🚀 go-pugleaf

The following are comments from Duncan Steel on some recent postings
on sci.astro and sci.space. Please address responses to his email
address. I am posting this for him as he does not have direct access
to UseNet.
Peter Brown 
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SOME COMMENTS ON RECENT DISPARATE POSTINGS W.R.T. TUNGUSKA AND P/SWIFT-TUTTLE


(1) Tunguska object being stoney: The work referred to is by Chris Chyba
   (now at NASA-Goddard SFC), Paul Thomas (U. Wisconsin-Eau Claire) and
   Kevin Zahnle (NASA-Ames RC).  They have a paper prepared which shows that
   the Tunguska event is consistent with the entry of a stoney asteroidal
   object about 50 m in size (i.e. there is NO NEED for a fluffy underdense
   object like a "comet" fragment - as if we know anyway what a comet is
   like; hell we don't even know P/Halley's density to within a factor of 3).
   I imagine that they would have given a paper on this at the DPS in Munich.

(2) Frequency of Tunguska-type impacts.  The thing which MAY be wrong with
   most of the discussions is that it is generally assumed that such objects
   hit Earth randomly in time.  This is daft in that we know that a good
   fraction - perhaps even the majority - of the mass influx of smaller
   meteoroids hit the Earth in showers (meteor showers).  These occur as
   the Earth passes through the the meteoroid stream produced by an 
   asteroid/comet.  They recur each year since the smaller particles produced
   by the cometary decay are spread around its orbit by ejection speeds from
   the comet nucleus, radiative forces etc.  However, there is a concenration
   close to the cometary nucleus (cf. cyclic meteor storms such as Leonids,
   Draconids, Perseids in 1992 - and in 1993/94?).  The large particles are
   also grouped close to the nucleus since ejection velocities are small,
   and radiative forces insignificant.  What if a giant comet fragments,
   leaving some km-sized lumps but many more smaller ones (50-100m)? These
   will be mostly in a clump.  At such time as the orbital precession/
   evolution gives a node at 1 AU, a shower of large lumps may occur: but
   not every year, only when the clump is at the right (wrong?) point in
   the orbit, like the Leonids recurring every 33 yr (the orbital period
   of the parent comet).  Anyhow, the above would lead to the following
   occurring: a few random incoming Tunguska-type objects every so often
   (every few centuries), but every millenium or so (time depending upon
   exact orbit because of precession rate) there will be a phase of a
   century or so in which every few years/few decades there is a large number
   of Tunguska-type events spread over a week or so, and this dominates the
   long-term (sporadic) influx.  This I call "Coherent Catastrophism", and
   catastrophic it would be.  Indeed, I believe, it "has been", judging
   from the historical record, since this is what is going on at the moment,
   with us now (late 20th Century) being in a hiatus between mass influxes.
   I would refer one and all again to V.Clube and B.Napier, "The Cosmic
   Winter", Blackwells, Oxford & NY, 1990, for the historical stuff.  For
   the nitty-gritty w.r.t. the meteoroids (small ones in showers observed
   now) see Steel, Asher & Clube, Mon Not Roy Astron Soc, 251, 632, 1991.
   Since then we have looked at the asteroids which we believe to be members
   of the complex (see Steel, The Observatory, 112, 120, June 1992) and
   shown that at the >95% confidence level there are 8/9 objects in the
   inventory of known Apollo asteroids which are members of this complex
   (Asher, Clube & Steel, presented at Meteoroids and their parent bodies,
   Slovakia, July 1992 and Mon Not Roy Astron Soc, submitted).  This would
   imply that there are many more smaller objects (50-100m?) which have
   orbits which do not have a node at 1 AU at present, but will precess
   so as to do so before too long!  Outliers in that respect include the
   Tunguska object itself, the 1975 lunar impactors (also detected on
   Earth as ionospheric disturbances: Kaufmann et al, Science in 1989 I
   think), and 1991 BA (that 5-10m near-miss asteroid/meteoroid).  As I
   have said before, I do not know when the world will end but I would
   bet that it'll be in the last week of June one year.  You all might
   like to note that the above ideas on Coherent Catastrophism were
   totally excluded from the NASA report on how to search for Earth-crossing
   objects (Report of the Near-Earth-Object Detection Workshop, ed. 
   D. Morrison) despite my best efforts as one on the committee members
   to have it included.

