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<DIV><FONT size=2 face=Arial>Thanks Jeremy.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>I am unsure about the basis for this paper, knowing
how difficult it is to measure the rotation period of small solar system bodies
and more especially comets.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>The problem I have is that they do not relate the
change in spin rate to an absolute change in the angular momentum of the
nucleus. At first sight, the enormous change from a period of 20 hr to 46 hr in
2 months strikes me as physically impossible. The reason is that in modelling
the change in rotation period (see Eqn. 1 under their Methods section) they have
employed an empirically derived constant, C multiplied by the water production
rate. However, in their analysis they have not tried to relate this
constant C to absolute values of mass, velocity, and angular momentum. If they
had done this they would have had an independent check on whether what they are
claiming has a reasonable physical basis.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>In reality, when water molecules escape by way of a
jet, they have a distribution of velocities related to temperature according to
the kinetic theory of gases (and to the speed of sound in a gas). This velocity
distribution sets an upper absolute limit on the amount of torque the escaping
gas can generate if we know the comet's water mass loss rate . So we
can make an accurate estimate of the upper limit of the momentum change possible
at the nucleus of 41P and then from an estimate of the mass of the nucleus we
should be able to determine the maximum rate of change of spin rate. The authors
should have included this analysis in their paper to give it more
credence.....</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Maybe one or two of us should check this
out?</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Richard</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=bunburyobservatory@hotmail.com
href="mailto:bunburyobservatory@hotmail.com">Jeremy Shears</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=comets-disc@lists.britastro.org
href="mailto:comets-disc@lists.britastro.org">comets-disc@lists.britastro.org</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, January 12, 2018 4:11
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [BAA Comets] A rapid decrease in
the rotation rate of comet 41P/Tuttle-Giacobini-Kresák</DIV>
<DIV><BR></DIV>
<DIV
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dir=ltr id=divtagdefaultwrapper>
<P style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px">A Nature pre-print on ArXiv
at: <A id=LPlnk100698 class=OWAAutoLink
href="https://arxiv.org/abs/1801.03870"
previewremoved="true">https://arxiv.org/abs/1801.03870</A></P>
<P style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px"><BR></P>
<P style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px">Jeremy</P></DIV>
<P>
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