General Information
    Mercury is best known as the closest planet to the sun. Mercury’s mean distance from the sun is 36 million miles (57.9 million km) which is about half the distance from the sun to Venus. The orbit of Mercury is elliptical, with an eccentricity of 0.2056, zero being a perfect circle. The Earth’s own eccentricity is only 0.0167, much more circular than Mercury's. Mercury’s orbital distance varies from 28.6 million miles (46 million km) to 43.4 million miles (69.8 million km). Mercury has a diameter of 3,031 miles (57.9 km) which is two-fifths of Earth’s diameter.

Mercury's Swift Movements
    Mercury moves faster than any other planet around the sun. Mercury travels 47.87 kilometers per second compared to Earth’s speed 29.79 kilometers per second. The reason why Mercury is named after the swift messenger of the Greek/Roman Gods because Mercury travels so fast. One time around the sun for Mercury is only 88 earth-days, only about 3 months only.
    Astronomers up to 1965, thought that Mercury only rotated once in every 88 earth-days, which is the same time it takes to go around the sun once. That would mean that one side of Mercury would always face the sun. As Mercury moves around the sun, Mercury would also rotate at the same rate so that the side that was facing the sun would catch up to the sun that shifted because of the change in position of Mercury. The side that would always face the sun be extremely hot, while the other side would be very dark and very cold. In 1965 they found out that Mercury rotates once every 59 earth-days. They found out by bouncing radar beams off of Mercury. The signals returned from one side of Mercury were different from another side. They measured and analyzed the movements of the different sides of Mercury and found that Mercury rotated once every 59 earth-days. So Mercury did not have one sizzling hot side and one cold dark side. Mercury rotates on its axis at a tilt of about 0.1 degrees.

Observing Mercury
   Mercury is a very hard object to see with the naked eye from Earth because it is obscured by the sun. Although, at certain times in the year Mercury can be seen low in the western sky just after sunset. At other times, Mercury can also be seen low in the eastern sky prior to  sunrise. Mercury has phases like the Moon that can be seen with a telescope. It will go from crescents to gibbous to a full Mercury. They come from having only parts or all of Mercury’s lit up side be visible to the Earth depending on the Earth point to view.

About the Rock
    The core of Mercury is appears to be made of iron and other heavy elements. When the Mariner 10 flew by, it detected a magnetic field which indicates a large iron core like the Earth. On the outside, Mercury is very much like our moon. It has little air or atmosphere, small, gray, reflects only 6% of the light it receives, craters, steep cliffs, and flat plains. The craters were caused by comets and meteorites colliding with the planet. Without a thick atmosphere to slow down and burn up incoming comets and meteoroids, Mercury and the moon are battered pieces of rock. The density of Mercury is only slightly smaller than Earth, at 5427 kg per cube meter compared to Earth’s density which is 5520 kg per cube meter. The similar densities indicate that Earth and Mercury are made of similar materials. Mercury has a smaller mass than Earth so Mercury’s surface gravity only a third as strong as the Earth's. (To learn why mass matters, see: Universal Laws of Gravity)
    Since Mercury is the closest planet to the sun, it gets very hot on Mercury. Temperatures skyrocket to 801° Fahrenheit (427° C) and dive to -279°F (-173°C) at night. The sun’s rays are 7 times stronger on Mercury and the sun seems 2 ½ times bigger in Mercury’s sky. That could cause one bad sunburn! Mercury doesn’t have enough gases to reduce the heat and light it receives from the sun. Also because of the lack of atmosphere, the Mercurial sky is black and the stars can be seen in broad daylight.
    The little atmosphere that Mercury does have consists of small amounts of helium, hydrogen, oxygen and sodium. Combined, the little amount of gases on Mercury make a possible atmospheric pressure of 0.00000000003 pounds per square inch (or approximately 10^-10 Pascals).   

Flights to Mercury
    The only closeup studies done on Mercury are have been the U.S.A.'s Mariner 10 mission. The Mariner 10 flew within 460 miles (740 km) of Mercury on March 29, 1974.  It took detailed pictures of the planet and detected the magnetic field of Mercury. The Mariner 10 also swept past it on Sept. 24, 1974 and March 16, 1975. This spacecraft also studied Venus, and was the first spacecraft to study two planets. Continue on to Venus....

This picture of Mercury was made by taking a lot of separate pictures of a specific region of Mercury then put together. This mosaic was taken of Mercury by the Mariner 10. It is made up of 48 images taken at 42 second intervals during the 13 minutes that the Mariner 10 was approaching Mercury.

This is also a mosiac of Mercury taken by the Mariner 10, but this is the other side when the Mariner was retreating from Mercury.

This is an artist's conception of the Mariner 10. The only spacecraft to study Mercury upclose. It used its solar panels as sails to catch the faint force generated by sunlight to control the crafts roll rate. It used many energy conserving techniques to complete a fly-by of Venus and two fly-bys of Mercury.