(3)  There has been a lot of bull written in these columns about
   P/Swift-Tuttle and whether it will hit the Earth and what the
   consequences might be.  Here is what led to the news stories.
   In IAU Circ 5636 dated October 15th Brian Marsden pointed out that
   with present orbital solutions and their acknowldeged uncertainties
   an impact on 2126 August 14th is/was possible.  Whenever something
   topical comes up I do some radio interviews with Australian Broadcasting
   Corp stations here, to keep the Australian public abreast of what is
   going on: good for astronomy, good for space, good for science all
   round.  I also informed a few newspapers here.  To that extent the
   Australian public knew all about this story a week or more ahead of
   the populace elsewhere, and with that in mind I would encourage others
   spread all over to do something to keep the public informed about what
   is happening; hell, they (mostly) pay for our salaries, equipment,
   education.  Getting back to Oz, I did my first radio interviews on
   Oct 16th (Darwin & Adelaide), then every other capital city on Oct
   19-21.  The first story in print was, I believe, in The Australian
   newspaper dated Oct 20.  That weekend there was a 3-day conference
   (National Space Development Conference) in Sydney and I gave a talk
   on our program here searching for near-Earth asteroids, NASA plans,
   international plans, SDI involvement, etc.  The case of P/Swift-Tuttle
   was/is an excellent example: a possible impactor, > 100 yrs notice,
   requires observations over the next 5/6 yr from the southern hemisphere
   for better knowledge of whether an impact is likely. (Despite that, it
   turns out that we have no funding as from December so that our program
   will be closed down, resulting in no southern hemisphere searches; I
   would also point out that the US searches have depended upon us for
   follow-up of objects moving south but we will be unable to do this from
   now on.  This is due to the idiocy of the Australian government; we
   have had great support from our north American co-workers in trying to get
   funding here).  Anyhow, back to the talk in Sydney.  The local Reuters
   correspondent was there and after discussions with me on the topic he
   put out a wire story which led to the furore.  Indeed he says that no
   story that he has previously filed has got as wide a response.  So far
   as I know the story he put out was accurate (saying that the CHANCE of
   an impact existed) except that he got the year wrong: 2116 instead of
   2126.  You would not believe where the story was carried: for example
   front page of THE TIMES (London) with an editorial, and a reply by
   Marsden (Oct 30), page 3 of THE SUN (London) with a semi-naked lady,
   front page of a paper in Fort McMurray (Canada) sharing top spot with a
   story about a moose.  I fielded phone calls from all over the world: 
   most of them based upon errors and misconceptions. "Such is life" (the
   last words of Australian anti-hero Ned Kelly).

(4) Impact velocity by P/Swift-Tuttle: this is known accurately.  If the
   comet does not hit this next time but it does at some time in the
   future then whenever that occurs the speed would be between 60 and 61
   km/sec (see my paper MNRAS, 227, 501, 1987) unless its orbit (a,e,i)
   changes appreciably.  The 60-61 km/sec ariation is due to
   the eccentricity of the Earth's orbit and hence terrestrial speed.
   For 2126 August 14 the speed can be found exactly, but I haven't done
   it; it's not important since the mass is so poorly estimated.

(5) Using an assumed sphere of diameter 5 km, density 1 gm/cm3, I get
   an energy of above 20 million Megatonnes, or around 1 billion times
   Hiroshima.



Duncan Steel,
Anglo-Australian Observatory,
Coonabarabran, NSW 2357.

"dis@aaocbn1.aao.gov.au"
 
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