THE mystery of Prometheus and Pandora has been solved. These tiny moons of Saturn refuse to stick to their calculated orbits. This had led astronomers to suggest that gravitational tugs from other satellites or from chunks in one of Saturn's nearby rings were responsible (New Scientist, 6 April, p 38).

But Peter Goldreich from the California Institute of Technology and Nicole Rappaport from NASA's Jet Propulsion Laboratory now say the misbehaviour is due to chaotic interactions between the two moons themselves. From detailed computer models they predict that the strongest disturbances should occur every 6.2 years when the moons' slightly elliptical orbits are closest together (www.arxiv.org/abs/astro-ph/0205330).

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