I wrote an article back in 2007 about how little was known about landing large payloads like a human mission on Mars, and how engineering experts in Entry, Descent and Landing (EDL) thought it might not even be possible. I checked in with Rob Manning from JPL for an update, and the outlook has changed considerably. https://www.universetoday.com/169697/the-new-mars-landing-approach-how-well-land-large-payloads-on-the-red-planet/