A spacecraft that launched a quarter century ago to study the sun has discovered its 4,000th new comet, continuing a spree of serendipitous science.
NASA and the European Space Agency (ESA) collaborated to launch the Solar and Heliospheric Observatory, or SOHO, spacecraft in 1995. Designed to last three years, SOHO was tailored to study the sun, in particular by imaging the sun's outer atmosphere, or corona, and the birth of the solar wind, a stream of charged particles that flows off the sun and across the solar system. But that work also gives SOHO a prime vantage point for spotting comets that dart perilously close to the sun, it turns out.
"Not only has SOHO rewritten the history books in terms of solar physics, but, unexpectedly, it's rewritten the books in terms of comets as well," Karl Battams, a space scientist at the U.S. Naval Research Lab in Washington, D.C., who works on SOHO and manages its comet-finding program, said in a statement.
Because SOHO's main aim is studying the sun, the comet-identification program is powered by amateur volunteers. Lucky comet number 4,000 was spotted by Trygve Prestgard, who has identified around 120 comets through the program, which also incorporates imagery from another NASA sun mission, Solar Terrestrial Relations Observatory or STEREO.
For SOHO, most of the discoveries come from data gathered by the spacecraft's Large Angle and Spectrometric Coronagraph (LASCO), which blocks out the brightest region of the sun to give scientists a better look at the faint corona.
And it's through the corona that sungrazing comets, also known as the Kreutz family of comets, pass, according to NASA. SOHO blocks out the sun well enough that its instruments can spot even small, faint comets, like the one currently called SOHO-4000, which scientists estimate is just 15 to 30 feet (5 to 10 meters) across.
Although scientists working on the project knew that SOHO was racking up its cometary discoveries, they expected it would be a bit longer before the spacecraft's data led to the 4,000th such identification.
But the spacecraft completed a special observing run in early June to coincide with the fifth sun flyby of NASA's Parker Solar Probe in order to double up on data of the same solar structures. The campaign also involved doubling SOHO's usual exposure time, allowing the spacecraft to spot smaller, fainter objects like SOHO-4000.
"I feel very fortunate to have found SOHO's 4,000th comet. Although I knew that SOHO was nearing its 4,000th comet discovery, I did not initially think that this sungrazer would be it," Prestgard said. "It was only after discussing with other SOHO comet hunters, and counting through the most recent sungrazer discoveries, that the idea sunk in. I am honored to be part of such an amazing collaborative effort."
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