Mercury

Mercury is the closest planet to the Sun. Therefore, it moves the fastest in its orbit, completing one trip around the Sun in only 88 days. Because of its rapid motion, Mercury is named after the fleet-footed messenger of the Roman gods.

Orbit, Observation, and Rotation

Mercury's average distance from the Sun is about 36 million miles (59 million km), or 0.39 AU. Mercury's orbit, with an eccentricity of 0.2056, is more elliptical than that of any other planet, and is inclined by about 7 degrees to the plane of the Earth's orbit.

From the Earth, Mercury always appears close to the Sun - about 28 degrees from it at most - and hence Mercury is only visible in twilight. Its magnitude varies from a little brighter than -2.0 to as faint as +5.5. A telescope shows that Mercury exhibits a pattern of phases, like the Moon's. Mercury transits the disk of the Sun 13 times each century, and those transits can be seen with the proper viewing equipment.

Due Mercury's small size and proximity to the Sun, it is difficult to observe any surface detail on the planet. Until the mid-1960s, it was thought that Mercury's rotation was tidally locked to Sun: in other words, that Mercury always showed the same face to the Sun, in the same way that the Moon always shows the same face to the Earth. But in 1965, astronomers discovered that Mercury rotates in a 3:2 resonance with its orbital period, meaning that Mercury rotates 3 times for every 2 orbits around the Sun. This means that a "day" on Mercury is about 58 Earth days.

Composition, Surface, and Exploration

With a diameter of about 3030 miles (4880 km), Mercury is midway in size between Mars and the Earth's Moon. Since Pluto was de-classified as a planet in 2006, Mercury is now the smallest planet in our solar system. However, Mercury is almost as dense as the Earth, and it also has a magnetic field. For these reasons, it is thought to have a large iron core.

The surface of Mercury looks similar to the Moon's, with heavy cratering. The largest impact feature on Mercury is the Caloris Basin, and is about 1,300 kilometers (800 miles) in diameter. There are also scarps or cliffs, some hundreds of miles long and up to a mile high, formed by early contraction of the crust. Mercury has no detectable atmosphere. This makes for extremes of temperature, ranging from about -280°F (-180°C) at night to about 800°F (430°C) during the day. Nevertheless, radar observations in 1991 showed that Mercury may have water ice at its north and south poles, inside deep craters that are perpetually dark and cold.

The first spacecraft to visit Mercury was NASA's Mariner 10 mission in 1974, which made three close flybys of the planet, and imaged 45% of its surface. The MESSENGER spacecraft is currently en route to Mercury, and will enter orbit around the planet in 2011.

Mercury has no known moons.