The effect of the giant asteroids hitting Mars would be devastating to any chance of colonization by humans for millions of years.
78. Well, there goes Trump’s plan for an upscale Trump Mars Hotel and Galactic Casino.
Unless you’re talking a Theia sized collision, Mars’ lack of a substantial atmosphere would mean the heat would rapidly dissipate into space. Theia was estimated to be about the size of Mars, which itself is only 1/10th the size of Earth.
https://en.wikipedia.org/wiki/Theia_(planet)
So optimally, there would be an asteroid, almost certainly from outside the solar system, about the size of Venus, which is 8/10ths the size of Earth, that would enter Jupiter orbit and break up before its fragments flew off to Mars. It might even have a catastrophic collision with one of Jupiter’s moons.
Only after that, a bunch of large comets would arrive, covering the still hot Mars with water.
Yes, it would take a while, but otherwise colonizing Mars would have no staying power.