Mass and propulsion implications for interstellar scientific observation by flyby

Author: David G Messerschmitt, Roger Strauch Professor Emeritus, Electrical Engineering and Computer Sciences, University of California at Berkeley

Description: In view of the vast distances to an interstellar target, the highest technologically achievable spacecraft speed is often assumed, thus minimizing the mass of the scientific payload and communication gear. This is generally a compelling assumption for human spaceflight,  where many considerations favor short time to target. These include biological constraints such as the lifetime of astronauts, life support and physiological needs, and even psychological well being and onboard culture. However, for the coming century interstellar missions are expected to be robotic and employ flyby observation (as opposed to landing) to avoid deceleration. We address missions in which a space probe performs a target flyby during scientific observations, with data conveyed back to earth by electromagnetic radiation. In this scenario the scientifically relevant metrics are the observation time, the total volume of data returned, the number or frequency of probe flybys, and the total data latency (time elapsed from probe launch until completion of data return). The four components of data latency are travel time to target, observation time at target, the subsequent data transmission time, and the signal propagation delay. The mission design should be optimized with respect to objective functions like maximizing data volume for a given latency or, when a swarm of probes is launched, launch cost per unit of data volume. While  reducing probe speed (increasing payload mass) does increase the travel time to target, for directed-energy propulsion with a fixed launch infrastructure the dependence is a weak quarter-power penalty. Offsetting this is an expected mass-squared increase in data rate, a longer scientific observation time, a moderation of the distance-squared decrease in data rate, and a smaller signal propagation delay at the end of transmission. Several examples of design optimization under appropriate criteria illustrate the benefits of substantially increased probe mass with a resultant lower speed. The scaling law governing the relationship of probe mass and transmission data rate is observed to be a significant issue in interstellar probe design.