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Bell Tone

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Narrator: All the comets that we can see from Earth are orbiting the sun, but some belong to

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a special group called sungrazing comets. Sungrazers are comets that

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come very close to the sun at their nearest approach, a point called perihelion.

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To be considered a sungrazer, a comet needs to get within about 850,000

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miles from the sun at perihelion. Many come even closer,

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even to within a few thousand miles. Being so close to the sun is very hard

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on comets for many reasons. They are subjected to a lot of solar radiation

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which boils off their water or other volatiles. The physical push

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of the radiation and the solar wind also helps form the tails. And as they get closer

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to the sun, the comets experience extremely strong tidal forces, or

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gravitational stress. In this hostile environment, many

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sungrazers do not survive their trip around the sun. They don't actually crash into the solar

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surface, but the sun destroys them anyway. Many sungrazing

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comets follow a similar orbit, called the Kreutz Path, and collectively belong to

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a population called the Kreutz Group. In fact, close to

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85% of the sungrazers seen by the SOHO satellite are on this orbital highway.

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Scientists think one extremely large sungrazing comet broke up

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hundreds, or even thousands, of years ago, and the current comets on the Kreutz Path

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are the leftover fragments of it. As clumps of remnants make their way back around

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the sun, we experience a sharp increase in sungrazing comets, which appears

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to be going on now. Comet Lovejoy, which reached

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perihelion on December 15, 2011 is the best known recent

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Kreutz-group sungrazer. And so far, it is the only one that NASA's

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solar-observing fleet has seen survive its trip around the sun. Comet

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ISON, an upcoming sungrazer with perihelion on November 28, 2013,

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is not on the Kreutz Path. In fact, ISON's orbit

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suggests that it may gain enough momentum to escape the solar system entirely, and never

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return. Before it does so, it will pass within about 40

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million miles from Earth on December 26th. Assuming it survives its trip

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around the sun. All comets are great laboratories for

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scientists to learn more about our solar system, but sungrazing comets can also help us learn

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about the sun. Their tails of ionized gas illuminate invisible

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magnetic fields, so they can act as a tracer, helping scientists observe these normally

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unseeable features. Such fields have even ripped off comet tails,

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allowing astronomers to watch them blowing in the solar wind. A wind that

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abruptly accelerates between one and five million miles from the sun.

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Because of this, researchers will be watching ISON, and other sungrazing comets

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very closely. And since we are in a period of high sungrazing comet activity,

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scientists can expect many more chances to watch these beautiful,

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natural research satellites in the coming years.