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THE ASTRONOMER Electronic Circular No 2129      2005 Jly 06 13.40UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK             GMH at AST.STAR.RL.AC.UK
WORLD WIDE WEB                         http://www.theastronomer.org
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COMET 9P/TEMPEL
Dr, Mark Kidger reports:
The last 36 hours have been extremely exciting, but ultimately 
disappointing with respect to Deep Impact and its consequences. Many 
people seem to think that in the absence of a major light curve event 
that there will be none. This may be premature because the third phase 
of the post-impact light curve has barely started.

Up to now the results have been somewhat unexpected. On impact the 
reaction from the comet was unexpectedly strong, certainly at the high 
end of expectations. Observations showed an increase of about 2 
magnitudes over 15 minutes after impact, although somewhat surprisingly 
the amplitude was only slightly larger in 2" aperture than in 5". 
Coincident with this, there was a brief enhancement of cometary emission 
lines such as CN. After this the comet has faded again and has 
stubbornly refused to brighten as the impact debris cloud expanded into 
the coma. Disappointingly, there is no information about the crater 
other than it is larger than a house (suggesting that it is well under 
the 100-m diameter that project experts were predicting as their 
baseline size for it) and no images of it have been released.

What this suggests is that the dust mantle of the comet was thick and 
also desiccated. When it expanded into space it carried little or no ice 
with it (had the dust been rich in ice we would have seen a major 
brightening from sublimation of ice trapped in the dust) and that only a 
small amount of ice underneath was volatilised when the crater formed. 
However, it seems possible that there was significant artesian gas 
pressure underneath the dust mantle and that when the mantle was 
breached the release of this pressure contributed to the formation of 
the plume.

Now, we enter the third stage of the light curve. A significant crater 
has formed which should have breached the dust mantle exposing fresh ice 
underneath. This should lead to the formation of a significant new 
active area on the surface of the nucleus. According to how rapidly heat 
enters this crater, how efficiently the insulating dust mantle has been 
cleared, and how much fresh ice has been exposed, the comet should start 
a slow brightening. If this happens the comet could brighten several 
magnitudes. A lack of such brightening would suggest that either fresh 
ice has not been exposed by the impact - either because the crater is 
too shallow, or because the nucleus's outer layers are poor in ice - or 
that heat is not reaching the crater floor efficiently, which could 
happen if the crater is very deep with respect to its diameter.

Observers should thus continue to monitor the comet for activity over at 
least the next week, although initial indications from post-impact CCD 
photometry by the "Observadores_cometas" group is that a sustained 
brightening has not occurred so far.

Late note added by Mark:
At present there are no signs that the expected brightening is 
happening. In fact, results from Swift also show the comet stable at 30% 
brighter than pre-impact, exactly the same as the amateur CCD results.

Guy M Hurst