Some of the technologies now being pursued by the In-Space Propulsion Program may not be ready to fly until well into the next decade. But others may fly sooner… In some cases, much sooner.

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The potential flight readiness of each technology depends on its maturity, as determined via a series of investigations validating the scientific principles involved, application of a prototype in a laboratory environment, and eventually flight-testing in an operating environment simulating the conditions faced by sustained exposure to space.

NASA calls this process the "Technology Readiness Level" scale, which is broken down as follow:

• TRL 9: Actual system "flight proven" via successful mission operations
• TRL 8: Actual system completed and "flight qualified" via test and demonstration (ground or space)
• TRL 7: System prototype demonstration in a space environment
• TRL 6: System/subsystem model or prototype demonstration in a relevant environment (ground or space)
• TRL 5: Component and/or breadboard validation in relevant environment
• TRL 4: Component and/or breadboard validation in laboratory environment
• TRL 3: Analytical and experimental critical function and/or characteristic proof-of-concept
• TRL 2: Technology concept and/or application formulated
• TRL 1: Basic principles observed and reported