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.
This has long been the promise of Chang-Díaz’s plasma-based VASIMR rocket engine. From a theoretical physics standpoint, the rocket has always seemed a reasonable proposition: generate a plasma, excite it, and then push it out a nozzle at high speed. But what about the real-world engineering of actually building such an engine—managing the plasma and its thermal properties, then successfully firing it for a long period of time? That has proven challenging, and it has led many to doubt the engine’s practicality.
The rocket engine starts with a neutral gas as a feedstock for plasma, in this case argon. The first stage of the rocket ionizes the argon and turns it into a relatively “cold” plasma. The engine then injects the plasma into the second stage, the “booster,” where it is subjected to a physics phenomenon known as ion cyclotron resonance heating. Essentially, the booster uses a radio frequency that excites the ions, swinging them back and forth.
As the ions resonate and gain more energy, they are spun up into a stream of superheated plasma. This stream then passes through a corkscrew-shaped nozzle and is accelerated out of the back of the rocket, producing a thrust.