SOHO, History's Greatest Comet Hunter, Discovers 1,000th Comet

Press Release From: Goddard Space Flight Center
Posted: Thursday, August 18, 2005


Toni Scarmato, a high school teacher from Italy, discovered Solar and Heliospheric Observatory (SOHO) spacecraft's 999th and 1000th comet on August 5, when two comets appeared in the same SOHO image.

Scarmato, an astrophysics graduate of Bologna University, said "I am very happy for this special experience that is possible thanks to the SOHO satellite and NASA- European Space Agency (ESA) collaboration. I want to dedicate the SOHO 1000th comet to my wife Rosy and my son Kevin to compensate for the time that I have taken from them to search for SOHO comets."

The SOHO spacecraft is a joint effort between NASA and ESA. It has accounted for approximately one-half of all comet discoveries with computed orbits in the history of astronomy. Many SOHO comet discoveries have been made by amateurs using SOHO images on the internet. SOHO comet hunters come from all over the world.

The SOHO team also held a contest over the internet to guess the time when the 1,000th comet would be discovered. The contest winner was Andrew Dolgopolov of Dublin, Ireland, who guessed the time of the comet's closest approach to the sun (perihelion time) within 22 minutes.

"Before SOHO was launched, 16 sun grazing comets had been discovered by space observatories. Based on that experience, who could have predicted that SOHO would discover more than 60 times that number, and in only nine years," said Dr. Chris St. Cyr. He is senior project scientist for NASA's Living With a Star program at NASA Goddard Space Flight Center in Greenbelt, Md. "This is truly a remarkable achievement!"

About 85 percent of the SOHO comets discovered so far belong to the Kreutz group of sun grazing comets, named because their orbits take them very close to Earth's star. SOHO's 999th and 1,000th comets also belong to the Kreutz group. The Kreutz sun grazers pass within 500,000 miles of the star's visible surface. Mercury, the planet closest to the sun, is about 36 million miles from the solar surface.

SOHO has also been used to discover three other well-populated comet groups: the Meyer, with at least 55 members; Marsden, with at least 21 members; and Kracht, with 24 members. These groups are named after the astronomers who suggested the comets are related, because they have similar orbits.

Because comets in a group have similar orbits, they are believed to be fragments from a larger comet that broke apart. Sun grazing comets can break up as they approach the sun due to its gravity and heat. It is likely that small fragments continue to break off all around their orbits, because SOHO observes a stream with tiny Kreutz members reaching the sun almost every day, and bits as small as these would have simply vaporized if this had happened near the sun. Most of these comet fragments are not visible from Earth because their small size makes them extremely faint. A typical comet nucleus is as big as a mountain, while most of the SOHO comets are only as big as a large room or small house.

However, since the Kreutz group is so numerous, the parent comet that shattered to create Kreutz comets is estimated to have been truly immense, about 60 miles across. The great comets of 1843 and 1882, with long tails that were spectacular to the naked eye were large Kreutz members, as was comet Ikeya-Seki in 1965. The 1882 and 1965 comets almost certainly broke off from each other the previous time they were near the sun, when the combined comet was likely seen as the comet of 1106.

Almost all SOHO's comets are discovered using images from its Large Angle and Spectrometric Coronagraph (LASCO) instrument. LASCO is used to observe the faint, multimillion-degree outer atmosphere of the sun, called the corona. A disk in the instrument is used to make an artificial eclipse, blocking direct light from the sun so the much fainter corona can be seen. Sun grazing comets are discovered when they enter LASCO's field of view as they pass close by the star.

"Building coronagraphs like LASCO is still more art than science, because the light we are trying to detect is very faint," said Dr. Joe Gurman, U.S. Project Scientist for SOHO at Goddard. "Any imperfections in the optics or dust in the instrument will scatter the light, making the images too noisy to be useful. Discovering 1,000 comets since SOHO's launch on December 2, 1995 is a testament to the skill of the LASCO team."

SOHO successfully completed its primary mission in April 1998. It has enough fuel to remain on station to keep hunting comets for decades, if the LASCO instrument continues to function.

For information about SOHO on the Internet, visit,

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