After the planets had formed, there was a lot of material left over in our solar system,  There were tiny grains of dust too small for you to see.  There were little gritty bits of minerals, ice crystals, small pebbles, and larger rocks.  There were bigger objects too -- some as big as cars, some as big as moons, some the size of small planets.  All these items were rushing around in space, some going very quickly, others just floating around.  Gravity was pulling them this way and that.  Collisions were occurring.  This is called the period of Intense Bombardment,

    Look up at the moon.  All those craters were made by collisions with floating objects.  We see only the big craters when we are on earth.  There are many smaller craters as well.

    Many scientists think that the moon itself was created as the result of a Mars-sized object crashing into the earth.  Pieces of the earth would have been tossed up into the atmosphere, and the other body probably broke up into pieces.  There would have been a lot of floating matter orbiting our planet after the collision..  Some of the pieces would have been inside the Roche Limit:  these would have formed rings of dust and gravel that orbited the earth.  The particles that made up these  rings of dust slowly drifted down into the earth's primitive atmosphere and fell to the surface or disintegrated in the air.  

    However, most of the fragments from the collision would have been outside the Roche Limit, and they gradually came together to form the moon.  The many craters on the surface of the moon tell us that the moon formed early in the history of our solar system and received many more blows from space debris. The really heavy bombardment occurred before life appeared on earth.  The surface of our world was a hot, inhospitable place, and huge rocks shooting out of the sky could have caused the early oceans  to boil or even lava flows to erupt in the still evolving surface.

    The moon was not the only target in this chaotic time,  Mars, Mercury, and some of the moons beyond the ice line are also peppered with craters.   We can see them with telescopes, and craters have even been seen on asteroids.  But where are the craters on earth?

     Earth was also hit many times, and there are a few craters or remnants of craters that we can still see.  The marks of most of the impacts have largely been erased by erosion.  However, bombardment from outside our atmosphere has not stopped.  Every day about 100 tons of space rubble enter our atmosphere.  The smallest and most numerous pieces burn up as the friction of shooting through atmospheric molecules heats the rock particles until they vaporize.  We call these meteoroids "shooting stars" when we see them.  Larger pieces fall to the surface of the earth, and are sometimes found and picked up by people.  We call these rocks from space meteorites.

    Are there still large pieces of space debris out there?  Yes, there are, and occasionally one of them will hit the earth, with disastrous results.  It is believed that an asteroid or meteoroid impact caused the extinction of the dinosaurs, 65 million years ago, and that meteoroids may have been responsible for other extinctions as well.   These events are very rare now, and so unlikely to occur in any specific lifetime.  However, over time, there will be impacts again..  They are interesting to think about, though.

    If you found this page interesting, go on to

Comets, Meteors, and Impact Craters

Asteroids and Planetary Spacing: Bode's Law