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A mission trajectory study estimated that a VASIMR-powered spacecraft could reach the red planet within 40 days if it had a 200 megawatt power source. That's 1,000 times more power than what the current VASIMR prototype will use, although Ad Astra says that VASIMR can scale up to higher power sources. The real problem rests with current limitations in space power sources. Glover estimates that the Mars mission scenario would need a power source that can produce one kilowatt (kW) of power per kilogram (kg) of mass, or else the spacecraft could never reach the speeds required for a quick trip. Existing power sources fall woefully short of that ideal. Solar panels have a mass to power ratio of 20 kg/kW. The Pentagon's DARPA science lab hopes to develop solar panels that can achieve 7 kg/KW, and stretched lens arrays might reach 3 kg/KW, Glover said. That's good enough for VASIMR to transport cargo around low-Earth orbit and to the moon, but not to fly humans to Mars. Ad Astra sees nuclear power as the likeliest power source for a VASIMR-powered Mars mission, but the nuclear reactor that could do the job remains just a concept on paper. The U.S. only ever launched one nuclear reactor into space back in 1965, and it achieved just 50 kg/kW.