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.
Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics.
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The longer the surface has been exposed to space, the more craters it will have.
If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. Select the photographs to display the original source in another window.There are still small chunks of rock orbiting the Sun left over from the formation of the solar system.Most of the impact basins---craters measured in hundreds of kilometers---were made during this time.It is noteworthy that about the time the heavy bombardment ended, life took hold.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.