Money quote: “There’s too much atmosphere on Mars to land heavy vehicles like we do on the moon, using propulsive technology completely,” said Manning, “and there’s too little atmosphere to land like we do on Earth. So, it’s in this ugly, grey zone.”
I believe that comment is more or less based on the shuttle landing and/or the soyuz capsules (eg parachutes) - but as far as I can tell, SpaceX is the only one currently capable (and using) propulsive landing on earth - so essentially they've already leapfrogged the grey zone to a large extent.
Makes me wonder whether testing a propulsive landing on an airborne drone barge is feasible? A blimp with a well-insulated large pad on the top - though I'm not sure how you'd effectively anchor it in the air - probably a active propulsion system as well.
Money quote: “There’s too much atmosphere on Mars to land heavy vehicles like we do on the moon, using propulsive technology completely,” said Manning, “and there’s too little atmosphere to land like we do on Earth. So, it’s in this ugly, grey zone.”