re: Why rescue? Colonize from the start!
There are a lot of unknowns about Mars we need to discover before colonizing. Like what happens to you in a lower gravity environment, what is the level of radiation on the surface and how toxic and abrasive is the dust.
We would need to grow food, find and purify water and make tools to be reasonably independent. Which would take some infrastructure being built prior to the colonization happening We don't want a bunch of corpse on Mars the first time out.