[BAA Comets] A rapid decrease in the rotation rate of comet 41P/Tuttle-Giacobini-Kresák

[Richard Miles]

Thanks Jeremy.

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.

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.

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.....

Maybe one or two of us should check this out?

Richard

  ----- Original Message ----- 
  From: Jeremy Shears 
  To: comets-disc at lists.britastro.org 
  Sent: Friday, January 12, 2018 4:11 PM
  Subject: [BAA Comets] A rapid decrease in the rotation rate of comet 41P/Tuttle-Giacobini-Kresák


  A Nature pre-print on ArXiv at: https://arxiv.org/abs/1801.03870




  Jeremy



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