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             Can 
              They Hit? 
              Most large 
              objects in the Solar System, including the planets and their moons, 
              move in circular orbits around the Sun. Their orbits are far apart 
              and change very little from eon to eon, so they will never crash 
              into each other. 
            However, there 
              are lots of small objects in the Solar System called asteroids 
              and comets that move in oval-shaped orbits 
              that sometimes cross the circular orbits of the planets. Most of 
              the time, the small objects cross a planet's orbit when the planet 
              is somewhere else in its orbit, so no collision occurs. It is like 
              crossing a railroad track when no train is present. But every now 
              and then, an asteroid or comet crosses a planet's orbit exactly 
              where the planet is, causing a huge crash! 
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