Mercury was named after the Roman messenger God, akin to the Greek God Hermes - which scientists believe was because the planet moved so fast across the sky. Being the first planet in our solar system from the Sun, it is also the smallest. Smaller than even some of the moons orbiting Jupiter and Saturn, Mercury is suspect to having a greater mass due to its iron core, which makes up for 47% of the planets mass. Although its core is suspected to be large in proportion to the planet's size, it is still too weak to hold an atmosphere.

The atmosphere itself is comprised of hydrogen, helium, oxygen, sodium, calcium and potassium, which is normally blasted away by solar winds. Regardless of this expulsion of atmosphere into space, Mercury continues to regenerate an atmosphere.

The surface of the Mercury resembles that of our own moon - a crater ridden field from constant bombardment of debris. With little to no atmosphere of its own, there is virtually nothing to burn up debris as it pummels into the planet. As the rotation period of the planet is 176 of our days (sunrise to sunrise), the "daylight" side of the planet is subject to extreme temperatures as hot as 840 ° Fahrenheit for long periods of time. Much too hot for any life that we know of. A rotational period of Mercury (one revolution on its axis) is 59 Earth days long - given its very fast orbit around the sun (88 Earth days), which gives it the extremely long "day". However, the deep recesses of the planets impact craters show signs of water ice. Since these craters are permanently shielded from sunlight, they never melt away.

In 1962, Russian scientists were the first to bounce a radar signal off of the planet, followed by American scientists 3 years later, confirmed the rotational period was only 59 days long, thus causing controversy over the previous evidence of Mercury's 88 day rotational cycle - meaning that one side would always face the sun. Later, came the explorations made by Mariner 10 - the first probe to visit Mercury. Mariner 10 gave us a small fraction of the planet's surface in what images it could collect. Later, it would be followed by MESSENGER in 2008, giving us even more images mapping out the planet and is expected to finish imaging the surface in 2011.

Mariner 10 detected UV radiation near Mercury and it was suspect that Mercury indeed had a moon. However, after a short period of time, the radiation stopped all-together and scientists speculate that it was radiation from a binary star, 31 Crateris. The actual source of the radiation is currently unknown.

BepiColombo is the joint effort of ESA (European Space Agency) and JAXA (Japan Aerospace Exploration Agency) of send two satellites to Mercury. Plans of a 3rd, lander-type, probe was scrubbed due to budget constraints. One orbiter will monitor the planets magnetosphere while the Planetary Orbiter will be getting a more detailed scan of Mercury's surface using laser altimeters and imagers. The orbiters are scheduled to arrive at Mercury in 2020.

For more information, check out the following links:
NASA.gov
WiKiPedia - Mercury (planet)
BepiColombo