Mercury is the closest planet to the Sun.  It has a very thin atmosphere and a diameter of 3032 miles at its equater.  Mercury’s surface is made up of deep craters and smooth plains.  Using telescopes, scientists can see it going through phases like the Moon.  Humans may establish a colony on Mercury, probably at one of the poles because of the extreme temperatures.

Mercury is the least explored of the Solar System’s inner planets.  Completing three rotations for every two orbits around the Sun, it has no moons.  It has an iron core that is partly molten, and a surface that looks like the Moon.  The ancient Greeks knew about Mercury before 300 BC.  It orbits the Sun every 88 days, and its average distance from the Sun is about 36,000,000 miles.

Rotating about once every 59 days, Mercury is the second densest body in the Solar System.  Only Earth is more dense.  Craters at Mercury’s poles have water ice, and its gravity is too weak to retain much atmosphere. Because of the heat and the lack of oxygen, plant and animal life could not live there. 

In 1974 and 1975, the Mariner 10 unmanned probe explored the planet in three flybys.  The National Aeronautics and Space Administration launched two Messenger probes in 2004 to perform flybys of Mercury.  The European Space Agency and Japan are planning a joint mission called BepiColumbo which will orbit the planet with two probes. 

In summary, Mercury is the smallest planet in the Solar System.  It has no moons and a cratered surface that looks like the Moon.  Several nations have sent unmanned probes to Mercury.