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## 1. Collisions with the Earth

The Earth collides all the time with other celestial bodies, in the shape of meteorites, but those meteorites are usually very small and not dangerous. It is possible that the Earth will someday collide with another planet, but the probability per year of that happening is exceedingly small, because the Earth has lasted for almost five thousand million years without hitting another planet. Moreover, we can nowadays predict the future motion of the planets for many millions of years into the future and those calculations show that the planets won't be a threat to the Earth during that period.

## 2. Meteorites, Meteors, and Meteoroids

A meteorite is a stone that came from space and fell to the ground. The stone gets hot by friction with the air when it travels through the atmosphere, and then starts to glow. Such a point of light that travels through the sky in the blink of an eye is called a meteor or falling star. Not every meteor produces a meteorite, because most meteors burn up completely before they reach the ground. Before the stone reaches the atmosphere, it is called a meteoroid.

If you see a rock from space glowing while it streaks across the sky, then it is a meteor. If you find it on the ground, then it is a meteorite. I'm not sure that everybody agrees how to tell when you should stop calling it a meteor and should start calling it a meteorite. Is that already when the rock is still in the air but has slowed down and cooled down so much that it doesn't glow anymore? Or is it when the rock first hits the ground? Or when it first hits anything other than the air (like a tree or your head)?

I think that it is clear that every meteor is a shooting star, but not that every shooting star is a meteor. It is certainly acceptable to say that you've seen a shooting star (i.e., a meteor), and I think it is also acceptable to say that you've found a shooting star (i.e., a meteorite). It is less acceptable to say that you've found a meteor.

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It is possible for a shooting star to fall on your head and hurt you, but it is very unlikely that this will happen to you. It is far more likely that you'll be hit by a car than that a shooting star will hurt you.

Most shooting stars that you see are about the size of a grain of sand or even smaller when they enter the atmosphere from space, and they burn up completely in the atmosphere when they are still very high above the ground.

Only shooting stars that look very bright have any chance of reaching the ground, but those are rare. Such a shooting star must get at least about four hundred times brighter than the brightest star, and even most shooting stars that are as bright as that don't reach the ground. //www.amsmeteors.org/fireball/faqf.html#7 says that even some shooting stars that look brighter than the full moon do not reach the ground.

I am not worried at all about being hit by a shooting star.

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## 3. Where in the Sky do Meteors Appear?

Meteors can appear in every direction in the sky. Where exactly they appear is entirely random. There is no preference or dislike for certain parts of the sky.

Some meteors belong to a meteor shower. If you extend the paths that those meteors follow backward in the direction from which they came, then those paths cross in the same small area of the sky, which is called the radiant of that meteor shower. Such a meteor shower is named for the constellation in which or the star near which the radiant of that meteor shower lies.

Even of meteors that belong to such a meteor shower you cannot predict with any accuracy where in the sky they'll appear, except that they won't appear exactly opposite the radiant in the sky.

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## 4. Meteorite Impacts

Each year innumerable stones and pebbles and dust specks enter the atmosphere of the Earth from space, but only a few of those reach the ground as meteorites, and those are almost always so small that they don't cause much damage and you can lift them with one hand.

Sometimes a meteorite hits the ground that does make a big crater, but the chance that one of those will fall on your head during the next 100 years is so small that you don't need to worry about it. The Earth Impact Database reported at the beginning of 2004 that 168 meteorite impact craters were then known on Earth, with diameters ranging between 15 meters and 300 kilometers, and ages between a few years and over a thousand million years.

However, craters on Earth eventually disappear, because wind and water and plants break down their walls and fill up the crater. There have been very many more craters than the 168 that are known now. When the Solar System was just formed, there were very many more stones flying through space than there are now, and at least as many of them hit the Earth as hit the Moon. On the Moon there is no erosion by wind or water, so craters are preserved much better there than on Earth. The surface of the Moon is littered with millions of craters, so there must have been as many on Earth as well.

What the consequences are of the impact of a meteorite on Earth or on another planet depends mainly on how big the meteorite is. The bigger the meteorite, the more serious the results of the impact are. Fortunately, the larger the meteorite, the more rare it is.

Small stones to a few millimeters in size are slowed down so much by the atmosphere that they end up drifting slowly to the ground.

Pieces of space rock of a few millimeters to a few centimeters burn up completely in the atmosphere of the Earth before they can reach the ground, and can be seen at night as falling stars. If it is sufficiently dark and clear outside, then you can see a few falling stars every night.

If the rock is bigger, then part of it can survive the trip through the atmosphere and reach the ground as a meteorite. If the stone weighs less than a few tons, then it doesn't fall very fast anymore in the end, and it makes a small dent in the ground.

If the meteorite weighs many tons, then it is hardly slowed down by the atmosphere, and then it hits the ground at a speed of at least some 40,000 kilometers per hour (and often much more than that). Because of the enormous speed, the impact is large. You can expect the following results from such an impact:

1. a crater is formed.
2. the material that was in the crater before is heated enormously and slung into the air, and can fall back down many miles away and cause fires and other damage, similar to what can happen with a volcanic eruption.
3. there's an enormous shock wave that can flatten trees and buildings over a wide area. If the meteorite hits the sea, then the shock wave can cause enormous waves that can cause floods thousands of miles away.
4. dust that is thrown up can block sunlight for years and cause plants to die for lack of sunlight, and animals to die for lack of food.

