Relaying Swarms of Low-Mass Interstellar Probes

Low-mass probes propelled by directed energy from earth are an early option for exploration of nearby star systems. A challenging aspect of such technology is returning scientific observational data to earth. We compare two configurations for achieving this. A direct configuration utilizes optical transmission from the probe to a terrestrial receiver employing a large photon collector. In a relay configuration, probes spaced at uniform intervals act as regenerative repeaters for the scientific data, which eventually arrives at a terrestrial receiver from the most recently launched probe. A number of advantages and disadvantages of the relay configuration are discussed. A numerical comparison approximates equal probe mass in the two cases by using the same optical transmit power and equivalent total transmit plus receive aperture area. When the total downlink data rate is equal, the relay configuration benefits from a smaller terrestrial receive collector, but also requires very frequent launches to achieve higher data rates due to the limitations on relay probe receive aperture area. The direct configuration can achieve higher data rates without such frequent launches by increasing terrestrial collector area. A single-point failure problem in the relay configuration can be addressed by introducing relay-bypass modes, but only at the expense of further increases in launch rate or reductions in data volume, as well as a considerable increase in design and operational complexity. Taking into account launch and collector area costs, the direct configuration is found to achieve lower overall cost by a wide margin over a range of cost parameter values and data rates.