On a Thursday afternoon in June, a 17-foot-tall rocket motor—looking like something a dedicated amateur might fire off—stood fire-side-up on the salty desert of Promontory, Utah. Over the loudspeakers, an announcer counted down. And with the command to fire, quad cones of flame flew from the four inverted nozzles and grew toward the sky. As the smoke rose, it cast a four-leaf clover of shadow across the ground.

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