Drawing on help from citizen scientists around the world, SOHO has become the single greatest comet finder of all time.
As people on Earth celebrate the holidays and prepare to ring in the New Year, a European Space Agency (ESA)/NASA spacecraft has quietly reached its own milestone. On December 26, the Solar and Heliospheric Observatory (SOHO) discovered its 2000th comet.
Drawing on help from citizen scientists around the world, SOHO has become the single greatest comet finder of all time. This is very impressive since SOHO was not specifically designed to find comets, but to monitor the Sun.
“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,” said Joe Gurman from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Of course, it is not SOHO itself that discovers the comets -- that is the province of dozens of amateur astronomer volunteers who daily pore over the fuzzy lights dancing across the pictures produced by SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) cameras. Over 70 people representing 18 different countries have helped spot comets over the past 15 years by searching through the publicly available SOHO images online.
The 1999th and 2000th comet were both discovered December 26 by Michal Kusiak from Jagiellonian University in Krakow, Poland. Kusiak found his first SOHO comet in November 2007 and has since found more than 100.
“There are a lot of people who do it,” said Karl Battams who has been in charge of running the SOHO comet-sighting web site since 2003 for the Naval Research Lab in Washington, D.C., where he also does computer processing for LASCO. “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 one of the spots in SOHO’s LASCO images looks to be the correct size and brightness and headed for the Sun — characteristics typical of the comets SOHO finds. He confirms the finding, gives each comet an unofficial number, and then sends the information off to the Minor Planet Center in Cambridge, Massachusetts, which categorizes small astronomical bodies and their orbits.
It took SOHO 10 years to spot its first thousand comets, but only 5 more 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, but also due to an unexplained systematic increase in the number of comets around 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.”
LASCO was not designed primarily to spot comets. The LASCO camera blocks out the brightest part of the Sun in order to better watch emissions in the Sun’s much fainter outer atmosphere, or corona. LASCO’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.
“But there is definitely a lot of science that comes with these comets,” said Battams. “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.” Indeed, says Battams, 85 percent of the comets discovered with LASCO are thought to come from a single group known as the Kreutz family, believed to be the remnants of a single large comet that broke up several hundred years ago.