Given a long enough time period, there is a nearly 100-percent chance an asteroid large enough to destroy most life on Earth will impact our planet.
The Tunguska event, a meteorite or comet explosion over Siberia in 1908, was unique in that it happened within modern cultural memory. We have newspaper interviews with eyewitnesses and photos and film footage of the aftermath, which was still a charred ruin 20 years after the event. But go back further, and it gets harder to know when and where impacts occurred on our planet. Many impacts likely occurred with no one there to witness them. They may have been recorded in ancient history as a period when the night was unusually bright. Or perhaps they were responsible for the sudden disappearance of an entire tribe or community, simply wiped off the planet in an instant, either by direct impact or tsunami caused by an over ocean impact.
In 1490 for example, Chinese astronomers recorded that 10,000 people in the Ch’ing-yang district were killed by stones falling from the sky. The stones ranged in size from chestnuts to goose eggs. However, despite occurring only 500 years ago, it is difficult to know exactly what happened. Go further back in history, and the damage done by asteroids striking Earth becomes even hazier. Researchers now believe that a huge comet may have impacted Earth some 13,000 years ago, possibly adding to a mini ice age that shapes the climate to this day. Estimates about how often a 10-megaton object hits the Earth have risen from once every million years to once every 1,000 years.
With this in mind, we can view our future with some certainty. Something big is going to smash into us, sooner or later, and probably sooner. Every night, there are over 100,000 shooting stars around the world. Each of those is a small asteroid burning up in our atmosphere. When a big rock hits Earth it has enough power to launch pieces of the Earth out into space, creating more asteroids which then smash into the moon and Mars.
We are however currently in a pretty good spot. Humanity made it far enough without being destroyed by a space rock that we can now acknowledge their danger and watch them as they approach. There are around 1,100 "Potentially Hazardous Asteroids, and we are currently tracking around 700. We should be able to see an asteroid headed towards us within a window of two to 40 ears. That's enough time to act. We even have the ability to alter their course, sending them gently off into space.
The Tunguska event was unusual but it was no anomaly. Over a longer period, a Tunguska scale event is not just normal, it is guaranteed. It's up to us to be ready for the next Tunguska.