You might expect to find our brightest hope for sending astronauts to other planets in Houston, at NASA's Johnson Space Center, inside a high-security multibillion-dollar facility. But it's actually a few miles down the street, in a large warehouse behind a strip mall. This bland and uninviting building is the private aerospace start-up Ad Astra Rocket Company, and inside, founder Franklin Chang Díaz is building a rocket engine that's faster and more powerful than anything NASA has ever...
