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(meteorobs) Excerpts from "CCNet 102/2000 - 10 October 2000 (LETTERS...)"
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 10 October 2000
Date: Tue, 10 Oct 2000 13:52:51 +0100
CCNet 102/2000 - 10 October 2000 (LETTERS TO THE MODERATOR)
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[...]
(4) HOW DO YOU RECOGNISE A WEAK COMET?
Duncan Steel <D.I.Steel@salford.acdot uk>
[...]
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(4) HOW DO YOU RECOGNISE A WEAK COMET?
>From Duncan Steel <D.I.Steel@salford.acdot uk>
Dear Benny,
Brian Marsden's nice discussion of the predictability=20
(or otherwise) of cometary trajectories, and his
differentiation between PHAs and PHCs as opposed to
grouping them together as PHOs, reminds me of the nursery tale
of Jack and the Beanstalk, and the giant saying:
"Fee, Fi, Fo, Fum, [Phee, Phi, Pho, Phum?]
I smell the blood of an Englishman."
As Brian and I are both Englishmen, one might enquire
as to the figurative smell of blood that he did not mention.=20
He said how Icarus and Phaethon (both small perihelion distance
objects) have observed ephemerides that do not differ from
point mass calculations over the time they've been known
(from 1947 for Icarus, 1983 for Phaethon), making the
case for the predictability of asteroid orbits as opposed
to comets influenced by non-gravitational forces (originating
from the sublimation of ices). He then went on to say that=20
an 'asteroid' might surprise us, in effect by suffering
non-gravitational perturbations sufficient to turn a calculated
near-miss into an impact trajectory. The question then is:
How do you recognise a weak comet? Rather than saying "It's=20
a PHA but it won't hit us soon", instead we'd soon be saying=20
"Oh, PHC."
In this context the case of Phaethon is interesting, because it
has an associated meteoroid stream (producing the Geminid meteor
shower in December, and the Daytime Sextantids in June when the
Earth makes a post-perihelion intersection of the stream). Such
an association is generally characteristic of cometary activity,
although there have been suggestions about how an asteroid might
produce a stream of small debris. Leaving that possibility aside,
the prima facie evidence is that Phaethon was once an active comet.
The timescale for dispersal of the meteoroid stream is of order
10^4 years, meaning that on the astronomical timescale Phaethon
has quite recently made the comet-asteroid transition. (Although
not as recently as 4015 Wilson-Harrington, say.) Or maybe it is=20
just cross-dressing for a while, and will soon burst forth into=20
splendid cometary activity, as 2P/Encke seems to have done some=20
time shortly before its discovery in 1786.
Obviously asteroids show no nebulosity due to possessing comae=20
or tails, and so simple imaging would not indicate the initiation
of outgassing. One wonders, then, how we should monitor 'asteroids'
that could be potential impactors within, say, a century, *if* they=20
were subject to moderate non-gravitational forces.=20
The UK NEO Task Force report has already evinced some confusion
amongst the ranks of astronomers due to its recommendation that
"spectroscopic" follow-up of asteroids should be conducted. Prior
to the finalisation of the report I pointed out that what was
really involved was *spectrophotometry*. That is, photometry
using the standard broad-band filters in order to determine colours
(B-V etc.) and hence provide categorisation of the asteroids into
the various classes (C-type, M-type etc.). The rejoinder to that,
quite correctly, was that the report was aimed at the public and
politicians, who would not understand "spectrophotometry" but might
comprehend "spectroscopy". (And we'll leave "spectrophotopolarimetry",
aimed at deducing surface texture too, out of this discussion.)=20
My point, though, is that *spectroscopy* might indeed be a useful
technique in our armory of observations of PHAs. Using a suitable
high-dispersion spectroscope on a large telescope we might identify
some characteristic emission line which would show that outgassing
was indeed taking place, without needing to wait years before the
ephemeris residuals indicated tiny non-gravitational accelerations.
When 5335 Damocles was found (as 1991 DA) we obtained such spectral
data using the Anglo-Australian Telescope specifically to look for
weak cometary activity. Likely others have done the same for other
suspicious objects. But should we be trying to do this as a matter
of routine for all PHAs, on a regular (every apparition) basis?
Duncan Steel
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