Mercury could have once orbited the Sun in a synchronous rotation, reports a study published online this week in Nature Geoscience. The work suggests that collision with a large asteroid could have knocked Mercury out of this stable set-up, causing the planet to slow into its present unusual rotation. Mark Wieczorek and colleagues examined different scenarios for Mercury’s rotational evolution using a numerical model. They show that, with a 68% probability, if Mercury initially spun in the opposite direction to its orbit, the planet would have been captured into a synchronous rotation with one side experiencing perpetual sunshine. Their calculations show that a large asteroid impact could have disrupted this pattern, causing Mercury to eventually evolve into its current state of rotating one and a half times for each orbit of the Sun.