Mercury (Planet) (Encyclopedia of Science)
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 Earth's mass. (Earth's diameter is about 7,900 miles [12,720 kilometers].) On average, Mercury is 36 million miles (58 million kilometers) from the Sun. The Sun's intense gravitational field tilts Mercury's orbit and stretches it into a long ellipse (oval).
The Mariner exploration
Little else was known about Mercury until the U.S. space probe Mariner 10 photographed the planet in 1975. Mariner first approached the planet Venus in February 1974, then used that planet's gravitational field to send it around like a slingshot in the direction of Mercury. The second leg of the journey to Mercury took...
(The entire section is 823 words.)
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