We don't have to worry about Mars, etc. hitting Earth. Is it that, being smaller, small asteroids have less inertia and so are more affected by, I guess, the gravity of various planets?
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3$\begingroup$ I’m no expert - but planetary collisions are theorised to have occurred in the past (see Theia) - and many asteroids are in stable orbits (see Trojans). So perhaps your observation is just a function of there being more asteroids than planets, and hence more of an opportunity to see them in unstable orbits? $\endgroup$– user438383Commented May 22, 2022 at 15:21
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A couple of factors. Firstly most small asteroids are in stable orbits. They don't approach any planets closely, and orbit in ellipses that are stable on timespans exceeding the age of the solar system. But there are lots more little asteroids than planets. If only a small proportion of objects are dislodged into Earth crossing orbits by the perturbation of planets then nearly all of those object will be very small, simply because there are lots more small objects.
Secondly small objects are particularly subject to non-gravitational forces, like radiation pressure or the Yarkovsky effect. While planets are bigger, gravitational effects are larger on bigger objects (Remember Galileo and dropping the two weights: they fall at the same rate). So planets are affected by gravity but much less by non-gravitational forces.
These forces will tend to move small asteroids from their orbit and can, over time cause them to approach a planet, or enter into a resonance with a planet. This can cause them to be scattered, and some will be scattered into an Earth crossing orbit. Planets are too large to be affected by these non-gravitational forces.
Finally there is an anthropic principle: If we had been on a planet which had collided with another planet after life had become established, we wouldn't be here to talk about it.
However the main reason it the first: There are more asteroids than planets, and so more asteroids are in potentially unstable orbits.
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2$\begingroup$ Yes, radiation pressure, never heard of the Yarkovsky effect, but interesting. Even though larger bodies receive more by radiation pressure, their surface area compared to volume is lower, right? I would also guess impacts by other small bodies, even if rare, would have a potentially large effect on their orbits/ $\endgroup$ Commented May 22, 2022 at 20:13