Discovery and exploration of the Solar System Soviet "Lunokhod 1", which landed on the Moon in 1970. The first to visit another planet was Sojourner, which travelled 500 metres across the surface of Mars in 1997. The only manned rover to visit another world was NASA's Lunar rover, which traveled with Apollos 15, 16 and 17 between 1971 and 1972. Overview of some missions to the Solar System. See also the categories for missions to , , , and . Crewed exploration of the Solar System ended in 1972. The first human being to reach space (defined as an altitude of over 100 km) and to orbit
Moons of Neptune formed "in situ". Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons, which are more like the outer irregular satellites of the other outer planets. Firstly, they are the largest two known irregular moons in the Solar System, with Triton being almost an order of magnitude larger than all other known irregular moons. Secondly, they both have atypically small semi-major axes, with Triton's being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the