As people on Earth prepare to mark the passage of another year, a sun-studying spacecraft has quietly reached a big milestone of its own: discovering its 2,000th comet.
The Solar and Heliospheric Observatory, a probe operated jointly by NASA and the European Space Agency, detected comet No. 2,000 on Dec. 26. SOHO, which draws on help from citizen scientists around the world, is the single greatest comet-finder of all time, researchers said.
This is an impressive show of versatility, since SOHO was designed to monitor the sun, not look for comets.
"Since it launched on December 2, 1995, to observe the sun, SOHO has more than doubled the number of comets for which orbits have been determined over the last 300 years," Joe Gurman, the U.S. project scientist for SOHO at NASA's Goddard Space Flight Center in Greenbelt, Md., said in a statement.
Amateur astronomers helping out
SOHO itself does not really discover comets. Scores of amateur astronomer volunteers make the finds, after poring over the pictures produced by SOHO's cameras.
More than 70 people from 18 different countries have helped spot comets over the last 15 years by searching through the publicly available SOHO images online, NASA officials said.
Comets No. 1,999 and 2,000 were both discovered on Dec. 26 by Michal Kusiak, an astronomy student at Jagiellonian University in Krakow, Poland. Kusiak found his first SOHO comet in November 2007 and has since found more than 100, according to NASA officials.
"There are a lot of people who do it," said Karl Battams, who runs the SOHO comet-sighting website for the Naval Research Lab in Washington, D.C., and also does computer processing for the spacecraft's cameras. "They do it for free, they're extremely thorough, and if it wasn't for these people, most of this stuff would never see the light of day."
Battams receives reports from people who think that a fuzzy blip in SOHO's images might be a comet. He confirms the finds, gives each comet an unofficial number and then sends the information off to the Minor Planet Center in Cambridge, Mass., which categorizes small astronomical bodies and their orbits.
It took SOHO 10 years to spot its first 1,000 comets, researchers said, but only five more years to find the next thousand. That's due partly to increased participation from comet hunters and work done to optimize the images for comet-sighting. [The Best Comet Photos of All Time]
But the acceleration in comet finds may also be due partly to an unexplained systematic increase in the number of comets circling the sun. Indeed, December alone has seen an unprecedented 37 new comets spotted so far, a number high enough to qualify as a "comet storm," NASA officials said.
A sun-studying spacecraft
SOHO's cameras were not designed primarily to spot comets. The craft's cameras block out the brightest part of the sun to better watch emissions in the sun's much fainter outer atmosphere, or corona.
SOHO's comet-finding skills are a natural side effect; with the sun blocked, it's also much easier to see dimmer objects such as comets, researchers said.
"But there is definitely a lot of science that comes with these comets," Battams said. "First, now we know there are far more comets in the inner solar system than we were previously aware of, and that can tell us a lot about where such things come from and how they're formed originally and break up. We can tell that a lot of these comets all have a common origin."
About 85 percent of the comets discovered by SOHO are thought to come from a single group known as the Kreutz family, Battams added. The Kreutz family is believed to be the remnants of a single large comet that broke up several hundred years ago.
The Kreutz family comets are "sungrazers" — bodies whose orbits take them so close to the sun that most are vaporized within hours of discovery. But many of the other comets discovered by SOHO boomerang around the sun and return periodically, researchers said. One frequent visitor is comet 96P Machholz, which orbits the sun every six years and has now been seen by SOHO three times.