The Great Christmas Comet of 2011, 2011 W3 (Lovejoy) |
The Great Christmas Comet of 2011, 2011 W3 (Lovejoy) |
Guest_Sunspot_* |
Dec 2 2011, 09:59 PM
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#31
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Guests |
http://sungrazer.nrl.navy.mil/index.php?p=.../birthday_comet
Possible very bright sungrazing comet coming mid December - Comet Lovejoy C/2011 W3 (Lovejoy) Information in the link above. |
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Jan 3 2012, 11:19 PM
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#32
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Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
Comet-ml post #19201 by Richard Miles:
QUOTE David Sargeant wrote:
As I wrote previously, I suspect that the initial intrinsic faintness of this comet was not so much a function of the small size of the nucleus, but of the presence of a surface crust of refractory material. If the nucleus was about 500 metres diameter (as against the 100 - 200 as initially estimated) and covered by an insulating crust, this might explain how it survived perihelion passage intact. If the insulating layer was blown off around perihelion, this may even have formed a "sun umbrella" of particles that shielded the freshly-exposed icy surface of the nucleus, rather as is thought to have happened to Seki-Lines in 1962 (analysis of the dust tail suggests that this comet shut down for a few hours at perihelion - q = 0.03 AU - which also helps to explain why there were no daylight sightings of this intrinsically bright object). In the case of Lovejoy, a similar event may have been a factor in preserving its existence. Once the meteoric cloud dispersed, the comet burst into furious activity, however by then the worst of its ordeal was already over. I agree that a temporary surface crust of material can form, but not as you have envisaged here. One key factor here is that time near closest approach is relatively short - i.e. the nucleus remained within 5 solar radii of the barycenter for about 6 h, and 2 solar radii for just 1.5 h. You have to consider both the solar electromagnetic radiation flux and also the flux of high energy baryons / charged particles, typically protons travelling at speeds of ~500 km/s. The initial effect of these is to strip away any "umbrella of dust particles" leaving the bare nucleus exposed to the 'onslaught' from the Sun. A more likely scenario may involve a Leidenfrost-type phenomenon. Here's how I see it: A large fraction of the near-surface material within the nucleus is likely to melt. The surface tension between the melt and any residual solids provides significant mechanical strength, more especially if most of the refractory solids are in the 1-1000 micron size range. This process temporarily inhibits physical break-up. Now if you assume a large fraction of the incident energy (electromagnetic radiation and particle kinetic energy) is absorbed by the surface, this will cause a proportion of the molten material to vaporize - but how much depends on the latent heat of vaporization of the material and the time-scale involved. The vapour boiled off from the melt is in effect a thin gaseous atmosphere, which will increase in pressure until a temporary bow-shock front develops. It is this bow-shock effect which may act as the "umbrella" - a "parapluie" could be more descriptive a word for this. If the gas pressure behind the bow-shock reaches a sufficient magnitude (Poiseuille conditions of P and T arise), particulates will also be entrained in the gas flow. Given this scenario, the surface of the nucleus can be shielded to a degree by two processes; (a) partial deflection of the intense oncoming solar wind by the bow-shock, and ( b ) particulates suspended in the temporary gas layer absorb some of the e-m radiation and re-radiate it back into space. Overall this creates a type of Leidenfrost effect and a temporary pseudo-steady state enabling the nucleus to survive perihelion passage. Remember, although H2O ice is an important constituent, as the thermal regime evolves to higher and higher temperatures, different materials which are normally solid will each begin to melt and play a significant role. What will be important now is to characterise the nature of any remaining particulates close to the centre of any debris field using large ground-based telescopes or the HST. Let's hope such observations are successful. The fate of the nucleus depends on what happened post-perihelion. Sufficient time has passed such that, given the very large thermal gradients, significant heat conduction to the central region of the nucleus would have occurred. You then have a complex situation in which solids melt, liquids vaporize and internal gas pressures develop leading to gradual disintegration of the nucleus. What debris remains will be to an extent an assay of the more refractory material from deep within the original nucleus. Richard Miles BAA |
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