Challenges in Scientific Data Communication from Low-Mass Interstellar Probe.

Near-term exploration of nearby star systems focuses on a swarm of low-mass probes accelerated to relativistic speed using propulsion from a terrestrial directed-energy beam. An optical downlink for the simultaneous return of scientific data from multiple probes at the distance of Proxima Centauri is studied. Transmission time/distance and probe mass are chosen to achieve the best data latency vs volume tradeoff. Challenges in targeting multiple probe trajectories with a single receiver are addressed, including multiplexing, parallax, and target star proper motion. Relevant sources of background radiation, including cosmic, atmospheric, and receiver dark count are identified and estimated. Direct detection enables high photon efficiency and incoherent aperture combining. A novel burst pulse-position modulation (BPPM) beneficially expands the optical bandwidth and ameliorates receiver dark counts. A receive aperture architecture combines minimum transmit power with constrained swarm-probe coverage. Theoretical limits on reliable data recovery and sensitivity to the various BPPM model parameters are applied, including a wide range of receive aperture areas. Such a downlink does not appear feasible with current technology. Enabling innovations include a high peak-to-average power ratio, a large source extinguishing factor, the shortest atmosphere-transparent wavelength to minimize target star interference, adaptive optics for atmospheric turbulence, very selective bandpass filtering (possibly with multiple passbands), very low dark-count single-photon superconducting detectors, and optical phased arrays for sky coverage shaping and coronagraph functionality.