Ad Astra Rocket Company

HOUSTON—Franklin Chang-Díaz bounds up a handful of stairs and peers through a porthole cut into the side of a silver, tanker-truck-sized vacuum chamber. Inside, a blueish-purple light shines, unchanging and constant, like a bright flashlight. “It looks kind of boring,” Chang-Díaz admits. “But that plume is 3.5 million degrees. If you stuck your hand in that, it would be very bad.”

Truth be told, the plume does not look impressive at all. And yet the engine firing within the vacuum chamber is potentially revolutionary for two simple reasons: first, unlike gas-guzzling conventional rocket engines, it requires little fuel. And second, this engine might one day push spacecraft to velocities sufficient enough to open the Solar System to human exploration.