odave said:
During it's formation and that of the Solar system.
Good answer. Earth was formed by the accretion of material. As Earths mass increased more and more material was attracted to it due to it's increased gravitational pull.
However, an event known as the "Late heavy meteorite bombardment" occured 3.8 to 4 billion years ago, and is considered to be the time era when the greatest number of meteorites struck Earth. (and the Moon) The origin of those meteorites is not fully resolved, but is thought to be material remaining after all the other planets had formed.
Also about every 62-65 million years, Earth passes through a field of asteroids causing many impacts, with asteroids large enough to cause an extinction event. The last occurring around 65 million years ago. A massive recurring pattern of extinction that has been happening once every 62 million years plus or minus 3 million years for over 500 million years and which is currently coming due!
Some interesting data:
Estimates for the mass of material that falls on Earth each year range from 37,000-78,000 tons.
Most of this mass would come from dust-sized particles.
A study done in 1996 (looking at the number of meteorites found in deserts over time) calculated that for objects in the 10 gram to 1 kilogram size range, 2900-7300 kilograms per year hit Earth. However, unlike the number above this does not include the small dust particles.
It is also estimated that between 36 and 166 meteorites larger than 10 grams fall to Earth per million square kilometers per year.
Over the whole surface area of Earth, that translates to 18,000 to 84,000 meteorites bigger than 10 grams per year.
Not sure about the scientific mind but I find astronomy and cosmology in particular, to be fascinating. Carl Sagan was an inspiration, may he rest in peace.
GG~