SOHO wasn't meant to search for comets. That's clear even in its name, which stands for the Solar and Heliospheric Observatory; it was designed to study the heliosphere, or the sun's extended atmosphere.
But in nearly 20 years of gazing at the sun, the joint NASA and European Space Agency mission has served as an accidental spy for comets on a death plunge -- Comet Lovejoy (in 2011), Comet ISON (in 2013) and nearly 3,000 others.
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The unofficial count as of Monday morning (Aug. 17) is in the low 2,990s, Karl Battams told Discovery News. Battams is the sungrazer project co-ordinator at the Naval Research Laboratory and works on many missions, including SOHO. He estimates that the 3,000th comet will be found in the next couple of weeks.
"The overwhelming majority (of comets) have been found by amateur astronomers in their spare time; it is a volunteering effort on their part," Battams said, adding that five or six people have together discovered roughly one-third of SOHO's comets. These volunteers scour SOHO's site for new pictures. The rules are that all comets are named after SOHO, but Battams and others acknowledge the volunteer efforts in publications.
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What makes SOHO unique among sun-gazing spacecraft is its coronograph (an instrument including an occulter that blocks out the sun's glare) is directly in the line between the Earth and the sun, allowing for a perfect view of sungrazing comets.
Most of them hail from a single group known as Kreutz comets, which are likely fragments of a huge comet that broke up in 1106. Kreutz comets have an extremely short perihelion (distance to the sun); due to their tiny size and closeness to the sun, only one comet has survived a scorching close-pass in SOHO's observations -- and that 2011 comet, Lovejoy, broke up within two weeks.
SOHO remains in excellent operational shape and has enough fuel on board to last for decades, Battams added. The 2016 NASA budget request asked for $2.2 million in funding annually for SOHO through 2020, out of a total budget request of $18.6 billion in 2016.
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Partner European Space Agency is prepared to stay on board for at least a few more years, Battams said. But a successor comet-hunting mission isn't on the books, at least yet.
"It is unique and extremely valuable," Battams said. "It's operating now at really low cost; they've managed to scrape by not on pennies, but figuratively speaking they have done it. My own speculation is they will continue funding it for as long as its operational, or until it gets replaced by something else."