@mangeurdenuage I was thinking about that recently. One wouldn't want to smash a large asteroid or comet into the planet, but if one could break up a bunch of smaller comets and asteroids into a cloud of small (like smaller than the size of a human body) pieces and direct those pieces onto the martian surface, it would be possible to build up enough mass to make holding onto a useful atmosphere possible.

(Mars has a thin atmosphere of about 95% carbon dioxide, but because the atmosphere is so thin, the planet is colder than one would expect at that distance from the sun.)