Space telescope SOHO—short for Solar and Heliospheric Observatory—has just discovered its 3,000th comet, nearly 20 years after it was launched by NASA and the European Space Agency to observe the Sun. 

Its amazing haul of so-called sungrazers—comets whose orbits bring them perilously close to our home star—was not part of the mission when the observatory was sent into space back in December, 1995.

But amateur astronomers were quick to appreciate that its cameras provide a continuous window on what is going on in the Sun’s surroundings as well as on the star itself. Background stars and planets in conjunction could be seen, plus unexpected icy visitors from the depths of the Solar System.

Some comets have made spectacular swings around the Sun that have been visible to SOHO’s cameras. In December 2011, Comet Lovejoy (C/2011 W3) was not expected to survive its close encounter but did, becoming a bright Christmas comet visible from the southern hemisphere and photographed from the International Space Station. In this case it had already been discovered by Australian amateur Terry Lovejoy the previous month as he made a photographic survey of the sky.

And who can forget Comet ISON, which promised to become one of the most spectacular comets ever seen when it was picked up deep in the Solar System from Russia in September 2012. Instead, ISON was seen to disintegrate as it rounded the Sun, becoming just a cloud of dusty debris.

NASA quickly backed the comet-hunting project when it became clear that amateurs were finding the cosmic wanderers in the data. They encouraged an army of amateurs to sift through SOHO’s images in an agency-funded Sungrazer Project. We reported in 2010 how a student at Nicolas Copernicus’s old Jagiellonian University in Krakow, Poland, spotted the 2,000th comet.

Now enthuiast Worachate Boonplod, of Samut Songkhram, Thailand, has identified the 3,000th sungrazing comet in the SOHO imagery. He said in a statement: “I am very happy to be part of a great milestone for SOHO’s comet project. I would like to thank SOHO, ESA and NASA for making this opportunity possible, including other fellow comet hunters who I have learned a lot from.”

An observatory launched into space 15 years ago to observe the Sun has this week discovered its 2,000th comet. It was spotted on an image taken by the satellite SOHO on December 26 by Michal Kusiak, a student at Nicolas Copernicus’s old Jagiellonian University at Krakow

SOHO watches the Sun’s disk itself plus its surrounding environment, tracking the flow outwards of charged particles known as the solar wind, as well as giant eruptions called coronal mass ejections, or CMEs, that produce space weather. Its LASCO (Large Angle and Spectrometric Coronagraph) cameras are particularly suitable for finding new comets as they have a wide angle of view. They are designed to observe the Sun’s atmosphere and space weather, with a disk blotting out the intensely bright Sun itself.

Joe Gurman, mission scientist for SOHO at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, commented: “SOHO has a view of about 12.5 million miles (20 million km) beyond the Sun. So we expected it might from time to time see a bright comet near the Sun. But nobody dreamed we’d approach 200 a year.”

Solar scientist Karl Battams, who runs the SOHO comet-sighting website, said: “The people who have found comets represent a very broad cross section as the program is open to anyone who has interest. There are scientists, teachers, writers. We have even had two 13-year-olds.”

Scientists believe that 85 per cent of the comets found are from one monster comet that broke up many hundreds of years ago. They are known as Kreutz sungrazers.

A NASA video reports on the variety of comets discovered by SOHO. Credit: NASA’s Goddard Space Flight Center/Bridgman/Duberstein