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Re: (meteorobs) Fwd Re: Phaeton directly imaged or spectrographed?
------- Forwarded Message
Date: Mon, 31 Aug 1998 22:11:11 +0200 (MET DST)
From: PRAVEC Petr <ppravec@sunkl.asudot cas.cz>
To: Lew Gramer <dedalus@latrade.com>
Cc: Meteor Observing Mailing List <meteorobs@latrade.com>,
Minor Planet Mailing List <mplist@bitnik.com>
Subject: Re: (meteorobs) Fwd Re: Phaeton directly imaged or spectrographed?
On Fri, 28 Aug 1998, Lew Gramer wrote:
> Thank you for forwarding that helpful information, Petr!
>
> So do I understand you correctly, that a rotational period of
> ~ 3.5 hours is simply too short for an object whose average
> density is similar to cometary material?
Yes, you understand me correctly. The only difference is that I would
formulate it not so definitely, since nobody really knows how a comet
nucleus looks like. But, if a current understanding of a nature of comets
is correct, then a bulk density of >1.1 g/cm^3 (needed for Phaethon to be
hold together by self-gravitation only) is too high for a cometary nucleus.
Whether there are internal cohesive forces able to hold a lower density
nucleus together against a centrifugal force of the fast rotation, remains
to be seen from further investigation. But it seems unlikely that a
cometary surface can resist such tension.
> In other words, are you saying Phaet(h)on MUST be a rocky (i.e.,
> "asteroidal") body, because it is not flying apart at its equator?
I cannot say that it must be rocky, but I think that it is
of a relatively high density for a normal cometary nucleus.
> If so, that would have fascinating implications not only for Phaeton, but for
> the Geminids as well. Thanks again, and keep in touch!
Here is a couple of further points indicating an abnormal character of
Phaethon and the Geminids. (I copy this from my file. If anyone is
interested I can send also the exact references.)
--- The Geminid meteoroids are of high density of 3--4 g/cm^3, as derived
by Spurny (1991, 1993) from an analysis of the atmospheric penetration behavior
of 41 Geminid fireballs (reanalysing also the 12 fireballs studied by Halliday,
1988), resembling the behavior of fireballs producing stony meteorites,
that are of asteroidal, not cometary origin.
--- Phaethon has a surface of high thermal inertia, suggesting an exposure
of bare rock or a coarse regolith (Green et al. 1985, Veeder et al. 1989),
that disagrees with the expected properties of a cometary nucleus.
Best regards,
Petr Pravec
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