Living on Mars?

[Lvl 2 Expert]

Oh, there's an interesting idea. It'd take a hellishly large amount of asteroids, though, and I'm not sure how well they'd coalesce into a moon. Wouldn't you need to smack 'em hard enough to melt them together?

Probably about all of them, if it's possible at all. Ceres contains a quarter of the mass of the main belt, and it's quite small compared to our moon.

And as you said, getting them to stick together is going to be a bit of a trick. The escape velocity on Ceres is about 500m/s. That's quite fast even compared to say the speed of sound (+-350m/s), but I'd still worry that landing nine asteroids gently and screwing the tenth attempt up would turn it into a bowling party. Getting the combined body to orbit Mars (after you've managed to weld them together or wait long enough for that to happen by itself) is also going to take a bit of energy.

But hey, if it saves more energy in the long run... I'm just not sure if giant engineering projects like this will ever be preferable to just building large space ships for people to live in. Maybe if you specifically want something from Mars, you can't automate the process far enough and you want the circumstances to be nice for the employees and closed Mars habitats are not an option for some reason (like you're mining using massive earthquakes or something.)