Power System for Miniature Interstellar Flyby Probe

Author: Geoffrey Landis, PhD, Researcher, NASA John Glenn Research Center

Abstract Background: In the last few years,the concept that an ultra-lightweight probe could be sent to one of the nearby stars pushed by a laser beam reflecting from a lightweight sail has moved from science fiction into conceptual design. The Breakthrough Starshot project envisions a two- to three-gram “starchip” micro probe, flying past a planet of Proxima Centauri after a 20 year voyage.

With the probe moving at 60,000 km/sec, the flyby encounter at the target planet lasts at most a few hours. With current technology, no power system exists that can produce the required power with a mass of less than one gram.

Abstract Objectives: Baseline requirements for the power system are:

Weight: 1 gram or less

Lifetime: 25 year cruise, followed by encounter phase.

The operational lifetime during the encounter phase can be a trade-off against the power level. The baseline requirement is 1 watt peak, 10 mW continuous, with higher power levels desirable.

Abstract Methods: Several power systems were analyzed.

Abstract Results: Radioisotope Power

Analysis shows that radioisotope thermal power system scale poorly to small sizes, and would be four to five orders of magnitude too heavy for such a microprobe.

An alternative proposal would be to use direct energy conversion, rather than thermal conversion. Betavoltaic decives scale well to low power levels. Betavoltaic cells have future specific power anticipated at 1 W/kg. The half life of tritium, 12.3 years, results in decay to about 25% of baseline power during the cruise.

An alternative uses the energy of alpha particles from spontaneous fission. Isotopes include plutonium-238 or curium 244. Semiconductor alphavoltaic converters, however, are subject to alpha-induced degradation, and may not make the lifetimes required.

Advanced Technology: The proposed flyby spacecraft has a kinetic energy of 10 million megajoules per gram. We can use the energy of the spacecraft’s motion through the interplanetary medium by using the ambient plasma and magnetic environments. The anticipated density of solar-wind generated plasma is ~6E12 protons per cm2 at an energy of ~5 MeV. Thiscould be turned into power with an electrostatic grid. Alternatively, for a target magnetic field of 1 nT, we can create an electric field by the spacecraft’s motion of about 60 volts per meter to generate power.

Abstract Conclusions: The power system for a small (Starshot-sized) interstellar probe is a major component that has, to date, not been well analyzed. The low mass requirement makes the problem very difficult. Several approaches to this power system are possible.