Musk retweeted, quoting his own 2011 statement to reiterate: reusable rockets are key to humanity becoming a multi-planetary species. He explained then that it was an extremely difficult engineering challenge because Earth's gravity made it "barely possible"; slightly lower gravity would make it easy, slightly higher impossible. He had long been unsure if it could be achieved, but a year before that conversation, he became convinced of its feasibility, and SpaceX would attempt to achieve this goal. At the time, there were simulated designs, but final testing still required real-world validation.