Comet Beginnings and Ends Comets formed 4.6 billion years ago along with the rest of the planets from the same solar nebula material. They were too small and cold to undergo any geologic activity (they did not differentiate), so they preserve the record of the early solar nebula composition and physical conditions. Those forming near the jovian planets were deflected outward, swallowed up or sent careening inward toward the terrestrial planets and the Sun. The water originally on the forming terrestrial planets may have evaporated into space, so the water now present on the terrestrial planets may have come from comets crashing into them. Short period comets make hundreds to thousands of passes around the Sun spewing out gas and dust. Over time a comet will leave bits of dust along its orbit, each piece of dust has an orbit close to the comet's orbit. The dust grains are the size of a grain of sand or smaller. If the Earth passes through the comet's orbit, the dust grains can hit the Earth's atmosphere to make the
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