Europa Lander is a concept for a potential future mission that would look for signs of life in the icy surface material of Jupiter's moon Europa.
The moon is thought to contain a global ocean of salty liquid water beneath its frozen crust, and if life exists in that ocean, signs of its existence, called biosignatures, could potentially find their way to the surface, where a spacecraft could sample and study them.
In order to investigate whether signs of life can be detected in Europa's surface material, a spacecraft would land on Europa and collect samples from about 4 inches (10 centimeters) beneath the surface. This is a depth at which the complex chemistry of materials from the ocean below would be protected from the damaging radiation that exists in space around Jupiter.
The samples would be analyzed by a miniature laboratory within the robotic lander, similar to the way samples on Mars have been studied by landers and rovers on the Red Planet. In addition to its onboard chemical analysis lab, a Europa Lander mission might also carry a microscope and a camera, along with a seismometer to detect geologic activity such as eruptions or the shifting of Europa's ice crust.
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The Europa Lander mission concept presents a number of unique challenges. First, the Europa Lander mission concept has a limited, non-rechargable battery supply, limiting the lander’s lifetime. In addition to this, Europa Lander would face long communication blackout periods (roughly half of every 3.5 day orbit of Europa around Jupiter) during which communication with Earth is not possible. The combination of these two factors results in a mission concept that requires significantly higher levels of autonomy than previous NASA surface missions. Whereas previous missions might be able to wait for ground instructions, the limited lifetime of Europa Lander means that it could not afford such a wait, especially during a lengthy blackout period.
In addition to requirements of higher levels of autonomy, Europa Lander would land in an environment with unprecedented unknowns. Environmental uncertainty is a large concern, since little is known about the surface of Europa. This severely limits the quality of a priori estimates of model parameters such as time and energy expenditures of activities. Another source of uncertainty is inaccuracy in measurement. As energy levels decrease, uncertainty in battery performance increases, resulting in energy usage that may differ from previous expectations. Finally, variability exists in the discovery of potentially valuable information. For example, discovering a biosignature in a given excavation site would drastically alter its value. Consequently, for the Europa Lander to be successful, it requires a planning and execution framework that is robust to unprecedented levels of uncertainty, can run onboard and in real-time to support efficient periodic replanning, and still maximizes its overall utility.
In planning for Europa Lander, we leverage domain knowledge and dependency structure to construct a hierarchical task network. We assign utility to all data that is collected and successfully downlinked back to Earth, so any data that is collected but not successfully downlinked has no utility. We then perform branch-and-bound heuristic search on the space of partial plans, using the utility to cost ratio as a heuristic.
To address sources of uncertainty in the Europa Lander mission concept, we take an integrated approach to planning and execution based on MEXEC. We use three major techniques here:
1. Flexible execution
2. Periodic re-planning
3. Online model parameter update
Flexible execution allows for minor plan changes at execution time without failure or replanning, for example, changing a task’s start time due to some small delay. This allows the system to drastically reduce time or resource constraint margins, thereby increasing overall productivity of the plan. Periodic replanning improves agility of the system, allowing updates to the plan during execution time, using up-to-date state knowledge. This allows reaction to stochasticity at execution time, using the measured values of resources rather than previous projections. Finally, model parameter update allows changes to task models online using knowledge gained at execution time, thereby improving future predictions. For example, the system changes expected energy use by the sampling task after discovering that sampling is more difficult than previously expected. The combination of these techniques results in a planning and execution system that is agile, robust, and capable of addressing the unique challenges of Europa Lander.
In more experimental work, we adopt a decision theoretic approach to representing uncertainty and utility. In this approach, we develop plans for carrying levels of energy remaining in the mission. We then evaluate these plans against contingencies where there may be more or less energy remaining than predicted. The plans that perform the best despite mis-predicted energy remaining are preferred. In this sense robustness can be preferred.
In a sister effort led by the JPL Mobility and Robotics Section, the system level autonomy team is also prototyping use of the TRACE executive to provide intelligent execution capabilities for the prototype. Part of the prototyping effort is to compare and contrast the procedural executive TRACE against the mode declarative models-based scheduler and executive MEXEC.
The system level autonomy prototype is intended to demonstrate that autonomy can reliably cover a wide range of operating conditions while still achieving mission goals when possible. The prototype is also intended to show that autonomy would enable achieving a greater level of science in many operating ranges. Autonomy reduces the need for ground in the loop (GITL) cycles and preserves valuable mission time, and therefore energy, thus enabling more science.
The prototyping effort is also working with the operations concept team to understand how an autonomous lander could and would be given guidance from the ground operations team to enable the best possible mission.
Missions such as Mars 2020 take the first steps toward integrating autonomy and AI onboard spacecraft in order to optimize resource utilization. Europa Lander is a natural extension to this work, intended to push the boundaries on onboard AI and maximize the science potential of exploratory missions.
The prototyping effort began in Summer 2019. Current plans are for a field deployment in early 2022 in the Arroyo Seco, with potential follow up deployment in Summer 2022 at the Matanuska Glacier in Alaska.
TRACE PublicationsJean-Pierre de la Croix and Grace Lim, “Event-Driven Modeling and Execution of Robotic Activities and Contingencies in the Europa Lander Mission Concept Using BPMN,” International Symposium on Artificial Intelligence, Robotics, and Automation for Space, October 2020.
Jean-Pierre de la Croix and Grace Lim, “Runtime Updates to BPMN Mission Models in the Europa Lander Mission Concept using TRACE,” International Workshop On Planning and Scheduling for Space, July 2021.
Jean-Pierre de la Croix and Grace Lim, “Runtime Updates to BPMN Mission Models in the Europa Lander Mission Concept using TRACE,” International Planetary Probe Workshop, July 2021.