Asteroids are always moving and plummeting throughout the galaxy. At some point in our future, an asteroid will most likely hit the Earth and alter the planet irreversibly. It has happened before, and it will likely happen again. 

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Scientists estimate that more than a million of these asteroids orbit the sun in the Asteroid Belt, a stretch of space between Mars and Jupiter. They range in size from dwarf planets nearly 600 miles (950 kilometers) across to chunky rocks less than half a mile (1 kilometer) wide. Some asteroids even have their own moons! These roving rocks are fine when they stay where they belong, orbiting the sun in the loose cluster of the Asteroid Belt. But when Jupiter’s gravity tugs one of the larger asteroids loose and sends it tumbling toward Earth, watch out!