In August 2002, ESA's SOHO spacecraft spotted its 500th comet (SOHO-500) as the comet passed close to the Sun. It seems strange that SOHO, designed to examine the Sun, should turn out to be the most productive comet finder in the history of astronomy! Bernhard Fleck talks about why working with SOHO excites him.
Dr Bernhard Fleck SOHO Project Scientist
Born: Germany PhD Physics, University of Würzburg
In 1993 Dr Fleck 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.
ESA: Congratulations on the comets, but what is SOHO supposed to be doing?
SOHO, or the Solar and Heliospheric Observatory, watches 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. I am responsible for the scientific aspect of the SOHO project and for optimising the scientific return. This means being in charge of the team of operations staff and scientists working here at the centre on this project.
For me this is very enjoyable and great fun. SOHO is the most exciting solar space mission ever launched and it's great to be a part of it. I work with a truly international team who come here from all over the world in search of the data they need for their research. To me this is the Mecca of solar physics, and it is a privilege to be at the centre of it all.
ESA: Do these comets ever distract you from those vital tasks?
Not at all. They turn up by chance in pictures we gather for 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.
ESA: 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.
ESA: 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.
ESA: Who are these people?
They live all over the world. The biggest tallies have come from Mike Oates in England, Rainer Kracht in Gemany who found SOHO-500, and Xavier Leprette in France. Mike 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.
ESA: 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.
ESA: 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.
ESA: 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.
ESA: Is SOHO-500 part of that debris?
No 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. But the story isn't over yet, as nearly every day there is something exciting happening and thanks to SOHO, so many new discoveries have been made about the Sun and its atmosphere. In science, you never know what you'll discover tomorrow. That's why we enjoy it!