They are tiny compared to the earth. I can hardly imagine they even wipe out lives within 1000km radiused area, which still is a tiny compared to the earths surface size. Asteroids are, just giant rocks right? not atomic bombs, wich gives an increasing explosion due to it’s chain reaction. So why?
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Because kinetic energy is mass times speed squared, and because space rocks are REALLY fast.
A 1-km rock has a lot of mass, and these things are flying at 15,000 mph (~24,000 km/h relative to Earth. The mass of a rock 1 km across times (24,000 km/h)^(2) gives an unfathomably large amount of energy, just from kinetic energy alone with no further “explosion” necessary beyond just dumping all of that kinetic energy into a relatively small impact area. That much energy, mostly converted to heat, immediately vaporizes a huge amount of rock into a gas cloud that rapidly expands because rocks (like everything else) take up more space in the gas phase. That makes a giant scale explosion with no conventional or nuclear “explosives” needed.
It’s really hard to grasp how much energy things have when they’re going this fast, but a visual helps. Here’s the damage from a **14-gram piece of plastic** [hitting a block of solid aluminum](https://www.reddit.com/r/pics/comments/7w50a7/this_is_what_happens_to_aluminium_when_a_12_oz/) at typical meteor speeds. The crater is about 5 inches deep. A little piece of plastic has vaporized like 100x its mass of solid metal. Now scale up to what a rock 1 km across going that same speed can do. They really are planet-killers.
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