Sen—A 10-year effort to test an innovative, commercially developed plasma engine on the International Space Station has come to quiet end, with NASA nixing plans to fly Ad Astra Rocket Company’s VASIMR thruster aboard the orbital outpost.

The Webster, Texas-based company, founded and overseen by seven-time shuttle astronaut and physicist Franklin Chang Diaz, has been working to fly a prototype Variable Specific Impulse Magnetoplasma Rocket, or VASIMR, engine on the space station.

But a series of agreements with NASA, dating back to June 2005, ended in December 2014, with the U.S. space agency determining that the space station “was not an ideal demonstration platform for the desired performance level of the engines,” NASA spokeswoman Rachel Kraft wrote in an email.

NASA cited VASIMR’s power consumption and test duration requirements as reasons for ending plans to fly the engine on the station.

Discussions between NASA and Ad Astra, aimed at a VASIMR station demonstration, have been ongoing since 2005 under previous agreements, notes a December 2008 Space Act Agreement signed by NASA associate administrator William Gerstenmaier and Chang Diaz.

“A number of joint studies and assessments have been conducted, which support the technical feasibility of such a test and have led to the present concept of a 200-kilowatt VASIMR package, operated in short bursts of up to several minutes from a battery pack periodically charged by the ISS power bus,” the agreement said.

Unlike traditional chemical rocket engines, VASIMR uses radio waves to ionize gas, such as argon, xenon or hydrogen, into superheated plasma, which is then herded through nozzles made of magnetic fields, not metal like traditional engines.

The magnetic fields also insulate nearby structures, so temperatures well beyond materials’ melting points can be achieved, and the resulting plasma harnessed to produce propulsion, Ad Astra notes on its website.

The company wanted to test a VASIMR engine on the station to see how it performs in the vacuum of space without having to shoulder the additional costs and complexities of flying communications systems, power supplies and other equipment to support the tests.

The engine would tap electrical power from the station to charge a large battery pack capable of powering the thrusters for approximately 15 minutes at full power, Ad Astra said.

Building a VASIMR to fly on the station, estimated to cost about $100 million, would be privately funded. Initially, the prototype was expected to fly between 2012 and 2015. It was later retargeted for launch in 2016.

NASA had earmarked an attachment point on the station’s main truss as the most desirable location for the test.

In an interview, Chang Diaz said Ad Astra was continuing to develop the engine, with technical and financial support from NASA. "I don't think any doors have been closed," he said.

An in-space demonstration will be needed before the engine can be incorporated into an operational system, but Chang Diaz said there are other options besides the International Space Station. He also said an opening on the ISS might be an option in the future.

"I don't think this is important to us right now," he said.

In 2009, the company successfully ground-tested a VASIMR prototype at 200 kilowatts for the first time.

It has proposed using VASIMR technology to remove orbital debris, reboost or reposition space stations and satellites, and to send spacecraft to asteroids, Mars and other destinations in the solar system.

“Advanced electric propulsion thrusters are an innovative technology that could propel NASA missions far into the solar system more efficiently than traditional chemical systems and provide crosscutting capabilities for commercial applications,” NASA said.

“In the case of high-power electric propulsion thrusters, NASA and Ad Astra had a Space Act Agreement to allow continued technical interchange related to the development and potential International Space Station demonstration of the VASIMR electric propulsion engine,” NASA said. “That came to an end in December 2014.”

“Multiple electric propulsion thruster technologies exist or are under development, spanning a wide range of power levels and performance parameters," NASA said. "The agency sees potential future applications for high-power electric propulsion thrusters such as those provided by (Ad Astra’s) engine development endeavors."

(This story has been updated with comment from Ad Astra)