Obviously Kookaburra meant there are no asteroids as big as your paranoia imagines there are, in this solar system. More than 350,000 asteroids are known about and Ceres, the first to be found in 1801, remains the largest that has been found,
To say "there could be a larger one" (in this solar system) fails to grasp why Ceres was the first to be found. It was one of the brightest ones, and thus able to be found more easily, because it was such a large one. Do you think it probable that a larger one would have escaped detection for 200 years when we have automated asteroid-hunting techniques that are now turning up 5,000 or more a month?
Asteroid belts or Kuiper Belts have been found round two other stars so far and in one case the belt has ten times the mass of ours and in the other case 25 times the mass of ours. Probably there are asteroids bigger than Ceres around these two stars, therefore. But how would they get here!?
You seem to have no grasp how the gravitational effect of massive bodies affects the movement/orbit possibilities of other bodies.
(1) the effect of Jupiter is to marshal the asteroids in the Main Asteroid Belt into bands separated by Kirkwood Gaps. It "keeps them in line".
Ceres will therefore not escape into the Inner Solar System and impact the Moon. Saying "it could do" is simply being uninformed and ignorant of how the Laws of Physics work.
(2) Asteroids and planets do not live in interstellar space and pop in to stellar systems they feel like visiting (as Nibiru mystics like to believe). They orbit a star.
Only in very rare circumstances like a supernova explosion do they leave their parent star. The gravitational attraction of other stars is not likely to pull them away from the star they orbit because other stars are so much further away, relatively speaking (except in a binary system).
(3) The only possible circumstances in which asteroids or planets might leave one stellar system and join another is if a massive star draws very close to another of much lower mass.
In about 1.4 million years time, the star Gliese 710 will approach the Sun to a distance of 1,1 light years away, It is a red dwarf and of lower mass than the Sun. This may cause some perturbation of comets in the Oort Cloud, and cause more to be dispatched towards the Inner Solar System. But its not close enough for us to gain any of Gliese 710's companions if it has any (none known as yet).
How do we know? The Hipparcos survey looks at the proper motion of all nearby stars to detect those that have been closer to us or will be in the future.
So saying "there might be other stars that could come closer" fails to grasp that they would have been picked up by the survey if that was so and obviously if they aren't close enough to be surveyed. they aren't (yet) close enough to have any prospects of coming close to the Sun.
If you read and understood more about astronomy, you wouldn't fall prey to believing that something you can imagine might happen has any realistic propsects of happening just because you thought it could perhaps happen.
Pigs might fly, but rational people like the Walrus and the Carpenter talk of many things and discuss the salient facts like "whether pigs have wings" and assess the probability that they might fly in the light of the known facts about pigs.
WHAT ARE THE KNOWN FACTS ABOUT ASTEROIDS?
No asteroids larger than Ceres are known in this solar system.
No star-hopping asteroids are known.
Therefore the assessment has to be that no asteroid not of Solar System origins will enter the Solar System and impact the Moon.
And therefore your fears are groundless.