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View Full Version : Comets that don't survive perihelion



tony873004
2013-Nov-25, 09:01 PM
I keep reading that IF comet ISON survives perihelion on Thursday, that it will put on a good show in December, and if it doesn't survive, it will be a dud. But how do you destroy a comet? There's still a conservation of mass going on. The nucleus is hard to see on its own. What we see in a tail is material that has already been ejected from the comet. So if the comet does not survive, shouldn't it put on a better show? No more nucleus = 100% of the mass now spreading out and forming a tail.

Swift
2013-Nov-25, 09:13 PM
There is a good explanation of the various possibilities here (http://www.isoncampaign.org/mmk/what-might-happen). Its even more complicated than destroyed or not, but if destroyed, exactly when it happens.

(my bold)

Assuming ISON survives the next few weeks in tact, it faces an even more daunting challenge: making it around the Sun. As you are no doubt aware, ISON will face extreme temperatures as it nears perihelion. At its closest point to the Sun, the equilibrium temperature approaches 5000 degrees Fahrenheit, hot enough to cause much of the dust and rock on ISON’s surface and in its coma to vaporize. While it may seem incredible that anything can survive this inferno, the rate at which ISON will likely lose mass is relatively small compared to how big it likely is (think of how a large pile of snow can last for weeks after a snowfall, even when the outdoor temperature has gotten much warmer than freezing). Furthermore, because it is moving very fast, about 400 km/sec at perihelion, it will not spend very long at such extreme temperatures. Assuming that ISON is bigger than about 200 meters in radius (current estimates suggest it is 500-2000 m in radius), it will likely survive mass loss due to sublimation of ices alone.

Unfortunately for ISON, it faces a double whammy from its proximity to the Sun: even if it survives the rapid vaporization of its exterior, it gets so close to the Sun that the Sun’s gravity might actually pull it apart! I discussed this in more detail in a previous post, but simulations found that ISON is more likely to survive than be pulled apart, although there is a very real chance that it could be pulled apart by these “tidal forces.”

If ISON is destroyed within a few hours to days of its close approach to the Sun, the most likely cause will be the temperature and gravitational stresses of the near-Sun environment. However, spontaneous disintegration (Case 1) could still be the culprit, it would just be hard to prove. In any event, if ISON meets it demise before its close approach to the Sun, it will likely be among the most spectacular comets observed by SOHO and STEREO, but there won’t be anything left to see from the ground after the close approach because all of the dust released during the breakup will be vaporized. On the other hand, if it is destroyed a few days after close approach when the temperatures are less extreme, the dust released when it disintegrates will likely form a spectacular tail visible from Earth.

redshifter
2013-Nov-25, 09:13 PM
deleted, swift has a much better explanation above.

tony873004
2013-Nov-26, 03:38 PM
So it seems like it has to do with vaporization? If the dust vaporizes, it is still there (conservation of mass), but now it is an invisible gas. This is similar to how you can't see water vapor in Earth's atmosphere, but drop the temperature a few degrees and it condenses into highly-visible clouds.

thoth II
2013-Nov-26, 04:02 PM
yes, if dust vaporizes, it would be invisible. It probably wouldn't scatter the sunlight it otherwise would scatter. I think it'll be a dud.