As the comet approaches perihelion on Nov. 28th, 2013, it has significantly brightened and its tail has started to bend . The NASA/ESA SOHO spacecraft has captured imagery.


Credit: NASA / ESA / SOHO / Mash Mix: SPACE.com

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Sungrazing comets are a special class of 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. Although they don’t actually crash into the solar surface, the sun is able to destroy 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 a perihelion of 730,000 miles 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.

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Category: Science, Weekend

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3 Responses to “Comet Ison, Sungrazing Comet”

  1. Iamthe50percent says:

    “To be considered a sungrazer, a comet needs to get within about 850,000 miles from the sun at perihelion.”

    To put that in perspective, that is about the diameter of the Sun. It is like calling an asteroid an Earth grazer if it comes within 8000 miles of the surface. Coming within a few thousand miles of the Sun is like coming within some 20 miles of the Earth. Close enough to skim the atmosphere. Drag from the Solar atmosphere is a major reason for uncertainty in the outward bound orbit. It’s like a bank shot in pool, but not as predictable.

  2. GeorgeBurnsWasRight says:

    Thanks, Jeff, for the Sky and Telescope link.

    My favorite quote was: “It is now clear that Comet ISON either survived or did not survive, or . . . maybe both,” tweeted Bruce Betts of the Planetary Society.