Mercury Facts

What's so special about Mercury?

© Kelly Whitt

Aug 23, 2006

Basic Data on Mercury

Mercury is the smallest of the inner, rocky planets. At 4,878 km in diameter, it is about three times smaller than Earth and just slightly larger than our moon. The closest planet to the sun is riddled with impact craters of asteroids that have bombarded it since its formation. The largest of these craters is known as the Caloris Basin and stretches 1,300 km in diameter. Mercury looks a bit like our moon with its gray color and pockmarked face.

Little Mercury is smaller than other nonplanetary objects in the solar system. Jupiter's moons Ganymede and Callisto and Saturn's moon Titan are all larger.

The Movement of the Fleet-footed Planet

Mercury is named after the Roman messenger god because it moves so fast around the sun. With the speediest orbit of any planet, it takes just 88 Earth days for Mercury to travel around the sun once and complete its year. But although Mercury races through space fast, its spin is quite slow. It takes 59 Earth days for Mercury to rotate once and complete one of its days. This means a Mercurian year is less than two days long!

Atmosphere and Temperature on the Smallest Inner Planet

Mercury has no moon and no atmosphere. Because of its lack of an atmosphere, there is nothing to stabilize the planet's temperature. Therefore Mercury experiences the greatest extremes in temperature for any planet. On the day side of Mercury, the sun, being only 36 million miles away, scorches the ground with a temperature of approximately 750 degrees Fahrenheit. Then as the planet slowly spins, when the ground reaches the night side of the planet and the heat of the sun is turned off, all the warmth escapes out into space and leaves the ground at a bone-chilling -300 degrees Fahrenheit.

Viewing Mercury: How to See It

Mercury is the hardest of the naked-eye planets to see. Mercury, Venus, Mars, Jupiter and Saturn are considered naked-eye planets because they do not require any optical aid to see in the night sky, just your eyes. But because of Mercury's position close to the sun, from Earth's point of view Mercury never gets very high in our night sky. When Mercury is visible it is always either at night when it is following the sun below the horizon, or in the morning when it is leading the sun up from behind the horizon.

Spacecraft Missions to Mercury

Some of our best views of Mercury have come from the Mariner 10 spacecraft, the only one to ever visit the innermost planet. In 1974 Mariner 10 flew by the planet three times, but every time it passed the same hemisphere. This means that there is one entire half of Mercury that we have never seen up close. Maybe future space probes to our fleet-footed planet will reveal mysteries yet unknown to us.

For more on the solar system, follow the links below.

Sun Venus Earth Mars Jupiter Saturn Uranus Neptune

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