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# Measurement of Probability

## Express ratio of favorable and possible outcomes as fractions, decimals, and percents.

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Deep Impacts

Credit: NASA
Source: http://commons.wikimedia.org/wiki/File:Meteor_Crater_-_Arizona.jpg

Throughout Earth’s history, meteorites and asteroids have struck our planet. 65 million years ago, an impact on the Yucatán Peninsula in southeastern Mexico may have wiped out the dinosaurs by causing an extreme change in temperature. 50,000 years ago, a much smaller meteorite left a huge crater in northern Arizona, pictured above. In 1908, a space rock estimated to measure only between 60-190 meters across demolished millions of trees across 2,000 square kilometers of Siberian forest. The explosion, known as the Tunguska event, was 1,000 times more powerful than the atomic bombing of Hiroshima during World War II. Are we safe from space rocks? What are the chances that we’ll be hit?

Every day, debris from space bombards our planet. Fortunately, our atmosphere usually protects us from damage. Most space rocks burn up in the atmosphere before they ever hit the Earths surface. As they burn, they leave trails of particles and streaks of light across the sky, which we see as shooting stars. Any pieces that make it to Earth are too small to do much damage. However, scientists know that far larger meteorites and asteroids, known as “city killers,” have struck our planet throughout history. NASA is racing to study these giant objects. Scientists hope to be able to accurately predict potential collisions far enough in advance in order to prevent them and avoid massive disasters.

Researchers have calculated the probability of major hits in the next century. There’s a 1100\begin{align*}\frac{1}{100}\end{align*} chance that an impact the size of the Tunguska event will occur in the next century.1 However, most of Earth’s surface is ocean, and most land on Earth is uninhabited. So chances are that if a large object from space strikes our planet, it won’t hit a population center. However, there’s also a 11,000,000\begin{align*}\frac{1}{1,000,000}\end{align*} probability that an asteroid greater than 10 kilometers in sizelarge enough to cause mass extinction and the end of civilizationwill collide with the planet in the next century.2 The Earth is as likely to see such a cataclysm within the next 100 years as you are to die in an earthquake.

Credit: NASA
Source: http://commons.wikimedia.org/wiki/File:Impakt.jpg

Private foundations and national governments are working together to find ways to protect mankind from impacts. Currently, NASA is trying to identify all near-Earth objects (NEOs) that could possibly hit our planet, and scientists are developing plans to deflect meteors and asteroids away from Earths orbit.

#### Explore More

At the links below, learn more about the Tunguska event, see the effects of a 20-meter meteor that exploded over Russia in 2013 (the largest recorded object to have struck the Earth since Tunguska), and find out what would happen to Earth in the aftermath of a large enough asteroid collision.

1. [1]^ Credit: NASA; Source: http://commons.wikimedia.org/wiki/File:Meteor_Crater_-_Arizona.jpg; License: CC BY-NC 3.0
2. [2]^ Credit: NASA; Source: http://commons.wikimedia.org/wiki/File:Impakt.jpg; License: CC BY-NC 3.0

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