Interstellar Mission Communications Low Background Regime.

Current attention on interstellar probes for near-term exploration of nearby star systems is focused on low-mass probes that can be accelerated to relativistic speed using propulsion from a ground-based DE beam. We consider the design of an optical communication downlink for the return of scientific data from such a probe at the distance of Proxima Centauri. The conditions under which background radiation can be neglected are quantified, and the design operates within that regime. Direct-detection is preferable to heterodyne, and in that context the transmitter should attain high peak-to-average transmitted power ratios. Based on available electric power sources, the downlink is expected to operate for years or even decades following target encounter, combined with low data rates. There are several areas in which technology innovations are needed, most of them related to Earth-based large-area aperture receiver design with direct detection. A major issue is the choice of multiplexing approach to support multiple probe downlinks and related challenges. Due to the interaction of trajectory parallax effects with field-of-view, we conclude that aperture synthesis with controlled optical beam forming may be required to reject radiation from the target star. Short visible wavelengths for laser communications are also highly advantageous in reducing that radiation. Highly selective optical bandpass filtering is needed to reject unnecessary background radiation, and a short-term data uplink is required to configure the transmit wavelength for variations in probe speed. Fundamental limits on the photon efficiency are compared to a concrete modulation/coding design in the presence of weather-based outages.