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The chance of an asteroid impact is 100%. The big question is when? Every object in the Solar System shows evidence of multiple impacts. The Earth has over one hundred verified impact craters and several more that have not been verified as of yet. The most recent was in 2008 when a small steroid hit our atmosphere. It burned up immediately and left nothing to hit the ground, but it goes to show that these events happen on a yearly basis.
Small objects hit the Earth with an amazing regularity. Astronomers have even gone to the trouble to predict these asteroid impact events. Asteroids with a 1 km diameter impact the Earth every 500,000 years on average. Large collisions with five kilometer objects happen approximately once every ten million years. Asteroids with diameters of 5-10 m impact the Earth’s atmosphere approximately once per year, but break up and most of the material is vaporized in the upper atmosphere. Objects of diameters of over 50 meters strike the Earth approximately once every thousand years. It is easy to believe these numbers when you take into account the millions of asteroids that are out there, then couple that with gravity anomalies, etc. The surprise is actually in that you would expect more frequent asteroid impact events.
The Chicxulub asteroid impact is thought to have changed the climate quickly and sufficiently enough to have destroyed the dinosaurs. That is why NASA has set up the Sentry and NEAT programs to keep an eye on asteroids that may come close to the Earth. There have been a couple of asteroid impact events that are larger than Chicxulub, so an event of that magnitude could happen again.
Asteroid impact events help us to learn more about space born objects and their content, but could be very dangerous for the peoples of the world if we are caught off guard. The complexity of the SENTRY and NEAT programs will prevent any unexpected strikes and will help us to defend any threatened cities. The movies have embellished on what might happen to prevent an asteroid impact, but they might not be too far off from the truth.
Here at Universe Today we have written about asteroid impact events on different occasions, including the one that wiped out the dinosaurs, as well as brought you NASA images of NEAs. There is also an Astronomy Cast episode on the subject.
References:
NASA: Will an asteroid hit the Earth?
NASA: Cosmic Earth Impacts
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