Coronagraph images at the Soho web site are updated every 30 minutes or so.
Michael Oates, a British observer, spotted the comet in an online image taken by the satellite-based Solar and Heliospheric Observatory (Soho). It is the 200th comet to be found using Soho.
Mr Oates is one of a growing number of part-time astronomers who use computer links to make observations that would not be possible using amateur equipment.
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The spacecraft's primary mission is to investigate the Sun's interior and atmosphere, and the solar wind.
Space weather forecasters rely on the satellite for advance warning of solar eruptions and geomagnetic storms. The comets are a bonus.
"No one expected to find all these comets when we launched Soho nearly 5 years ago," said Doug Biesecker, a solar physicist at the Goddard Space Flight Centre.
Only comets that pass perilously close to the Sun catch Soho's attention.
The vast majority, like Soho-200, don't survive the encounter. They swoop so close to the Sun that their icy cores vaporise completely.
Most of Soho's 200 comets no longer exist - they disintegrated hours after they were discovered.
Dodging the glare
The key to spotting comets so close to the Sun is Soho's brace of coronagraphs.
A coronagraph is a device that blocks out the Sun's blinding glare so that the faint corona is visible.
Coronagraph images at the Soho web site are updated every 30 minutes or so.
About once each week the photos include a faint comet that anyone can discover if they happen to be the first to look.
Soho's impressive spate of comet finds can be attributed in large part to the efforts of amateur astronomers
Soho data is freely available to anyone with an internet connection and a computer. Both real-time and archival images are accessible at the Soho web site, a popular destination for comet hunters.
"Amateurs have even taken the lead on real-time discoveries," said Dr Biesecker.
"If a comet zooms through the chronograph's field of view at 2 a.m. here at Goddard, someone in Europe is probably looking at the web site while we're asleep!"
By convention, comets discovered in Soho data are named after the spacecraft rather than the astronomer.
But for amateur comet hunters, being the first to spot a comet streaking past the Sun can be a real thrill.