Line-of-Sight Navigation Observability Analysis for Near-Earth Asteroids Exploration with CubeSat

Abstract

CubeSats represent a promising solution for low-cost deep-space exploration, especially for Near-Earth Asteroids (NEAs) missions. In this framework, autonomous navigation is a viable option to improve CubeSat capabilities for deep-space exploration. The state of a deep-space cruising spacecraft can be estimated with a Line-of-Sight (LoS) navigation approach, which is based on the observation of visible celestial bodies direction with a camera. It is attractive for small-spacecraft missions, as it does not require additional instrumentation, since cameras or star trackers are usually carried on-board. Feasibility of exploiting LoS navigation for NEAs exploration is investigated, by analysing the relative geometry between observable planets and NEAs at their ascending and descending nodal passages. In literature, a Figure-of-Merit (FoM) has been formulated to quantify analytically the navigation performance for simultaneous tracking of two bodies, and it is used here to investigate LoS navigation applicability to NEAs exploration scenarios. The FoM depends on the relative observation geometry. The lower its value, the higher the expected navigation accuracy. The NEAs dataset has been retrieved from NEODyS-2, a web-based database containing 26822 orbital parameters. NEAs ephemerides have been propagated for the period 2023-2033. The spacecraft has been assumed coincident with the target NEA at the nodes, as the distance asteroid-spacecraft during the encounter is in the order of few hundreds of kilometers. The observation of the six smaller semi-major axis planets (from Mercury to Saturn) is considered. The FoM has been calculated for each pair of planets at each nodal passage, assuming all the bodies whose line-of-sight direction has an angular separation of at least 30° from the Sun direction are visible (common Sun exclusion angle for star trackers). The result consists in a list of NEAs, whose geometry is appealing for LoS navigation, and that can be used for further analysis to design trajectories to reach these bodies with CubeSats. The FoM ranges from ∼ 10¹⁵ to ∼ 10²⁴. Analysing a total of 265067 node passages, in less than 1% the FoM could not be computed because either none or only one planet was visible, while in almost 13% of the cases all planets were observable. A direct link between FoM and expected state estimation error cannot be established, so two test cases are analysed, using CubeSat hardware characteristics. The first, with FoM ∼ 10¹⁵, is characterised by a final position estimation error around 100 km, while for the second, with FoM ∼ 10²², it is around 2000 km.