Near-Earth asteroids and comets range in size between a few metres to tens of kilometres. They are essentially primordial material left over from the solar system's creation 4.6mn years ago. Ceres, now classified as a dwarf planet, was the largest asteroid with a diameter of 1,000 km. Comets consist of dust, water ice and frozen gases such as carbon dioxide and methane, that were formed in the outer solar system while rocky asteroids, also containing water ice and possibly other frozen gases, formed in the warmer space between the orbits of Jupiter and Mars. Planetary gravitational forces nudged them close to the Earth and Jupiter's gravity prevented them from accreting into planets.
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