In fact, the name Mercury derives from its speed in moving around its orbit.
We began to learn more about Mercury with radar imaging from the Earth in the 1960s, and obtained most of what we know about the planet from the Mariner 10 space probe was placed into a complicated orbit involving Venus and Mercury and which passed close to Mercury and sent back information three times in the period 1974-1976.

Distance from the sun: 0.4 AU [46 - 70 million kilometers range]
Rotational period: 59 days [length of time to rotate on its axis]
Mercurian year [length of time to revolve around sun]: 88 days
Orbit: Elliptical
Mass: 5.5% of the earth's. 5x the mass of the moon
Density: 5.5 g/cc--the same as earth's
Magnetic field: 1% of the earth's
Temperature range: -180 degrees Celsius at night...425 degrees Celsius daytime