We integrated the orbital evolution of 30,000 Jupiter-family comets , 1300 resonant asteroids , and 7000 asteroidal , trans-Neptunian , and cometary dust particles .
For initial orbital elements of bodies close to those of Comets 2P , 10P , 44P , and 113P , a few objects got Earth-crossing orbits with semi-major axes a < 2 AU and moved in such orbits for more than 1 Myr ( up to tens or even hundreds of Myrs ) .
Four objects ( from 2P and 10P runs ) even got inner-Earth orbits ( with aphelion distance Q < 0.983 AU ) and Aten orbits for Myrs .
Our results show that the trans-Neptunian belt can provide a significant portion of near-Earth objects , or the number of trans-Neptunian objects migrating inside the solar system can be smaller than it was earlier considered , or most of 1-km former trans-Neptunian objects that had got near-Earth object orbits for millions of years disintegrated into mini-comets and dust during a smaller part of their dynamical lifetimes .
The probability of a collision of an asteroidal or cometary particle during its lifetime with the Earth was maximum at diameter d \sim 100 \mu m. At d < 10 \mu m such probability for trans-Neptunian particles was less than that for asteroidal particles by less than an order of magnitude , so the fraction of trans-Neptunian particles with such diameter near Earth can be considerable .