An amateur astronomer using a home-made backyard telescope has discovered an Earth-sized smudge in Jupiter’s surface.

Anthony Wesley of Murrumbateman, Australia was viewing Jupiter at around 1am local time yesterday when he spotted the damage. He told SPACE.com that he saw a dark spot rotating near the planet’s south pole; his records showed the same area two days earlier had been clear. He realized it was too solidly black to be a polar storm, and moving too slowly to be a moon. An image produced by Wesley (right) shows the black spot just below the top of the picture.

Wesley then e-mailed Nasa scientists in California who used a remotely controlled infrared telescope atop a mountain in Hawaii to explore the finding. They believe it is from the impact of a comet, but are not yet sure. Another theory is that it was caused by a falling block of ice.

The discovery came on both the 40th anniversary of the initial human moon landing, and the 15th anniversary of Jupiter being hit by a comet. On that occasion the impact had been widely anticipated and it was the first time such a collision had ever been seen as it occurred.

Nasa says it was only through fortunate timing that the damage became visible on earth as it occurred at both the right hour and on the right side of Jupiter for optimal viewing. That said, it was hardly a coincidence that Wesley should spot the scar: he spends up to 20 hours a week viewing Jupiter. ScienceNOW notes that he uses a 37 cm diameter telescope (pictured), whereas professional astronomer use at least a 1000 centimeter model.

Because Jupiter is a gaseous planet, the spot will likely soon be torn apart by winds, meaning scientists will have to be fast to catch it. That rush will spark emergency applications for access to some of the world’s leading telescopes, including the space-based Hubble.