Every 26 months, the celestial mechanics of our solar system hand spaceflight engineers a gift: a roughly one-month stretch when Earth and Mars align closely enough that a spacecraft can make the trip using the least possible fuel. Miss it, and the next chance is more than two years away. This November, that window opens again — and this time, for the first time in human history, a private company is aiming to send a fleet of rockets through it toward another planet. SpaceX h