On Monday 12 August 2002, about 16.05 UT, ESA’s SOHO spacecraft spotted its 500th comet as the comet passed close to the Sun.
It seems a little strange that SOHO, designed to examine the Sun, should turn out to be the most productive cometfinder in the history of astronomy, and by a very wide margin. We interviewed ESA’s project scientist for SOHO, Bernhard Fleck, about that.
“Congratulations on the comets – but what’s SOHO’s job supposed to be?”
“To watch the solar weather, 24 hours a day. To find out how the Sun works, all the way from its hot core, through its stormy surface, to the solar wind that buffets the Earth.”
“Don’t all these comets ever distract you from those vital tasks?”
“Not at all. They turn up by chance in pictures we gather for quite different reasons. An instrument called LASCO (Large Angle and Spectrometric Coronagraph) routinely monitors a huge region of space around the Sun, watching for its eruptions. Most of SOHO’s comets have simply flown unexpectedly into LASCO’s field of view.”
“But they must mean a lot of extra work for your team, surely?”
“Again, no. Just one team member, Doug Biesecker, runs our comet discovery programme at SOHO headquarters. But more than 75% of the discoveries have come from amateur comet hunters around the world. To me, that’s the most exciting aspect.”
“How do you involve the amateurs? ”
“It’s very simple. Since 1999, up-to-date SOHO-LASCO pictures have been available to anyone on the Internet. Take a look yourself. But it’s best if you know something about the subject, otherwise you may be misled by odd white flashes due to cosmic rays. And remember you’ll be competing with people who spend several hours a day searching the latest images.”
“Who are these people?”
“They live all over the world. Confirmed discoveries of comets have come from Australia, Canada, China, France, Germany, Italy, Lithuania, Russia, the United Kingdom, and the United States.”
“Have any individuals done especially well?”
“The biggest tallies have come from Mike Oates in England, Rainer Kracht in Gemany who found SOHO-500, and Xavier Leprette in France. They went back over pictures from 1996-99 and found dozens of comets that the professionals had overlooked. Mike Oates runs an electroplating business in Manchester, but thanks to SOHO and the Internet he is also the highest-scoring discoverer of comets ever, with 136 to his name.”
“Did you expect all these comets to show up, when you were planning SOHO?”
“Not at all. They were a wonderful surprise, of the kind we get in science sometimes. I remember the possibility being mentioned when we were preparing the spacecraft that we might discover one or two comets each year. We had two comet experts on the original science team, Philippe Lamy from Marseille and Jean-Loup Bertaux from Paris. Philippe was in charge of the design of the LASCO C2 coronagraph. Jean-Loup is responsible for the French-Finnish SWAN instrument on SOHO, which studies the solar wind by its effects on hydrogen atoms all across the Solar System. Jean-Loup expected SWAN to see dense clouds of hydrogen around big comets discovered by other people. Sure enough, it did and Comet Hale-Bopp in 1997 had the biggest cloud ever seen, 100 million kilometres wide. But now SWAN has also discovered a couple of comets for itself.”
“What was so surprising about the hundreds of SOHO-LASCO comets? Did you learn something new?”
“Yes, we found out that comets can be both extremely large and extremely small. Nearly all of the SOHO discoveries are what we call sungrazers. They hit the Sun’s atmosphere and disappear. They are quite small, typically only about 10 metres in diameter. And most of them come from the same direction in space, because they’re fragments of a really huge comet.”
“How do you know that?”
“Big fragments were seen by the ancient Greeks, more than 2000 years ago. The original comet must have been enormous to create so much debris – probably more than 100 kilometres in diameter and well visible even during daylight. And quite scary really, if you think what damage a comet like that could do if it ever hit the Earth.”
“Is SOHO-500 part of that debris?”
“As a matter of fact it’s not. Just this year, among the SOHO comets, we’ve discovered three smaller families of near-Sun comets, each with different orbits: the Meyer, Marsden, and Kracht groups. Kracht’s SOHO-500 belongs to Meyer’s group. The experts are busy trying to work out where these new families came from.”
“So the story isn’t over yet?”
“In science, you never know what you’ll discover tomorrow. That’s why we enjoy it!”
Note
Rainer Kracht of Elmshorn in Schleswig-Holstein, Germany, spotted a small object in an image from SOHO received via the Internet. It has been officially confirmed as Comet 2002 P3 (SOHO). It is the 500th comet discovered with the ESA-NASA solar spacecraft and it made its closest approach to the Sun at 16:05 UT on Monday, 12 August 2002. Diane McElhiney won a contest run by the SOHO science team for guessing that date and time for SOHO-500. Her prediction was too early by only 103 minutes.
SOHO Project Scientist Bernhard Fleck received his PhD in physics in 1991 from the University of W¸rzburg, Germany. In 1993 he joined ESA’s Space Science Department at ESTEC in Noordwijk, the Netherlands, to work on the SOHO project. With the launch of SOHO in December 1995 he moved to the SOHO operations centre at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. His main research interests include the dynamics of the solar atmosphere, in particular, wave propagation characteristics in the chromosphere.
