How are gravitational orbits maintained

How is it that the orbits of the planets and moons can remain so relatively stable when they are under constant bombardment by comets, asteroids, solar wind, etc.

It is said that an orbit is maintained when the force of gravity as a 1/r^2 force equals the centripedal force of the object in orbit. Or, it has also been stated that the object in orbit is actually just falling fast enough to curve around the object. In either case, this would seem that the equality of the forces would have to be pretty exact in order to maintain a stable orbit.

This exact balance would be like trying to balance something on a knife edge. Any disturbance woud lose the balance. It would seem to me if an asteroid hit the moon in the direction of the orbit, that would have to slow the moon down and suddenly the gravity force would be greater than the centripedal force and cause an irreversable spiral of the moon into the Earth.

But historically, the moon and the Earth have had major impacts in the past, none of which have caused the Earth or Moon to suddenly spiral into the sun or fly out of the solar system. If the moon got hit by something that slowed it down, it would seem that the moon would either have to move out to a farther orbit so that gravity would be reduced to the same amuont as the decrease in the centripedal force or the moon would have to speed up so as to increase the centripedal force so that gravity = centripedal force.

There would almost have to be some kind of feedback loop so that small disturbances are compensated for and the object will always settle in to the point where gravity = centripedal force and a relatively stable orbit is maintained. It just can't be that orbits are maintained by just the balancing of the gravitational aginst the centripedal force.

A potential answer is found in this web site reference:

http://msowww.anu.edu.au/~pfrancis/roleplay/MysteryPlanet/Orbits/?vm=r

It turns out that if you nudge the moon to slow it down, gravity does become stronger than the centripedal force, but as the moon drops lower due to gravity, the law of conservation of angular momentum causes the moon to speed up at (just like a skater pulling in their arms) and this increases the centripedal force at a rate proportional to 1/r^3 which is faster than the increase in gravitational for of 1/r^2, so that the cetripedal force catches up with the gravitational force as r (the radius) decreases.

So – orbits are stable – if you nudge them, they will come back into balance. This can only happen because there is a balance between the force which is causing the moon to go down (gravity) and the even stronger force (centripedal) which causes the moon to go up. There must be this balance of un-even forces to maintain the balance. If the centripedal force only increased by a 1/r^2 force, then it would never be able to overtake the gravitational force and stable orbits would not be possible in the face of frequent collisions.