Spacecraft are gaining unexpected speed during planetary flybys

Space · Published 18 October 2026 Swipe more like this →

In short: When spacecraft swing past planets like Earth, they often gain more kinetic energy than our best physics models predict. This mysterious nudge, first noticed during the 1990 Galileo mission, involves tiny velocity jumps that persist across multiple space probes, leaving researchers to wonder if they are witnessing a fundamental shift in physics.

First detected in 1990 during the Galileo mission to Jupiter, the flyby anomaly involves a discrepancy between predicted and observed velocity increases. While the largest recorded discrepancy is a tiny 13.46 mm/s (observed during the NEAR spacecraft's 1998 encounter), the fact that it persists across multiple missions—including Cassini-Huygens and Rosetta—has left scientists searching for a definitive cause. The initial detection occurred during a meticulous review of Deep Space Network Doppler data. This specific observation revealed a 66 mHz shift, which translated to a velocity jump of 3.92 mm/s at perigee. Despite extensive investigations by the Jet Propulsion Laboratory and the University of Texas, no satisfactory explanation has emerged. While the 1992 Galileo flyby showed expected drag, the error bars remained too large to rule out hidden forces.

The mystery is complicated by inconsistent results. For instance, the MESSENGER mission to Mercury showed no significant anomaly, possibly because its symmetrical approach and departure relative to Earth's equator suggest the effect might be linked to the planet's rotation. Conversely, the Juno flyby in 2013 yielded no anomalous acceleration, though researchers noted that using a low-precision gravity model could mimic an anomaly of 4.5 mm/s. The 2005 Rosetta flyby resulted in a gain of 1.82 mm/s, while the 1999 Cassini–Huygens encounter saw a much smaller increase of 0.11 mm/s. In 2018, the interstellar object 'Oumuamua also exhibited a small excess velocity as it moved away from the Sun. Scientists initially suspected outgassing for this object, but no such activity was detected. These varied results make it difficult to establish a universal rule for the phenomenon.

Proposed explanations range from the mundane to the revolutionary. Some suggest The Céspedes-Curé hypothesis suggests that moving through varying gravitational energy density fields might alter the refractive index of space. This would cause unaccounted corrections in Doppler data that assumes a constant speed of light. Other theories involve topological torsion currents, which might only affect spacecraft moving in a retrograde direction. Some researchers also point to range-proportional excess delays in telemetry signals as a potential culprit. These models attempt to reconcile the fact that the anomaly appears inconsistently across different missions.

hidden

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Spacecraft steal a little of a planet's motion to speed up
• How spacecraft use planetary gravity to slingshot through the deep solar system
• NASA’s Lucy spacecraft performs a daring, low-altitude swing dance around Earth
• Using a planet's atmosphere to brake into a stable orbit
• The grueling six-year odyssey to reach our solar system's smallest planet
• Why your weight changes while your mass remains constant
• The mathematical recipe that tells the universe how to curve
• Every journey through the Earth could take exactly forty-two minutes
• Could a 120-year-old experiment finally reveal the existence of extra spatial dimensions?
• Visualizing the invisible architecture of space-time
• Why is gravity so much weaker than every other force?
• Edmond Halley predicted his comet's return but died before it came back.

More in Gravity · More in Space · Latest · A–Z · All topics