Batteries in the Outer Solar System

The advent of small, highly integrated spacecraft – also known as CubeSats – in the last decade has opened up the possibility of doing planetary missions in new ways, such as networked swarms. A major driver of this trend has been the improvement of batteries, from heavy lead-acid batteries in the 1960s to the svelte lithium-ion ones used today.

For my master’s in space science, I am working on a possible spacecraft mission to Saturn’s rings. It would consist of a small craft – probably no bigger than a CubeSat (~1 kg, 10 cm long) that detaches from a mothership in Saturn orbit to approach a few ice boulders in the rings for close-up study.

WANDERERS_ringsurf_01
Artist’s Conception of Saturn’s rings, from Wanderers by Erik Wehrquist

There are *many* factors that go into a good design. One such factor is the energy storage system. Spacecraft batteries have improved a great deal since Vanguard 1 52 years ago. But one drawback has prevented their wide use in the outer solar system – their narrow temperature range. Even the most advanced lithium-ion batteries shut down below -30 C. At the Saturnian system, where temperatures can get as low as -200 C, batteries are impossible to operate without sufficient insulation and heating. That’s why the Galileo and Huygens atmospheric entry probes had to carry radioisotope heater units (RHUs) to keep their batteries warm.

Smaller spacecraft wouldn’t have the internal volume to carry multiple RHUs, so insulation would be even more crucial. A big part of my thesis is mathematically modeling the relationship between internal and external heating and the temperatures throughout the spacecraft, during all phases of a Saturn orbit.

So far, it seems that just a few millimeters’ thickness of multi-layer insulation (MLI) will keep a CubeSat at an acceptable temperature during the sunlit portion of an orbit. Internal heating only becomes crucial during the dark portion when the sun is blocked by the planet. This period can be over an hour, so it may affect the size/mass of the batteries required.

The narrow temperature range does not seem to be a terrible showstopper for including batteries on outer solar system missions!

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