It is generally assumed that few massive asteroids were formed the first million years following the big bang, because it is thought that only small amounts of lithium were available to make solid bodies. Asteroids and proto planets however began to form near here about 4.7 billion years ago, which is 9000 million years after the big bang, so likely our galaxy had, and still has trillions of asteroids and comets not orbiting a star, plus millions of asteroids that are in the asteroid belt.<br />About 2000 asteroids have solar orbits that cross the orbit of Earth, so they will likely strike Earth sometime in the next billion years, unless their orbits change. Others that orbit the galaxy have a very low probability of hitting the Earth, but they likely number in the trillions. The probability of a massive hit of Earth has likely decreased at least slightly over the past 3 billion years. Neil