Triton (moon) discovery; he later successfully suggested the name Hyperion, previously chosen by John Herschel, for the eighth moon of Saturn when he discovered it. Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e. it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons. However, these moons are all much more distant from their primaries, and are small in comparison; the largest of them (Phoebe) has only 8% of the diameter
Mercury (planet) 2015. The "BepiColombo" spacecraft will arrive at Mercury in 2025. Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and possibly a solid inner core. Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's