Re: What would it take to keep the Moon?

How big would a collision against the Moon need to be in order to cancel it's tendency of gradually moving away from the Earth, keeping it at aproximatly the same distance from the Earth forever (or until the Sun blows over)?

The Moon is moving away from the Earth because of tidal effects. The Moon raises tides on the Earth, which slows the rotation of the Earth, and transfers angular momentum to the Moon's orbit. This produces the gradual increase in Moon's orbital radius.

This tidal effect will not go away until the Earth is tidally locked to the Moon (that is, it keeps the same face towards the Moon, just as the Moon now does to the Earth). That won't happen for billions of years, however. So all an impact on the Moon could do is change it to another orbit, a little closer or farther from the Earth. From that position, it would still be subject to the forces that are causing it to move away.

In reality, any impact large enough to significantly alter the orbital radius of the Moon would release so much energy that the Moon would be fragmented. Some of the resulting debris would produce severe impacts on the Earth, some would be sent towards the Sun, and what was left would probably coalesce to form a new, smaller satellite around the Earth.