Landing on planetary bodies remains one of the most demanding phases of deep-space missions, particularly when attempting soft touchdown on environments that are either poorly characterized or impose stringent planetary-protection constraints. Enceladus and Mars exemplify these challenges. Enceladus, a small icy moon with active plumes emerging from its south-polar fractures, offers a unique opportunity to investigate subsurface ocean habitability and potential biosignatures. Mars, in contrast, presents a dynamic atmosphere, high-velocity atmospheric entry, and a long heritage of missions that demonstrate both the promise and difficulty of precision landing. Both targets are of high scientific priority for NASA and ESA, with Enceladus identified as a prime candidate for future flagship missions. Despite this strong scientific motivation, the landing technologies currently baselined for these missions often rely on complex and costly terrain-relative navigation (TRN) architectures, which may be unnecessary when medium landing accuracy, on the order of 1–5 km, is sufficient to achieve mission objectives.
We propose an alternative landing paradigm, that is currently being developed under contract with the European Space Agency, that enables medium-accuracy touchdown using a simple, robust, and cost-effective autonomous navigation architecture. The concept eliminates terrain-relative navigation entirely and instead exploits a hybrid solution combining inertial measurement unit (IMU) data with radio-frequency observables already available in the spacecraft’s radiometric tracking subsystem. The approach capitalizes on the high-precision orbit determination of a carrier spacecraft (“mothership”) and the availability of satellite-to-satellite link measurements during descent. These data provide relative positioning and allow the lander to maintain a dynamically consistent estimate of its descent trajectory without relying on optical sensors or lidar, thus significantly reducing system complexity, mass, and power.
The project evaluates this navigation concept through a representative end-to-end analysis framework. The study encompasses definition of representative mission scenarios for Enceladus and Mars; derivation of navigation performance requirements; orbit determination analysis for the mothership and secondary spacecraft; descent trajectory design and optimization; and development of a high-fidelity dynamical simulation environment. The resulting environment supports detailed modelling of radiometric observables, IMU measurements, plume and atmospheric perturbations. Navigation filters and data-fusion architectures are then selected and optimized through trade-off analysis. Performance is initially validated via orbit determination simulations, performed with GODOT, followed by deployment of algorithms onto representative OBC and subsequent Processor-in-the-Loop tests to raise the solution to TRL-4.
The paper presents initial performance results showing that a simplified autonomous navigation system can support medium-accuracy landing on both icy moons and atmospherically complex terrestrial planets. Leveraging existing spacecraft subsystems and avoiding TRN reduces system complexity while preserving robustness and enabling more cost-effective planetary landing architectures.