A Mars-crosser is an asteroid whose perihelion lies within part or all of the orbit of Mars. Mars-crossers are subdivided into shallow Mars-crossers, with perihelia between 1.58 and 1.67 astronomical units (AU), and deep Mars-crossers, with perihelia between 1.3 and 1.58 AU; by comparison the distance of Mars from the Sun varies between about 1.38 and 1.67 AU. Mars-crossers are believed to have originated as main-belt asteroids that fell into a 3:1 orbital resonance (at a heliocentric distance of 2.5 AU) with Jupiter. Objects that go around the Sun three times for every orbital period of Jupiter meet up with the giant planet at the same point every third orbit. Jupiter's powerful gravity then perturbs the asteroid's path, increasing its eccentricity with each encounter. Over a period of about 100,000 yr, as perturbations accumulate, the asteroid becomes a Mars-crosser. Further perturbations from Mars, over a much longer period of several tens of million years, can then transform the asteroid into an Earth-crossing asteroid – and a potential hazard.