The Solar System's eight planets, and its nine most likely dwarf planets, are known to be orbited by at least 231 natural satellites, or moons. At least 20 of them are large enough to be gravitationally rounded; of these, all are covered by a crust of ice except for Earth's Moon and Jupiter's Io. Several of the largest ones are in hydrostatic equilibrium and would therefore be considered dwarf planets or planets if they were in direct orbit around the Sun and not in their current states (orbiting planets or dwarf planets). Moons are classed in two separate categories according to their orbits: regular moons, which have prograde orbits (they orbit in the direction of their planets' rotation) and lie close to the plane of their equators, and irregular moons, whose orbits can be pro- or retrograde (against the direction of their planets' rotation) and often lie at extreme angles to their planets' equators. Irregular moons are probably minor planets that have been captured from surrounding space. Most irregular moons are less than in diameter. The earliest published discovery of a moon other than the Earth's was by Galileo Galilei, who discovered the four Galilean moons orbiting Jupiter in 1610. Over the following three centuries only a few more moons were discovered. Missions to other planets in the 1970s, most notably the Voyager 1 and 2 missions, saw a surge in the number of moons detected, and observations since the year 2000, using mostly large, ground-based optical telescopes, have discovered many more, all of which are irregular. Moons by primary Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of . For a very short time in 1974, Mercury was thought to have a moon. Venus also has no moons, though reports of a moon around Venus have circulated since the 17th century. Earth has one Moon, the largest moon of any rocky planet in the Solar System. Earth also has more than 20 known co-orbitals, including the asteroids 3753 Cruithne and 469219 Kamoʻoalewa, and the occasional temporary satellite, like 2020 CD3; however, since they do not permanently orbit Earth, they are not considered moons. (See Other moons of Earth and Quasi-satellite.) Mars has two known moons, Phobos and Deimos ("fear" and "dread", after attendants of Ares, the Greek god of war, equivalent to the Roman Mars). Searches for