Elon Musk admitted he is not sure this one is possible. Musk: “This absorbs more of my mental energy than probably any other single thing. But it is so preposterously difficult that there are times where I wonder whether we can actually do this.” He does not talk like this. Mars gets a date. Robotaxis get a date. This gets a maybe. Stern: “What is so hard about it for just a normal person to understand?” His answer comes down to a single number. Musk: “In order to make a rocket fully reusable, you’ve got to basically create a rocket that can do about 4%, if not more than 4%, of its lift-off mass to orbit, which hasn’t happened before.” Four percent. Everything else standing on that pad is fuel, tank, and engine. The landing gear, the heat shield, the extra structure that survives reentry, all of it comes out of that same four. None of it can be left behind. The vehicle has to come home. Musk: “So that means you have to have basically A-pluses across the board: incredibly efficient engines, incredibly efficient structure.” Across the board is what makes this brutal. Most engineering lets you buy a weakness in one place with strength somewhere else. The margin here is thin enough that one component landing at merely excellent drops the whole vehicle below the line. Nothing carries anything. Musk: “But if full and rapid reusability can be achieved, it reduces the cost of access to orbit by a factor of 100 or more.” A hundred times cheaper turns nearly every question about space from whether into when. Telescopes, stations, factories, missions that never made it past a spreadsheet. Musk: “So we’ve got to get rockets to the point where we simply refuel the rocket and we don’t throw it away.” Every other vehicle humans build already works that way. Space is the one place we still build the machine, use it once, and let it fall into the ocean. Musk: “It’s the difference between humanity being a single-planet species and a multi-planet species. It’s really that big of a deal.” Now put his doubt in context. Landing an orbital booster upright was considered impossible by most of the industry. He spent years being told so while boosters exploded on barges. Those landings became routine, then boring, then unwatched. Then they stopped using legs. A twenty story booster came back from the edge of space, slowed itself, and hovered next to the tower it launched from. Two arms closed around it and took its weight in midair. Nothing touched the ground. That was a headline for about a day. He has been on this side of impossible before, more than once. Doubt from him measures how hard the thing is at the start rather than how it ends. Most people never attempt anything where failure is genuinely on the table. He found the hardest problem available and made it the center of his life. That is what real ambition looks like from the inside. Not certainty. A number nobody has ever hit, and showing up to it every morning until it falls.