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What Is the Planet Mercury?
March 30, 2011
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Mercury is the smallest and fastest planet in the solar system. It is also the closest planet to the sun. It is named after the Roman messenger god Mercury, the fastest Roman god. The planet Mercury was known by ancient people thousands of years ago. It has no moons.

How Big Is Mercury?
Mercury is the smallest of the eight planets in our solar system. It is only a little bigger than Earth's moon. It would take more than 18 Mercurys to be as big as Earth. If you could weigh Mercury and the moon, Mercury would weigh much more. Mercury has a rocky surface, but deep inside is a heavier material, probably iron.

Where Is Mercury?
Mercury is the closest planet to the sun. On average, it is a little more than one-third the distance from the sun than Earth is. Or, put another way, Earth is almost three times farther from the sun than Mercury. Mercury orbits the sun once every 88 Earth days. So one year on Mercury is 88 Earth days. But a day on Mercury is longer than an Earth day. One Mercury day lasts for 59 Earth days.

What Is Mercury Like?
The surface of Mercury looks much like Earth's moon. It has a lot of impact craters. Mercury has almost no atmosphere. Because it is so close to the sun, it can be very hot. On its sunny side, Mercury can reach a scorching 800 degrees Fahrenheit! (But Mercury is not the hottest planet in the solar system. The hottest planet is Venus.) On its dark side, Mercury gets very cold because it has almost no atmosphere to hold in heat and keep the surface warm. The temperature can drop down to minus 300 degrees Fahrenheit. Sunlight never reaches into the bottoms of some craters near Mercury's poles. That could mean that ice may be inside those craters, because they always stay cold.
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How Has NASA Studied Mercury?
Because it is so close to the sun, Mercury is hard to study from Earth. No people have ever gone to Mercury, but the first robotic spacecraft to visit Mercury was Mariner 10. It flew by Mercury in 1974 and 1975. Mariner 10 was able to take pictures of less than half of Mercury's surface. No spacecraft visited Mercury for more than 30 years. Then NASA's MESSENGER flew by Mercury in 2008 and 2009. On March 17, 2011, it began its orbit of Mercury. MESSENGER will map Mercury by taking pictures of the planet's surface, including some areas that have not been seen before. It will also collect data on the composition of the surface rocks, and measure the heights of mountains and depths of craters and valleys. Some data collected by MESSENGER will help scientists to understand what the inside of Mercury is like. MESSENGER will let people learn more about Mercury than they ever have before.

More About Mercury:
› Space Place in a Snap: The Solar System's Formation   →
› Solar System Kids: Mercury   →
› MESSENGER for Students


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David Hitt/NASA Educational Technology Services

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The planet Mercury
This picture of Mercury was taken by NASA's MESSENGER spacecraft.
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NASA
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Artist's drawing of MESSENGER in front of Mercury
MESSENGER has given scientists a better look at Mercury than they've ever had.
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NASA
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Page Last Updated: January 27th, 2014
Page Editor: NASA Administrator