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Now that Pluto is no longer classified as a planet, Mercury is the smallest planet in the solar system. Mercury has a rocky surface and an iron core. The iron core in Mercury is very large compared to other rocky planets like Earth and Mars. This makes Mercury's mass very high compared to its size. Click above to watch a video on Mercury Mercury is a barren planet covered with craters from impacts of asteroids and other objects. It looks very similar to the Earth's moon. Mercury has virtually no atmosphere and rotates very slowly in relationship to the sun. A single day on Mercury is as long as almost 60 Earth days. As a result of its long day and little atmosphere, Mercury has some wild extremes in temperature. The side facing the sun is incredibly hot (800 degrees F), while the side away from the sun is super cold (-300 degrees F). How does Mercury compare to Earth? Mercury is much smaller than Earth. It's actually a lot closer to the size of Earth's moon. It has a shorter year, but a much longer day. There is no air to breath and the temperature changes wildly each day (even though it's a really long day!). Mercury is similar in that it has a hard rocky surface like Earth's. You could walk around on Mercury if you had a space suit and could take the extreme temperatures. 
From left to right: Mercury, Venus, Earth, Mars How do we know about Mercury? There is evidence that the planet Mercury has been known since 3000 BC by civilizations such as the Sumerians and Babylonians. Galileo was the first to observe Mercury by telescope in the early 1600's. Several other astronomers since then have added to our knowledge about the planet. Since Mercury is close to the Sun, it is very difficult to send a space craft to explore the planet. The gravity from the sun is constantly pulling on the space craft causing the ship to need lots of fuel in order to stop or slow down at Mercury. There have been two space probes sent to Mercury. The first was Mariner 10 in 1975. Mariner 10 brought us the first close up pictures of Mercury and discovered that the planet had a magnetic field. The second space probe is Messenger. Messenger is just now (2008 - 2009) reaching Mercury. Scientists will learn a lot more about Mercury in the coming years. 
The Mariner 10 Space Probe Mercury is tough to study from Earth because it is inside the Earth's orbit. This means that when you try to look at Mercury, you are also looking at the Sun. The bright light of the Sun makes it nearly impossible to see Mercury. Because of this Mercury is best seen right after the Sun sets or just before it rises. Interesting Facts about the Planet Mercury
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