Home > Encyclopedia of Science > Mercury (Planet)

• The Mariner exploration
• Discovery of water on Mercury
• Future exploration

Mercury, the closest object to the Sun, is a small, bleak planet. Because of the Sun's intense glare, it is difficult to observe Mercury from Earth. Mercury is visible just above the horizon for only about one hour before sunrise and one hour after sunset.

Mercury is named for the Roman messenger god with winged sandals. The planet was so named because it orbits the Sun quickly, in just 88 days. In contrast to its short year, Mercury has an extremely long day. It takes the planet the equivalent of 59 Earth days to complete one rotation.

Mercury is the second smallest planet in the solar system (only Pluto is smaller). Mercury's diameter is about 3,000 miles (4,800 kilometers), yet it has just 5.5 percent of...