• Every planet in the solar system has been smashed by asteroids (and the occasional comet). 

• Not all asteroid strikes are bad for us Earthlings. Some rocks contain organic compounds that may have kickstarted life on Earth. Smaller impacts leave behind meteorites, letting scientists study other worlds without having to blast off. 

• Sci-fi movies portray asteroid belts as spaceship-smashing jumbles of rock. Our solar system’s real Asteroid Belt isn’t nearly so crowded. The average distance between rocks is more than a million miles (1.2 million kilometers), giving space pilots plenty of maneuvering room.

If you think of the eight planets as members of Earth’s family—friendly cousins who swing by regularly and never cause any grief—then asteroids are the total strangers lurking in the neighborhood. And like strangers, their visits aren’t always welcome. Asteroids are the rubble left over from the solar system’s formation roughly 4.6 billion years ago. Scientists estimate that more than a million of these itty-bitty worlds 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! More than a thousand people were injured in 2013 when an asteroid just 62 feet (19 meters) wide exploded high in the atmosphere above Chelyabinsk, Russia. And that was a near miss! An asteroid impact 65 million years ago may have wiped out the dinosaurs. But don’t worry too much. Astronomers are scanning the skies for space rocks, hoping to give us warning before the next stranger arrives.