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Of the meteoriet that probably cause the dinosaurs to go extinct 65 million years ago, it is thought that it was a few miles in size. That impact formed a crater (the Chicxulub crater near the Mexican coast) of 180 km in diameter and about a kilometer deep, and caused dinosaurs to go extinct all across the world. The impact probably caused large amounts of dust to be blown into the atmosphere, which hid the Sun for many years so that many plants could not grow anymore so that many animals had no food anymore and even dinosaurs far from the impact point were affected by it.

Meteorite impacts on other planets work similar to those on Earth, except that on planets without an atmosphere (such as Mercury and the Moon) small meteorites also hit at full speed, because there's no atmosphere to slow them down or to burn them up.

## 5. The Chance of a Meteorite Impact

How large the chance is that a meteorite will impact depends very much on how large that meteorite is. It is estimated that about 100 million stones of 1 gram (1/1000th of a kilogram) or more reach the atmosphere of the Earth each year, but almost none of them reach the surface. About 10,000 stones of 1 kilogram or more reach Earth a year, but hardly any of those reach the ground, either. Only about 10 stones of 1000 kg or more reach the Earth each year, so the chance of getting one of those on your head is still very small. Meteorites like the one that caused the Chicxulub crater in Mexico (and presumably also the demise of the dinosaurs) are thought to hit the Earth at a rate of about one per 100 million years. If another one of those comes our way, then we need to worry about it.

There are astronomers that try to locate all big stones in the Solar System that might perhaps one day hit the Earth, and to determine their orbits, so that we'll at least have some advance warning if one of them comes our way.

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## 6. Crater Size

When a meteorite hits the Earth, then the size of the resulting crater depends on many things, including the mass density of the meteorite, the speed of impact, the angle of impact, and the gravity of the planet where it impacts. For a large iron meteorite that impacts straight down at the minimum speed (11 kilometers per second) and hits land, the diameter of the resulting crater is something like 25 times the diameter of the meteorite. If the meteorite impacts at the more reasonable speed of 40 km/s, then the diameter of the crater is about 50 times the diameter of the meteorite. For predictions of the size of a crater, you can go to //www.lpl.arizona.edu/tekton/crater.html.

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## 7. Deflecting of Meteoroids

If we discover that a certain large meteoroid is on its way to hit the Earth, then we'd like to prevent that impact. You can only avoid an impact by deflecting the meteoroid or by breaking the meteoroid into pieces that are each so small that they burn up completely in the atmosphere.

Deflecting a meteoroid is difficult because they always move very fast and because they usually rotate around their axis, so just attaching a rocket and starting it won't help. And the really dangerous meteoroids are as large as a mountain, so you'd have to push really hard to deflect it.

To disintegrate the meteoroid is also hard. You can blow up the meteoroid (if you use enough explosives, which may be impossible if the meteorite is too large), but then you are probably left with a number of large pieces that can hit the Earth, and then you may have a hundred rocks to worry about instead of just one. Then you may get a hundred smaller craters spread over a bigger area, instead of a single crater. Is that better?

Perhaps. If you blow up the meteoroid long enough before its impact, then there is a chance that some of the pieces will miss the Earth completely. Of the pieces that still reach the atmosphere of the Earth, a larger fraction will burn up than would have been the case for the original meteorite. All in all, less of the meteoroid will reach the surface.

If you blow up the meteoroid, then you don't know beforehand how it will fall apart, so you can't predict where the pieces will go. You can predict where the whole meteoroid would impact, because you know its orbit (or else you could not send a rocket to it to blow it up).

This is a difficult political problem. If you have to choose between many small craters of which a few will probably be in your area, or one large crater in another continent, then what would you choose? And if the large crater would be in your area, what would you choose then?

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## 8. Shooting Stars and Satellites

Sometimes at night you can see a satellite pass overhead as a small point of light that moves silently in a straight line, taking a few minutes to cross the sky, and often getting a bit brighter and dimmer again at a fixed rate. Sometimes at night you can see a shooting star, a point of light that appears, brightens, and fades again, while moving across the sky in a fraction of a second. A shooting star sometimes leaves a glowing trail behind for a few seconds, and sometimes flares up at the end.

Sometimes a shooting star crosses the path of a satellite, but satellites and shooting stars have nothing to do with each other. Shooting stars only flare up when they enter the atmosphere, while satellites must stay far above the atmosphere to avoid falling back into the atmosphere and turning into shooting stars themselves. Shooting stars show up at random places in the sky, so it is a coincidence if one happens to show up in the same direction as (but much closer to you than) a satellite.

However, shooting stars come from space as (small) meteoroids, and there they might hit a satellite. That's why satellites have a thick skin. Most meteoroids are smaller than a grain of sand and cannot do much damage to such a satellite. There are also large meteoroids that could destroy a satellite, but those are rare. If you make the skin of the satellite thick and strong enough, then you can make the chance as small as you want that during the planned lifetime of the satellite it will be hit by a meteoroid that can do damage to it.