Avionics for Hibernation and Recovery on Planetary Surfaces

Robots are challenged to endure extreme cold during nights in planetary environments such as the moon and Mercury, and polar winters on Mars. A process for robots to endure cryogenic temperature, and then recover when the environment has warmed, is essential to solar system exploration. We present a novel power system that uses a mechanical thermostat to de-power all electronics to hibernate when temperatures fall. The thermostat automatically restarts the electronics when warmed to operating temperature. Environmental testing shows reliable recovery of the prototype after repeated freezes as cold as the -173°C temperature of lunar night. Landers and rovers endure on the Martian equator but experience avionic failures in the cryogenic temperatures of lunar nights and Martian winters. The greatest vulnerability is battery failure due to dielectric breakdown. Integrated circuits and semiconductors will exhibit unexpected behavior as temperatures fall because the resistance of N-type and P-type transistors is not linearly correlated with temperature [1]. Other components fail, and interconnects break from fatigue. The Surveyor program demonstrated partial success reactivating landers after lunar night, but their batteries and electronics were damaged by the extreme cold [2]. Five Surveyors landed on the moon, one of which is pictured in Figure 1. Of these, only Surveyors I and V could be reactivated (two and three times respectively), and the reactivated spacecraft had reduced capability due to component failures. The current state of practice is for planetary robots to keep their electronics above cryogenic temperatures by using radioisotope heater units (RHUs) and electrical heaters from stored energy [3]. Stored heat and resistive heating are viable for bridging short, mild cooling over a Martian night, but storage is not viable for seasonal winters or long lunar nights. Isotopic heating is viable for some missions, but isotopic decay cannot be turned off and becomes an overheating problem when the cold of night turns to the heat of day. The processes, permissions, and costs required to implement an isotopic solution are a severe risk to budget and schedule. Moreover, the plutonium-238 used in RHUs and RTGs is scarce because production of the isotope ceased after the Cold War. The global stockpile of plutonium-238 is nearly depleted and new material will not be available before 2017 [4]. For these reasons, isotopic solutions are not viable for commercial missions. Small, near-term surface systems must recover after enduring cryogenic hibernation. This ambition requires freeze-tolerant batteries and durable devices. Freeze tolerant batteries are electrochemical cells that endure repeated thermal cycling to cryogenic temperature without losing the ability to store energy. Durable devices are electronic components and interconnects that neither break nor change behavior after repeated thermal cycling to cryogenic temperature. A system with freeze tolerant batteries and durable devices does not require isotopic heating because the entire system can withstand storage at cryogenic temperatures without damage. This research asserts that a system constructed using commercially available electronics and Lithium Iron Phosphate batteries can withstand cryogenic temperature when unpowered and recover automatically when the temperature warms. This work affirms that hibernating through long nights and seasonal winters, then recovering at the start of the next period of operation is a viable alternative to isotopic heating for planetary surface missions. The recovery of consumer grade electronics and batteries after exposure to -173°C indicates production process improvements implemented over the last forty years have resulted in durable devices capable of withstanding cryogenic temperature without modification. This result enables low cost planetary science by supporting mission planning with lower cost spacecraft that have longer mission duration.