Even when the idea of terraforming Mars was originally put forward, the idea was daunting. Changing the environment of an entire planet is not something to do easily. Over the following decades, plenty of scientists and engineers have looked at the problem, and most have come to the same conclusion—we're not going to be able to make Mars anything like Earth anytime soon. A new paper available in pre-print on arXiv from Slava Turyshev of NASA's Jet Propulsion Laboratory is a good explainer as to why.
