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Some of them have orbits that cross the orbits of the planets and moons.When they get close enough to a planet or moon, they will be pulled in by the large body's gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet.This assumes, of course, that the cratering rate has been fairly constant for the last few billion years.The heavy bombardment of about 3.8 billion years ago must be taken into account when using the crater age dating technique. Select the photographs to display the original source in another window.

Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month.The first frame of a 3D movie flyover of Tycho Crater on the Moon as seen by Kaguya. It displays the classic terraced crater walls and central peak of a complex crater.Select the image to go to the Selene website for a great flyover of the crater.Earth has over 170 impact craters on its continents with the 1.2-km diameter Meteor Crater in northern Arizona being one great, well-preserved example.Even Venus with its thick atmosphere has impact craters, though they all have diameters measured in kilometers because smaller projecticles burn up in its atmosphere.

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