This is an article about space. There are many asteroids, especially big asteroids, that have heavy element cores and lighter element outer layers, just like planets. It is common that asteroids are shattered into pieces, and pieces of shattered layered asteroids obviously comes from only one layer, just like pieces of planets blown into space. This means that their chemical element signature is not that of undifferentiated asteroids. And since undifferentiated asteroids are the standard by which traditional geology identifies asteroids that have once hit Earth (chemical signatures like "the iridium signature"), lots of asteroid impacts are therefore unknown. The low asteroid impact rate in present time can be explained by impacts being concentrated to time periods when a big asteroid have