When there is a collision and fragments are ejected outwards, I notice that they are not affected by the gravity of bodies that they pass, even if I make them into full objects (attracting bodies). So if I crash Pluto into Earth and immediately make fragments into full bodies, the ones that shoot towards the sun or towards a large body like Jupiter or Saturn just go in a straight line forever. This also happens when you have a body (like a rogue planet) passing through the orbit of planets or through the solar system.
Unless I'm not understanding this correctly, this is not realistic. Bodies that shoot off in a straight line would quickly change course as they are affected by the gravity of larger bodies.
Basically what I want to do is to setup a rogue moon scenarios where I have multiple moon-sized objects pass through the solar system slowly and get captured by planets, which they will continue to orbit. Is this not possible or just extremely difficult to do?