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A Mars crosser (MC or Mars-crossing asteroid or MCA) is a solar system object orbiting the Sun such that its orbit crosses the orbit of Mars, but the term is generally meant to exclude objects that also cross the orbits of other planets. Exact criteria varies, one specifying a perihelion between 1.67 AU and 1.3 AU (the 1.3 AU criteria is meant to keep the classification distinct from that of near-Earth objects, NEOs; Mars' eccentric orbit roughly spans this range, 1.3-1.67 AU). The terms shallow Mars crosser and deep Mars crosser are sometimes used, the typical criteria being whether the perihelion is greater than or less than 1.58 AU. Today a huge number of SSSBs are known with all kinds of relationships to the orbit of Mars (e.g., barely reaching it from the inside) but a population of substantial Mars crossers with aphelions clearly outside Mars and perihelions as described above are believed to be objects with orbits evolved out of the 3:1 Kirkwood gap: asteroids initially in that gap migrated into this orbital space.