Martian Mayday: The Evolution of Curiosity’s Safe Mode Communication Over Ten Years

Abstract—When the Curiosity Mars rover encounters a serious fault, it enters a configuration called safe mode. Curiosity's autonomous safe mode behavior must keep the rover communicative and energy safe until the operations team can triage the issue and send recovery commands. In safe mode, Curiosity follows a preset schedule of windows that specify the antenna, transmitted telemetry, and solar time of each communications opportunity. The timing of these windows, called system fault protection (SFP) windows, is crucial for Curiosity to autonomously maintain its safe mode goals of communication and power-positivity. Not only do SFP windows dictate when Curiosity attempts to communicate over its ultra-high frequency and X-band antennas, they also largely control the rover’s power consumption by dictating when its primary computer powers on and off, known as wake and sleep states. Because Curiosity is power-negative when awake, sleep time is necessary to charge its batteries to a higher state of charge than the previous sol. During Curiosity’s prime mission, fault protection engineers regularly updated SFP windows to accommodate changing Mars relay orbits and relative geometry of Earth and Mars. The initial SFP windows at landing allowed a hearty nine Martian hours of communication which left Curiosity power-positive by 22% increased state of charge over a single sol in safe mode. But as the mission progressed, updates to SFP windows were additionally driven by Curiosity’s aging power source, which continuously requires increased sleep time to maintain power-positivity. As of 2023, 6.6 hours of safe mode communication leaves Curiosity only 5.5% power-positive. Secondary factors that were not present during Curiosity's prime mission—including changing uplink strategies and Mars relay infrastructure—have also introduced new challenges to updating SFP windows. The process of selecting SFP window times has evolved into an iterative analysis that must consider several dynamic operational factors to achieve a balance of communication and power. By using this analysis to converge on a design that satisfies energy requirements and desired communication time, fault protection engineers ensure Curiosity’s safe mode configuration in its extended mission remains truly safe. This paper discusses how Curiosity uses SFP windows to communicate in safe mode, the factors that influence how SFP windows are designed, and how both the design process and the windows themselves have changed over ten years on the surface of Mars.