Flyby anomaly

Partly settled. Some of it is understood, some is still argued about.

Between the early 1990s and 2000s, several spacecraft performing gravity-assist flybys of Earth showed small but statistically significant unexplained velocity changes, and no single conventional effect (atmospheric drag, solar radiation pressure, tidal forces) fully accounted for the pattern across missions, fueling proposals ranging from modified inertia theories to relativistic frame-dragging effects. However, more recent, more precisely modeled flybys — Juno's 2013 Earth flyby and BepiColombo's 2020 Earth flyby — showed no significant anomaly within measurement uncertainty, suggesting the earlier anomalies may have stemmed from incomplete spacecraft modeling (e.g. thermal or attitude-control effects) rather than new physics, though a fully agreed-upon accounting for every historical case is still lacking.

For a couple of decades, some of humanity's most precisely tracked machines seemed to pick up an inexplicable extra nudge from Earth's gravity every time they swung past it, and the mystery quietly faded as our tracking got better.

Explanations that circulated

• That the flyby anomaly reflects a real breakdown of general relativity or evidence of a dark matter halo around Earth — later, more precisely tracked flybys (e.g. Juno 2013, BepiColombo 2020) showed no significant anomalous velocity change, undermining exotic-physics explanations

References

• The flyby anomaly: A multivariate analysis approach, Various authors, arXiv preprint, 2017