Bell Tone
Narrator: All the comets that we can see from Earth are orbiting the sun, but some belong to
a special group called sungrazing comets. Sungrazers are comets that
come very close to the sun at their nearest approach, a point called perihelion.
To be considered a sungrazer, a comet needs to get within about 850,000
miles from the sun at perihelion. Many come even closer,
even to within a few thousand miles. Being so close to the sun is very hard
on comets for many reasons. They are subjected to a lot of solar radiation
which boils off their water or other volatiles. The physical push
of the radiation and the solar wind also helps form the tails. And as they get closer
to the sun, the comets experience extremely strong tidal forces, or
gravitational stress. In this hostile environment, many
sungrazers do not survive their trip around the sun. They don't actually crash into the solar
surface, but the sun destroys them anyway. Many sungrazing
comets follow a similar orbit, called the Kreutz Path, and collectively belong to
a population called the Kreutz Group. In fact, close to
85% of the sungrazers seen by the SOHO satellite are on this orbital highway.
Scientists think one extremely large sungrazing comet broke up
hundreds, or even thousands, of years ago, and the current comets on the Kreutz Path
are the leftover fragments of it. As clumps of remnants make their way back around
the sun, we experience a sharp increase in sungrazing comets, which appears
to be going on now. Comet Lovejoy, which reached
perihelion on December 15, 2011 is the best known recent
Kreutz-group sungrazer. And so far, it is the only one that NASA's
solar-observing fleet has seen survive its trip around the sun. Comet
ISON, an upcoming sungrazer with perihelion on November 28, 2013,
is not on the Kreutz Path. In fact, ISON's orbit
suggests that it may gain enough momentum to escape the solar system entirely, and never
return. Before it does so, it will pass within about 40
million miles from Earth on December 26th. Assuming it survives its trip
around the sun. All comets are great laboratories for
scientists to learn more about our solar system, but sungrazing comets can also help us learn
about the sun. Their tails of ionized gas illuminate invisible
magnetic fields, so they can act as a tracer, helping scientists observe these normally
unseeable features. Such fields have even ripped off comet tails,
allowing astronomers to watch them blowing in the solar wind. A wind that
abruptly accelerates between one and five million miles from the sun.
Because of this, researchers will be watching ISON, and other sungrazing comets
very closely. And since we are in a period of high sungrazing comet activity,
scientists can expect many more chances to watch these beautiful,
natural research satellites in the coming years.
Beeping