Satellite re-entries & bright meteors

Tony Beresford (starman@camtech.net.au)
Tue, 14 Oct 1997 21:33:19 +0900

The event Reported by Wayne X of Sydney doesn't seem to be the same as this
object over NSW at local sunset of the 11 of Octeber,1997,
which Rob McNaught has has collected reports and interviewed
eye-witnesses. However 2 things indicate that it too was a meteor.
Firstly the short time it was bright.[ 5 seconds isnt out of question],
secondly its path was to the west. There are few objects in really
retrograde orbits and none of them are currently near decay.
I checked up on the Celestis rocket+ payload , and its currently
doing very close to 15.0 revs/day.

r NSW on Oct 11,1997
From:	"Robert H. McNaught" 11-OCT-1997 19:47:17.87
Subj:	Fireball 97Oct11, Australia

Twilight fireball, 1997 October 11, 08:15 UT +/- 1 min
About three minutes after sunset this evening, a fireball of around
mag -7 to -10 was over NW, New South Wales, Australia (149E, 31S).
1.
Six separate reports have been received from Siding Spring Observatory,
Coonabarabran and Tamworth and even from 300km away on the NSW coast.

General Characteristics
Mag -7 to -10 (much brighter than Venus, but not brighter than the Moon)
Duration ~3 secs.
Green or blue
Trail in sky for up to 3 mins; initially straight, then distorted into
"small patches of cirrus"
Trajectory roughly West to East and radiant <= ~20 deg

Eye-witnesses will be interviewed tomorrow.

Robert H. McNaught


2.
Fireball 1997 Oct 11 08:15.5 UT +/- 1 min, NSW, Australia

Reports of yesterday's fireball appeared in the media and it appears to
have been seen from much of NSW to as far south as Canberra (450 km
distant).  The initial magnitude reported (-7 to -10) was based on a
report by an astronomer from the Anglo-Australian Observatory, but he
suggests now that it was probably brighter.  Many reports suggest brighter
than the Moon, and given the extent of visibility and press interest, I would
guess the absolute mag was in the region of -10 to -15.

A preliminary trajectory based on 4 sightings (3 of which are in situ
interviews by myself and Gordon Garradd), give the following rough
track
           Long (deg E)     Lat (deg S)    Height (km)

Start         149.4            -30.9           55
End           149.8            -31.0           40

Radiant  RA 230d  Dec +10d    Az 295d   El 20d

There are no reports of flaring, but the object fell to pieces towards the
end.  No conclusive reports of sounds have been recieved and I myself heard
nothing; the fireball having passed about 50km distant, high in the sky
over my house.  Other witnesses from close to the calculated end point
have still to be interviewed in situ, but no reports of sounds were
made by them.  It would appear that the end point height is above 30km.

Robert H McNaught  (rmn@aaocbn.aao.gov.au)

3.
More on Saturday's fireball.

Initially, no eye-witnesses reported hearing any sounds after the passage
of the fireball, but two people reported hearing loud sounds but had not
seen the fireball.
I now have two witnesses from Bugaldie who both saw the fireball and shortly
thereafter heard a "boom".  As I live in Bugaldie, so much for me as an
(aural) observer!

Rob McNaught

[The following discussion from Rob, nicely puts the procedure to
distinguish between satellite re-entries and natural objects.
The tendency for the media or ignorant spokespersons to suggest,
quite incorrectly that such events are all "space junk" doesnt
help the NEO detection cause. To quote Rob,"I think the perception
that such objects are man-made is part of the reason the NEO hazard
is ignored.  Natural objects are much more common than re-entries
and much bigger." Of course both of us think that the NEO hazard
shouldnt be ignored.

Date: Mon, 13 Oct 1997 12:11:03 +1000 (EST)
From: "Robert H. McNaught" 
Subject: Satellite or natural object

Why it is unlikely to be a satellite re-entry.
* Re-entries are usually of very shallow angle of entry (few degrees)
  This object was close to 20 degrees and I'd be surprised if this is in
  error by more than 5 degrees.
* The shallow angle of entry of satellites usually gives durations of
  many tens of seconds.  This object was less than 10 and most reports
  give about 5.
* The object was very bright.  Only a VERY large satellite (booster etc)
  could produce a re-entry this bright.  As I understand it, the Shuttle
  re-entry is much fainter than -10, typically -6ish.
* There is only around one or two predicted re-entries every day, but
  worldwide there are probably many tens of fireballs brighter than these
  re-entries each day.  Fireball networks have only photographed 3 re-entries
  I believe, but hundreds of fireballs and 3 meteorite falls.

To be a satellite re-entry it is necessary to show
a) a long duration (which implies shallow angle)
b) A PREDICTED OBJECT IN THE CORRECT PATH!

posted by
Tony Beresford in Adelaide So. Australia
(34.97S, 138.67E)