In December 2010, as people on Earth celebrated the holidays and prepared to ring in the New Year, a European Space Agency (ESA)/NASA spacecraft 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 all the more impressive since SOHO was not 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 three hundred years,” says Joe Gurman, the U.S. project scientist for SOHO at 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 the dozens of amateur astronomer volunteers who daily pore over the images produced by SOHO’s LASCO (or Large Angle and Spectrometric Coronagraph) cameras. More than 70 people representing 18 different countries have helped spot comets over the last 15 years by searching through the publicly available SOHO/LASCO images online. The 1999th and 2000th comets were both discovered by an astronomy student at the Jagiellonian University in Krakow, Poland.
“There’s an ever-growing community of amateur astronomers who contribute to this project,” says Karl Battams, who has been in charge of running the SOHO comet-sighting web site since 2003 for the Naval Research Laboratory, where he does software development, data processing, and visualization work for NRL’s solar physics missions. “These volunteers are absolutely fundamental to the success of this program. Without them, most of this tremendously valuable astronomical data would never see the light of day.”
Battams receives reports from people when they find a feature in SOHO’s LASCO images that has the correct location, brightness, speed, morphology, and other characteristics to be a comet. He confirms the finding, gives each comet an unofficial number, and sends the information off to the Minor Planet Center in Cambridge, Massachusetts, which categorizes small astronomical bodies and their orbits. It took SOHO ten years to spot its first thousand comets, but only five more to find the next thousand. That is 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. December 2010 saw an unprecedented 40 new comets spotted. During one short period, the rate of discoveries was so great as to be labeled a “comet storm.”
“The storm began on December 13 and ended on the 22nd,” explains Battams. “During that time, SOHO detected 25 comets diving into the Sun. It was crazy!”
LASCO was not designed to spot comets. The LASCO camera blocks out the brightest part of the Sun in order to better watch emissions in the
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.
“There is a significant science return from these ‘sungrazer’ comets,” says Battams, “as they can be considered probes of solar wind conditions near the Sun. One of the main objectives of the SOHO/LASCO mission is to investiagate the solar wind and its acceleration processes.” (See page 14 of Spectra more on LASCO.)
“Also,” continues Battams, “now we know there are far more comets in the inner solar system than we were previously aware of, and this can tell us a lot about how they’re formed and how they break up. We can tell that many of these comets have a common ori- gin.” Indeed, a full 85% 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.
Sungrazers like the Kreutz family comets approach so near the Sun that most are vaporized within hours of discovery. But many of the other LASCO comets boomerang around the Sun and return periodically. One frequent visitor is comet 96P Machholz; orbiting the Sun approximately every six years, it has now been seen by SOHO three times.
SOHO is a cooperative project between ESA and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA. NRL’s Dr. Russell Howard was principal investigator for the LASCO instrument.
Learn how to find your own comet!