The innermost planet in our solar system, Mercury is named for the Roman messenger of the gods.  It moves the most swiftly about the Sun. It is a terrestrial-type planet and the second smallest planet of the solar system (after Pluto), in both size and mass. Mercury has a diameter of 3,031 miles (4,878 km), about two-fifths the Earth’s diameter. Its average distance from the Sun is about 36 million miles (57.9 million km) compared to 67.3 million miles (108.2 million km) for Venus, the second closest planet.

Because of Mercury’s size and nearness to the brightly shining Sun, the planet is often hard to see from the Earth without a telescope. At certain times of the year, Mercury can be seen low in the eastern sky just before sunrise.

Mercury travels around the Sun in an oval-shaped orbit. The planet is about 28.6 million miles (46.0 million km) from the Sun at its closest point, and about 43.4 million miles (69.8 million km) from the Sun at its farthest point. Mercury is about 57 million miles (91.7 million km) from the Earth at its closest approach.

Mercury moves around the Sun faster than any other planet. The ancient Romans named it Mercury in honor of the swift messenger of their gods. Mercury travels about 30 miles  (48.3 km) per second, and goes around the Sun once every 88 days. .

Mercury’s surface resembles that of the Moon. It reflects about 6 percent of the sunlight it receives, about the same as the Moon’s surface. Like the Moon, Mercury is covered by a thin layer of mineral particles called silicates. It also has broad, flat plains; steep cliffs; and many deep craters similar to those on the Moon. Many astronomers believe the craters were formed by meteorites crashing into the planet. Mercury does not have enough atmosphere to slow down meteoroids and burn them up by friction.

Mercury is dry, extremely hot, and almost airless. The Sun’s rays are about seven times as strong on Mercury as they are on the Earth. The Sun also appears about 2 1/2 times as large in Mercury’s sky as in the Earth’s. Mercury does not have enough gases in its atmosphere to reduce the amount of heat and light it receives from the Sun. Temperature on the planet may reach 801^o F (427^o C) during the day. But at night the temperature may drop as low as -279^o F (-172^o C). Because of the lack of atmosphere, Mercury’s sky is black. Stars probably would be visible from the surface during the day.

The plant and animal life of the Earth could not live on Mercury because of the lack of oxygen and the intense heat. Scientists doubt that the planet has any form of life.

The United States Mariner 10 was the first and only spacecraft to reach Mercury. The unmanned spacecraft flew to within 460 miles (740 km) of Mercury on March 29, 1974. It swept past the planet again on September 24, 1974, and on March 16, 1975. During those flights, the spacecraft photographed portions of the surface of Mercury. Mariner 10 discovered the largest impact sites in the solar system called Caloris Basin in March, 1974. It also detected Mercury’s magnetic field. In the future, unmanned space probes may go into orbit around Mercury, crash into it, or land on it. Manned exploration of Mercury would be difficult because of the extreme temperatures on its surface.

Mercury Facts

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