Two good ideas there, g.

The first idea is the use of modifying the course of a comet to terraform Mars quite quickly - the comet carries water, and the impact releases CO2 - and by the way, the technology of large nuclear charges already exists, as does that of the reaction motors needed to place the charge(s) -

Mars already has plenty of impact minerals just lying on its surface -

The second idea is that you could steer an approaching comet or asteroid into our Moon as a means of planetary defense - of course, debris from the Moon would fall to the Earth, and take out most of the world's satellites, but compared to the alternative this might be a quite useful technique -

EP