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Bill Steigerwald                June 10, 2003
NASA Goddard Space Flight Center
(301)  286-5017

Release 03-65

HEADLESS COMETS SURVIVE PLUNGE THROUGH SUN'S ATMOSPHERE

A run through the jungle is too easy; for the ultimate reality show 
contest, try a race through the Sun's atmosphere, where two comets 
recently lost their heads. The tails from a pair of comets survived a 
close encounter with the Sun, even after the Sun's intense heat and 
radiation vaporized their heads (nuclei and coma), an extremely rare 
event photographed by the Solar and Heliospheric Observatory (SOHO) 
spacecraft.

On May 24, 2003, a pair of comets arced in tandem towards the Sun, 
their paths taking them to just 0.1 solar radii above the Sun's 
surface, deep within the searing multimillion-degree solar atmosphere 
(corona).

They belong to the Kreutz family of sun-grazing comets, often seen by 
the SOHO spacecraft while diving towards their final rendezvous with 
the Sun. But as in humans, twins are rare. Even more so, this pair 
showed another very unusual trait: What looks like a faint tail (or 
"puff of smoke") can be seen moving away from the Sun, seemingly 
emanating from a point in the orbit beyond the comet's closest 
approach. Normally, sungrazers simply fade and disappear at an 
earlier stage, obliterated by the Sun's intense heat and radiation 
pressure.

Another pair of Kreutz sungrazers with such a "headless tail" was 
observed in June 1998, when the observing geometry was very similar. 
But out of more than 600 sungrazing comets observed during more than 
six years by SOHO, this is only the third showing any signs of such 
behavior. However, this seems now likely to confirm the existence of 
such comets.

"Everyone who's seen this agrees it's a very interesting 
observation," said Dr. Douglas Biesecker, a solar researcher at the 
National Oceanic and Atmospheric Administration's Space Environment 
Center in Boulder, Colorado, and the head of SOHO's comet discovery 
program. SOHO has become the most prolific comet finder in history.

The tail is most likely the dusty remains of the comet's nucleus, 
being pushed out by sunlight (radiation pressure) after all the ice 
in the nucleus has evaporated, thus eliminating the processes 
maintaining a bright coma surrounding the nucleus. Studies of the 
dust cloud may reveal clues to the size distribution of the dust 
grains.

"The fact that the tail 'holds together' so well probably means that 
the dust is mostly the same size," said Biesecker.

Comets are chunks of ice and dust that zoom around the solar system 
in elongated orbits. This "dirty snowball" is the nucleus of the 
comet; it ranges in size from a large boulder to a large city. As the 
comet gets close to the Sun, solar heat and light liberate gas and 
dust from the nucleus, forming the coma, which is an extensive, 
bright cloud around the nucleus, and one or more tails. A comet's 
dust tail can be millions of miles (kilometers) long and is pushed 
away from the Sun by sunlight. Comets also have a tail of 
electrically charged particles (ions) that is usually fainter and is 
pushed away from the Sun by the solar wind, a thin stream of 
electrified gas that blows constantly from the Sun. Both tails point 
away from the Sun, even for comets that are traveling back outwards 
in the solar system. Studies of the tails can reveal changes in solar 
wind structure and radiance of the Sun.

SOHO is a project of international cooperation between the European 
Space Agency and NASA. For images and movies of this event, refer to:

http://soho.nascom.nasa.gov/pickoftheweek/old/27may2003/

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*****************************
Mark S. Hess
Chief, Office of Public Affairs
NASA/GSFC, Code 130

E-Mail:  Mark.S.Hess{at}nasa.gov
Phone:  301 286-6255
Fax:  301 286-1707
*************

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