system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it currently less common for the earth to encounter a large comet or asteroid than it was millions of years ago?

I was reading Astrophysics for People in a Hurry and one of the lines gave me pause "earth's occasional encounters with large comets and asteroids, a formerly common event"

NDT doesn't explain why this is, simply that it was formerly a common event.  

I assumed that it was either the universe was expanding and now there is more room and less collisions.  Or perhaps that there are now less comets and asteroids since over time they have collided and been destroyed?  Or something else I didn't think of at all?  Thanks!
gpt
Asteroids and comets are predominantly remnants leftover from formation of the solar system. Over the billions of years that our solar system has been around, these leftover bits have mostly had an unsteady orbit which caused them to collide with the sun, or have already collided with another object in the solar system.