A vast cosmic collision that left a dark scar the size of the Earth on the surface of Jupiter has been discovered by an amateur astronomer using a home-made telescope.

Anthony Wesley spotted the extraordinary impact on Sunday night while watching Jupiter from the backyard of his rural home in Murrumbateman, near Canberra, Australia. He nearly missed it because he was also watching the final rounds of the Open Championship on television.

Mr Wesley, 44, an IT consultant who designed his telescope himself, told The Times how he saw a strange black blob creep across the planet’s surface.

“About 11pm I went inside to have a break and watch the golf, and by the time I came back out at about 1am the impact point had rotated around into view,” he said. “I couldn’t believe it. I thought, ‘That wasn’t there before’, and then I realised Jupiter had actually been hit by something.”

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Mr Wesley immediately set about alerting professional astronomers to his discovery, some of whom trained more powerful telescopes on Jupiter after seeing his e-mailed images.

Scientists at Nasa confirmed that his observations were of an impact rather than a storm. It is thought to have been caused by a small comet or cometary fragment, about 1km in diameter, which would have struck the planet at a speed of about 60km per second (about 135,000mph).

It is only the second impact of its kind to be documented by astronomers and comes exactly 15 years after Jupiter was bombarded by fragments of the comet Shoemaker-Levy 9 between July 16 and July 22, 1994. It also coincided with the 40th anniversary of the Apollo 11 Moon landing on Monday.

The scar that appeared on Sunday near the planet’s south pole is similar in size to those left by the larger pieces of Shoemaker-Levy, which reached up to 2km in diameter. The precise size and nature of the object that struck this time remains to be determined.

Glenn Orton and Leigh Fletcher, of Nasa’s Jet Propulsion Laboratory in Pasadena, California, had booked time on the Infrared Telescope Facility on Mauna Kea, Hawaii, when Mr Wesley’s tip came through and immediately began to watch Jupiter. They detected bright particles welling up in the blot – recognisable signs of an impact.

Dr Orton said: “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. It could be the impact of a comet, but we don’t know for sure yet.”

Dr Fletcher said: “These are the most exciting observations I’ve seen in my five years of observing the outer planets.”

The shape of the spot, and that Mr Wesley had not seen it earlier, suggests that the impact happened a few hours before he observed it. The light from Jupiter takes 43 minutes to reach Earth.

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