|A comet impacting on Mars|
That might kick-start a colony, but the surface gravity of Mars isn't sufficient to retain a reasonable atmosphere and water over thousands of years.
We could be optimistic and hope our descendants will fix this problem, but why not a little insurance?
I propose we choose some large ice-comets, preferably in the outer system where gravitational perturbations from the likes of Jupiter and Saturn are minimised, and adjust their orbits to intercept Mars in, say, 5,000 years time.
The downside: if there's a civilisation on Mars at that time which lacks technological sophistication, they're going to have some real bad days.
The upside: some would survive. But if Mars loses its water and oxygen, they're all toast anyway.
I checked. The JPL ephemeris program can currently model the position of the planets out to 3,000 AD. Their positional accuracy for Mars is around plus or minus one kilometre.
I think we can do this, people.
You know they'll thank us in the end.