Comets that stray too close to the sun have a very uncertain future. Some live to tell the tale, while others literally lose their tails. And as we get excited for ISON's hellish dive deep into the solar corona, astronomers are hoping that the potential "Comet of the Century" will also get the chop.
Comets are composed of rock and ice that form their nucleus. As they approach the sun, their icy masses begin to sublimate into space, creating a large cloud of gas known as a coma. As they fall ever deeper into the sun's gravitational well, the solar wind "blows" back these gases to produce a bright tail. In actuality, comets usually have two tails; one that is composed of neutral particles and created purely by solar wind pressure and another that is composed of charged particles and blown along the sun's interplanetary magnetic field (IMF).
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As one would expect, while comets travel through the tumultuous stream of solar gas that flows throughout the solar system, any transient bursts in energy or changes in solar wind density can impact a comet's tail.
In 2007, solar astronomers tracking the progress of Comet Encke as it pushed deeper into the corona (our sun's hot atmosphere) saw a rare event. A coronal mass ejection (CME) erupted from the sun and collided with the comet. The energized gas embedded inside the CME's magnetic bubble wasn't dense enough to impact the comet's nucleus in any measurable way, but it did have a dramatic effect on Encke's tail: it was ripped off.
Now the sun is at solar maximum, solar activity is much higher than it was in 2007 (despite the fact that this solar cycle is fairly weak in comparison to previous cycles), so astronomers are waiting with bated breath just in case our nearest star should hurl a magnetic tantrum in Comet ISON's direction.
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"I would absolutely love to see Comet ISON get hit by a big CME," said astronomer Karl Battams, of the Naval Research Lab and NASA's Comet ISON Observing Campaign (CIOC). "It won't hurt the comet, but it would give us a chance to study extreme interactions with the comet's tail."
Space weather activity is one factor, but ISON will also dive much deeper into the corona than Encke did -- some 30 times closer, with a close approach (perihelion) of a little over a million kilometers from the sun's 'surface' (photosphere) on Nov. 28.
"The CME that ran over Comet Encke back in 2007 was slow, barely creating a pressure pulse by compressing the solar wind ahead of it," said Angelos Vourlidas also of the Naval Research Lab and CIOC. "It was this compression which caused the Encke's tail to fly off."
In ISON's case, a CME impact will likely be far more dramatic. "Any CME that hits Comet ISON close to the sun would very likely be faster, driving a shock wave with a much stronger magnetic field," added Vourlidas. "Frankly, we can't predict what would happen."
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Coincidentally, Comet ISON has the veteran Comet Encke for company on its first dive into the corona and a fleet of solar observatories will be watching the pair's tails being buffeted by the solar wind.
We currently have no direct means to probe the inner corona, so observing any object interacting with the superheated plasma deep inside the sun's atmosphere is a valuable science opportunity. But will the sun oblige and fling a CME at ISON? That will be left to chance, but both comets are in a prime spot for a CME collision.