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SOHO Celebrates 1,500th Comet Discovery
06.27.08
 
It’s the most successful comet catcher in history. The Solar and Heliospheric Observatory (SOHO) spacecraft has just reached a new milestone. It has discovered its 1,500th comet, making it more successful than all the other discoverers of comets throughout history put together. Not bad for a spacecraft that was designed as a solar physics mission.





SOHO's 1500th comet The Kreutz-Group comet SOHO-1500 was spotted on June 25th 2008 in images taken by the LASCO C2 coronagraph. Most SOHO comets are small and faint like this one. Credit: The European Space Agency and NASA.
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> ESA page with animation of 1500th comet
SOHO’s history -making discovery was made late on June 25, 2008.





When it comes to comet catching, SOHO has one big advantage over everybody else: its location. Situated between the sun and Earth, it has a privileged view of a region of space that can rarely be seen from Earth. From the surface of the planet, the space inside our orbit is largely obscured because of the daytime sky, and so we only clearly see close to the sun during an eclipse.





Roughly eighty-five percent of the SOHO discoveries are fragments from a once great comet that split apart in a death plunge around the sun, probably many centuries ago. The fragments are known as the Kreutz group and now pass within 1.5 million kilometers (about 930,000 miles) of the sun’s surface when they return from deep space.





Comet Bradfield seen by SOHOComet Bradfield passed through the SOHO C3 coronagraph's field of view from April 16 through April 20, 2004. It created quite a sensation with its bright head and long white tail trailing behind it. A mask in the coronagraph (dark disk in center) blocks out the sun's intense light to allow us to see faint objects in the corona, the sun's tenuous outer atmosphere. The white circle on the coronagraph disk represents the size of the sun. Credit: The European Space Agency and NASA.
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At this proximity, which is a near-miss in celestial terms, most of the fragments are finally destroyed, evaporated by the sun’s fearsome radiation – all within the sight of SOHO’s electronic eyes. One of twelve instruments, the Large Angle and Spectrometric Coronagraph (LASCO) takes the images.





Of course, LASCO itself does not make the detections; that task falls to an open group of highly skilled volunteers who scan the data as soon as it is downloaded to Earth. When SOHO is transmitting to Earth, the data can be on the Internet and ready for analysis just 15 minutes after it is taken.





Enthusiasts from all over the world look at each individual image for a tiny moving speck that could be a comet. When someone believes they have found one, they submit their results to Karl Battams, Naval Research Laboratory, Washington DC, who checks all of SOHO’s findings before submitting them to the Minor Planet Center, where the comet is catalogued and has its orbit calculated.





The wealth of comet information has value beyond mere classification. "This is allowing us to see how comets die," says Battams. When a comet constantly circles the sun, it loses a little more ice every time, until it eventually falls to pieces, leaving a long trail of fragments. Thanks to SOHO, astronomers now have many images showing this process. "It is a unique data set and could not have been achieved in any other way," says Battams.





Two sungrazing comets seen by SOHOTwo "sungrazing" comets are seen heading in tandem towards the sun’s corona. They do not reappear on the other side. The comets follow similar but not identical orbits and enter the corona – the tenuous outer atmosphere of the sun. Comets, composed of ice and dust, characteristically have particles streaming out behind them. In the image, the shaded disk in the center is a mask in the coronagraph instrument that blots out direct sunlight. The white circle added within the disk shows the size and position of the visible sun. Credit: The European Space Agency and NASA.
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All this on top of the extraordinary revelations that the solar physics mission has provided over the thirteen years it has been in space, observing the sun and the near-sun environment. "Catching the enormous total of comets has been an unplanned bonus," says Bernhard Fleck, European Space Agency SOHO Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.





Anyone with spare time can help to search for SOHO’s comets by visiting:

http://ares.nrl.navy.mil/sungrazer/

















Comet seen by SOHO on December 23, 1996The LASCO C2 frame was selected to show Comet SOHO-6 as its head enters the equatorial solar wind region on 23 December 1996. The comet was one of numerous sungrazing comets discovered using SOHO's Large Angle and Spectrometric Coronagraph (LASCO) instrument. The comet eventually plunged into the sun. In the image, the shaded disk is a mask in the LASCO instrument that blots out direct sunlight. The white circle added within the disk shows the size and position of the visible sun. Credit: The European Space Agency and NASA.
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The Minor Planet Center operates under the auspices of the International Astronomical Union, and is located in Cambridge, Massachusetts. SOHO is a cooperative project between the European Space Agency and NASA.

 
 
Stuart Clark
European Space Agency