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janrinok writes:

The Solar Heliospheric Observatory spacecraft known as SOHO is set to cross the 3,000 comet discovery threshold this month [phys.org]. Launched atop an Atlas II rocket on December 2nd, 1995, SOHO is a joint NASA/ESA mission, and has observed the sun now for almost 20 years from the sunward L1 lagrange point. That fact is amazing enough, as SOHO has already followed the goings on of our tempestuous host star for nearly two full solar cycles.

And though SOHO wasn't initially designed as a comet hunter extraordinaire, it has gone on to discover far more comets than anyone—human or robotic.

The U.S. Naval Research Laboratory's (NRL) sungrazer website lists the discovery count as 2,987 as of July 31, 2015, with more comets awaiting verification daily. "In the past, SOHO has often discovered as many as four or five comets in a single day," Karl Battams, a solar scientist at the NRL told Universe Today. "Suffice to say, it really could be any day now, given how close we are to 3,000! I actually expected it to be a month ago, so I'm surprised it's dragging out like this. Predicting comets is fraught with uncertainty!"

Part of what gives SOHO an edge is its LASCO (the Large Angle and Spectrometric Coronagraph) C2 and C3 coronagraphs. With a field of view about 15 degrees wide, the C3 imager monitors the faint corona of the sun, while blocking its dazzling disk. The corona is the pearly white outer atmosphere of the sun, and is about half as bright as a Full Moon. On Earth, we only see the corona briefly during a total solar eclipse. SOHO routinely sees sungrazing comets 'photobomb' the view of its LASCO C3 camera, sometimes to the tune of more than 200 a year.

SOHO has rewritten the history of sungrazers. How far we've come: flashback to 1979, and less than a dozen sungrazers were known, one being the famous Comet Ikeya-Seki in 1965. Early space-based platforms such as Solwind and SMM sported early coronagraphs, and paved the way for SOHO. Think about that for a moment; a vast majority of the cometary population of the solar system was simply sliding by, unobserved from the ground. And this was only a generation ago.

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