Background: During the navigation of Rosetta’s first Earth swing-by in 2005, a small yet unexplained asymptotic velocity increase was detected, necessitating an artificial prograde delta-V at perigee.[1] Similar instances of this phenomenon have been seen by various missions[2]; however, subsequent Rosetta Earth flybys did not exhibit this effect. Despite numerous investigations of the – so called Earth fly-by anomaly – over the last decades, the underlying cause remains unresolved.

Idea: To address the problem, we perform an Orbit Determination using ESOC’s new system for Deep Space navigation[3], utilizing the original radiometric tracking data and spacecraft model. This approach enables the incorporation of physical effects on the actual flight trajectory in an effort to identify the root cause of the anomaly.

Purpose: Achieving high-accuracy navigation during gravity-assist manoeuvres, while refining physical models and addressing unresolved dynamical effects, integrates operational and scientific objectives, that are of significant relevance for current and future missions. 

Results: We successfully reproduced the anomaly using the new navigation software. Subsequent analysis ruled out explanations involving ephemeris errors, gravitational field modelling, relativistic contributions, and propagator configurations. Additional dynamical variations, such as the solar radiation pressure or albedo radiation, were evaluated but found insufficient to account for the observed discrepancy.

Outlook: During investigations of Earth’s albedo effect, a possible explanation for an artificial velocity increase was identified. Albedo-induced illumination on thermally sensitive spacecraft surfaces could have triggered outgassing, consistent in direction and magnitude with documented outgassing events during Rosetta’s cruise phase. To facilitate independent analysis, we plan to provide a standalone data package of the Rosetta Flyby to the interested community.

[1] Morley, T. and Budnik, F., “Rosetta Navigation at its first Earth Swing-by”, ISTS 2006-d-52, 2006.
[2] Antreasian, P. G. and Guinn, J. R., “Investigation into the unexpected Delta-V increases during the Earth Gravity Assist of Galileo and NEAR”, AIAA 98-4287, 1998.
[3] Castellini, F. et al., “ESOC’s new Orbit Determination System for Deep Space Mission Operations”, ISSFD 2022.