- guardian.co.uk, Tuesday 21 July 2009 10.27 BST
An amateur Australian astronomer looking through his backyard telescope has discovered that a large comet or asteroid has crashed into Jupiter, creating a hole the size of the Earth in the planet's atmosphere.
Anthony Wesley, 44, a computer programmer who lives in a small town outside the capital, Canberra, discovered a large scar on Jupiter when he was photographing the giant gaseous planet.
He tipped off Nasa about his discovery, and images taken by the US space agency's infrared telescope in Hawaii show a scar in the atmosphere near the south pole of the planet.
In a remarkable twist of fate, the discovery was made on the 40th anniversary of the Apollo 11 moon landing and the 15th anniversary of another large comet strike on Jupiter.
Wesley, who spends about 20 hours a week on his passion of watching and photographing Jupiter, spotted the strike using the telescope at the bottom of his garden in Murrumbatema at about 1am yesterday (4pm BST on Sunday).
But he almost missed making the discovery because he was watching the Open golf championship and the second Ashes test match. "I'm a keen golf watcher and unfortunately we were being flogged in the cricket," he told the Guardian. "I was imaging Jupiter until about midnight and seriously thought about packing up and going back to the house to watch the golf and the cricket. In the end I decided to just take a break and I went back to the house to watch Tom Watson almost make history.
"I came back down half an hour later and I could see this black mark had turned into view."
He recorded the moment in his observation log: "I noticed a dark spot rotating into view in Jupiter's south polar region [and] started to get curious," he said. "My next thought was that it must be either a dark moon ... or a moon shadow, but it was in the wrong place and the wrong size.
"By two o'clock I'd come back up to the house and was sending alerts to all the people I could think of that should be looking at this and especially the professional astronomers with specialised instruments for measuring this," he said.
Wesley emailed scientists at Nasa's jet propulsion laboratory in Pasadena, California. Using Nasa's infrared telescope facility at the summit of Mauna Kea, Hawaii, they gathered evidence indicating an impact.
"We were extremely lucky to be seeing Jupiter at exactly the right time, the right hour, the right side of Jupiter to witness the event. We couldn't have planned it better," said Glenn Orton, a scientist at the Pasadena lab.
"It could be the impact of a comet, but we don't know for sure yet," he said. "It's been a whirlwind of a day, and this on the anniversary of the Shoemaker-Levy 9 and Apollo anniversaries is amazing."
Wesley has been an ardent star-gazer since the age of 10 when he was given a small telescope. But over the past five years he has been in Jupiter's thrall. "It's one of my passions. It's such a dynamic planet, it's changing all the time. To take a photograph of this type, really it's a dream come true for me."
Leigh Fletcher, another Nasa scientist, told the New Scientist: "The impact scar we're seeing is about the same size as one of Jupiter's big storms ... That, I believe, is about the size of the Earth."