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When the planets were forming, this was common. When they were young, collisions were still pretty common, but over time things have settled down quite a bit and thankfully this is pretty rare. We’ve been circling the sun for 4 billion years. There’s only so much stuff in the solar system, and there’s no new stuff coming in, so we’re kind of in a steady state now.
That’s not to say that it couldn’t happen. It’s thought that the dinosaurs were wiped out by a big meteor, and if a big one were to come now, we’d not be ready to deal with it. It’s important that we are able to watch the skies with powerful instruments to look for things like this!
The size is amazingly small, actually. A big meteorite that will do damage is about the size of a football or something? I think? Something the size of a car would be a major event. Something a few hundred metrres across would be disastrous. It’s not the size so much as the speed. The speed creates a massive shockwave in the air that effectively makes the meteor much larger than it is (and very hot), and that’s what causes the trouble.
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Meteors move so fast that the friction heats up the meteor and causes the surface to combine with air. This is why they look as if they are burning and leave a trail through the air. Most meteors “burn up” in the atmosphere. Meteorites are big enough that some of the meteor remains after all the outside has “burnt off”, so meteorites are those meteors that make it all the way down to the surface of the earth.
How often and how big? You’d need to ask an astrophysicist – I don’t know.
When we hit a nail with a hammer, the impact of the hammer heats up the nail and the hammer. When a “big” meteorite hits the ground, the impact can release so much heat that an explosion occurs.
The Woodleigh crater in Western Australia was caused by one such really big meteorite. Since the crater is a few hundred million years old, it is hard to tell exactly how big the crater is, but it is some between 40 KILOMETRES and 120 km in diameter. It is estimated that the meteor was between 5 km and 60 km in diameter. HUGE.
Most meteors are small. Scientists who collect meteors often collect them from glaciers. Most of these meteors are just centimetres in diameters. The big ones are very very rare.
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Well…it really depends on what size of meteorites you want to look at,
There’s a lot of small stuff that enters the Earth’s atmosphere every single day but these a meteoroids and probably not the types of things that you want to know about.
There is this graph which shows how often certain sizes of meteorites hit or enter Earth’s atmosphere. http://www.tulane.edu/~sanelson/images/impactrecurrence.gif Luckily for us, if the meteorite is larger, the less often they are expected to hit. The way scientists work this out is to record all the known meteorites hitting Earth to use in their calculations
Meteorites around 1mm in size hit the Earth every 30 seconds. When they enter the Earth’s atmosphere, they’re travelling really fast and experience friction against the particles in the air heating up. The temperature gets so hot that they start to melt and vapourise. This also lets quite a bit of light, so much so that you can see this with your eyes at night as shooting stars.
Check out this video to see what else a shooting star could be at night straight from the mouth of a NASA astronaut. http://www.youtube.com/watch?v=OAGC61LOTTY
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