Almost everyone recognizes that if humans are truly to go deeper into the Solar System, we need faster and more efficient propulsion systems than conventional chemical rockets. Rocket engines powered by chemical propellants are great for breaking the chains of Earth's gravity, but they consume way too much fuel when used in space and don't offer optimal control of a spacecraft's thrust.

NASA recognizes this, too. So in 2015, the space agency awarded three different contracts for development of advanced propulsion systems. Of these, perhaps the most intriguing is a plasma-based rocket—which runs on Argon fuel, generates a plasma, excites it, and then pushes it out a nozzle at high speed. This solution has the potential to shorten the travel time between Earth and Mars to weeks, rather than months.

But to realize that potential, Houston-based Ad Astra Rocket Company must first demonstrate that its plasma rocket, VASIMR, can fire continuously for a long period of time. The three-year, $9 million contract from NASA required the company to fire its plasma rocket for 100 hours at a power level of 100 kilowatts by 2018.

This week, Ad Astra reported that it remains on target toward that goal. The company completed a successful performance review with NASA after its second year of the contract, and it has now fired the engine for a total of 10 hours while making significant modifications to its large vacuum chamber to handle the thermal load produced by the rocket engine.

When Ars visited Ad Astra early in 2017, it was pulsing its rocket for about 30 seconds at a time. Now, the company is firing VASIMR for about five minutes at a time, founder Franklin Chang-Diaz told Ars. "The limitation right now is moisture outgassing from all the new hardware in both the rocket and the vacuum chamber," he said. "This overwhelms the pumps, so there is a lot of conditioning that has to be done little by little."

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

Ad Astra Rocket Company

As the company continues to test the new hardware, it is gradually building up to longer and longer pulses, with inspections in between. Ad Astra remains on target to perform the 100-hour test in late summer or early fall of 2018, Chang-Diaz said.

Initially, the company foresees the plasma rocket as a means for pushing cargo between Earth and the Moon—or on to Mars. With solar powered panels, the rocket would have a relatively low thrust and therefore would move loads slowly but efficiently. But with more power, such as from a space-based nuclear reactor, it could one day reach much higher velocities that would allow humans to travel rapidly through the Solar System.