NASA credited Anthony Wesley of Murrumbateman, Australia, a computer programmer with a good reputation among professional astronomers.
"We owe a huge debt to him for picking up on these things," said planetary scientist Leigh Fletcher at NASA's Jet Propulsion Laboratory.
The discovery began as a routine night-gazing exercise. Wesley pointed his new 14.5-inch backyard telescope to the cosmos on Sunday and started snapping images.
Conditions weren't ideal and when the sky got murkier, Wesley almost wanted to quit.
Instead, he took a half-hour break. When he peered back into the telescope, he noticed a curious black spot on Jupiter.
Was it a moon or a shadow? Can't be, he thought.
Wesley checked images that he took just two days earlier and found no sign of the spot.
Was Jupiter struck by something?
"I had no real idea, and the odds on that happening were so small as to be laughable, but I was really struggling to see any other possibility given the location of the mark," he wrote in an online posting. "If it really was an impact mark, then I had to start telling people, and quickly."
Wesley fired off an e-mail to a group of amateur and professional astronomers.
Among those who received the tip were NASA scientists, who were preparing to look at Jupiter using their infrared telescope on Mauna Kea, Hawaii.
"The clue from him was we had an unusual and very black-looking scar or blemish," said JPL scientist Glenn Orton.
Images released by NASA show the impact occurred near the south pole that caused debris to fly into the upper atmosphere. The scar is pale-looking from the reflection of debris.
Since scientists did not see the actual collision, they do not know the size and mass of the object that hit. However, based on the size of the gash, whatever slammed into Jupiter was "much, much smaller" than Earth, said Fletcher of the Jet Propulsion Lab.
The impact scar was only the second time astronomers have observed the aftermath of an object hitting Jupiter. Fifteen years ago, the planet was bombarded by pieces of the comet Shoemaker-Levy 9.
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