An international observation campaign comprising amateur and professional astronomers alike, and even the famous Hubble Space Telescope, now wants to crack the mystery.
Comet 29P is a mind-boggling object. More than 37 miles (60 kilometers) across, it is one of the largest comets known — about as big as the famous Hale-Bopp that streaked the sky in the 1990s.
It is one of only a handful of comets known as Centaurs, which orbit the sun between Saturn and Jupiter, slowly making their way through what astronomers call a gateway, waiting to eventually be flung by Jupiter's gravity closer to the sun (or out of the solar system entirely).
And 29P brightens up periodically with powerful eruptions that make it the second most active body in the solar system after Jupiter's moon Io.
But why is it erupting?