The innermost, fastest, and smallest solar system planet, known to the ancients as two different planets. It is composed mostly of a large iron core. It rotates three times for every two orbits around the Sun. Its daytime side is one of the hottest places in the solar system, while its night side is one of the coldest. One of the only rocky bodies known to have its own magnetic sphere. Has a minimal atmosphere and no moon. Visited by the Mariner 9 spacecraft three times and is currently orbited by Messenger. Several other international missions are planned.