The precarious journey of a primitive icy body through the solar atmosphere
Since it was first discovered in September 2012, comet C/2012 ISON has stirred up a lot of interest, to the point of becoming a global event. This is mostly because of its billing as possibly the brightest comet since 1680. In that year a bright comet swept across the sky near the horizon, shining brightly and casting shadows just like the Moon. The greatest scientist of all time, Issac Newton, analyzed its orbit in his famous Principia. Looking back through historical records we find that very bright comets tend to appear near the horizon, indicating their sun-grazing nature. Some of them are now believed to belong to the same cometary group called the Kreutz family, which is composed of many fragments produced by the breakup of a parent body. Some of them could be as large as the 2-km wide comet ISON. Thousands of smaller ones in sizes down to tens of meters have been detected by the coronagraph of the European SOHO space solar telescope when they came into the field of view of the instrument within about 10-20 solar radii. The theoretical scenario developed from orbital statistics suggests that these sun-grazing comets must have originated in the cometary Oort cloud which is distant from the sun on the order of several ten thousand astronomical units. (One astronomical unit or AU is the distance between our Earth and the Sun.) Because of the gravitational perturbations of passing stars, some of the Oort cloud comets are occasionally sent into kamikaze trajectories diving towards the Sun. Many of these have stopped short of plunging into the planetary region, that is, inside the orbit of Neptune; but some of them like the Kreutz family comets have reached solar distances as small as 2 or 3 solar radii.
As a comet approaches the sun, its surface temperature gradually increases because of solar radiation. Frozen water, which makes up the bulk of its composition. begins to sublimate at a distance of about 3 AU from the Sun thus generating a coma and dust tail of majestic proportions. Since August this year Dr. Zhonyi Lin and Hung-Chin Lin of the Institute of Astronomy, National Central University, which is a partner university of the University System of Taiwan, have been monitoring the activity of comet ISON using the telescopes at the Lulin Observatory. Dr. Lin explains that the green color of the cometary coma is basically due to the emission of CN (cyanogen) molecules which might have been produced by photo-dissociation of HCN or C2N2.
There have been a variety of guesses as to what would happen when comet ISON reached its closest distance to the sun of 2.7 solar radii on November 28. Would it disappear because of the explosive sublimation process caused by the intense heating? Would it be broken into a million pieces because of the strong tidal effect of the Sun? Or would it be able to remain intact? The whole world was asking these questions. The latest imaging observations by the American STEREO space solar telescope showed that comet ISON has survived the rite of passage and returned to view again. Its brightness has diminished some but this should be surely regarded as an example of “beaten but not broken”. How did comet ISON manage to sail through the hellfire of the Sun, and what is the physical nature of the remaining nucleus? Is it still of icy composition or more of a rocky structure? The real game has just started, says Dr. Lin – stay tuned.
Vocabulary
comet [n.] 慧星
horizon [n.]地平線
coronograph [n.]日冕觀測儀
solar radius (plural, radii) [n.] 太陽半徑
ISON [n.]艾桑彗星
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Source from：Taiwan News
http://www.taiwannews.com.tw/etn/news_content.php?id=2363630