The Electron Technologies, Inc. (ETI) model 999H is a high-power high-efficiency space traveling-wave tube (TWT) that will provide high-rate, high-capacity, direct-to-Earth communications for science data and video from the 2009 Mars Telesat Orbiter spacecraft and will also provide communication relay support for surface-based probes on Mars. The ETI model 999H Traveling-Wave Tube has demonstrated 144 watts of continuous wave power over the 31.8–32.3 GHz frequency band with 60% average overall efficiency. The extreme power limits were not tested, but we expect that 50 to 150 watts of continuous wave power could be readily obtained over the frequency band. This contrasts with 35 watts and 50% overall efficiency of the prior highest power deployed space TWT at this frequency developed for the Mars Reconnaissance orbiter. Additionally, the TWT has been designed for flexible output power depending on mission requirements, and has demonstrated high efficiency over the power range of 54 to 144 watts. This improvement in state-of-the-art output power at Ka-band frequencies enables science data and video to be sent from Mars to Earth at more than four times the rate previously possible.