Mercury has a lot in common with Earth’s moon. They’re about the same size and both have solid, heavily-cratered ground. Instead of a thick atmosphere, both have a thin layer of atoms generated by solar wind. Most interestingly, Mercury and the moon might both contain water. But obviously Mercury isn’t a satellite like the moon. Mercury also has a strong magnetic field while the moon’s magnetic field is weak. Unlike the moon, Mercury can’t usually be seen from Earth because it’s too close to the sun. Also, no spacecraft has ever landed on Mercury, although two have captured fly-by photographs. If humans did land on Mercury, the sun would appear three times larger than it looks from Earth.