The smallest planet in our solar system and nearest to the Sun, Mercury is only slightly larger than Earth's Moon, with a radius of 2,440 km. Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system – that title belongs to nearby Venus, thanks to its dense atmosphere. But Mercury is the fastest planet, zipping around the Sun every 88 Earth days. Mercury is appropriately named for the swiftest of the ancient Roman gods. NASA's MESSENGER spacecraft orbited Mercury for more than four years, from March 17, 2011 to Apr. 30, 2015.