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Cambridge Conference Comments - February 27, 1998

CAMBRIDGE-CONFERENCE COMMENTS, 27 February 1998
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From: Rob McNaught 

VISUAL FIREBALL SIGHTINGS: A CONSUMER WARNING!

I applaud the comments of David Morrison and Jeremy Tatum. With about
two decades of interest in following up fireball reports and 25 years of
operating all-sky fireball cameras, I have come across this sort of hype
and exaggeration regularly.

The appearance of a story in the media need not have anything to do with 
the magnitude of the event. In the USA with such a high population 
density, it would be difficult for a fireball to occur away from some 
major centre of population. Here in Australia, it is commonplace for a 
bright fireball to be photographed over western NSW and have no mention 
in the media. Also, an evening fireball is much more likely to be 
reported than a morning fireball. When however, a fireball occurs over 
a major town and sonic booms are widely heard, the event invariably 
makes the national news within hours. Early maps of tornado 
distribution in the USA showed a similar bias due to the uneven 
population density.

Other news items that compete for space can oust a fireball from being
covered. During the recent huge Pilliga forest fire near here, a bright
fireball was photographed by the one camera station operating that night
(others were affected by smoke). It was weeks before I knew of it when the
film was processed (the long delay caused by the fire).

Once a journalist takes interest in a fireball, it is easy to hunt down
reports of other events. After all, fireballs that light up the ground
occur monthly from any dark location. Thus one fireball suddenly 
becomes several with the attendant speculation about increased rates.

It is probably unfair to critise a journalist for having no scientific
training, but it is certainly fair to critise the organisations they work
for, the larger ones at least, if they either do not have scientifically
trained journalists on their staff, or they ask a scientifically illiterate
journalist to cover a fireball report. This results in 
misinterpretations and accepting highly questionable statements 
from highly questionable sources.  As an example of the former, I 
was once quoted as saying a fireball had exploded causing 
devastation similar to a nuclear explosion and I was seeking help 
in finding where it had occurred!!! Well, to be fair, I had said 
that, but only when asked about Tunguska.  More recently, a  
potential meteorite fall near here was widely reported in the 
press (at sunset, no cameras operating, over 50 in-situ eyewitness 
interviews indicate an end height below 20km and low velocity).  A 
local newspaper ran a front page story about it, having spoken to 
two "experts".  One, a local amateur astronomer who operates an 
all-sky fireball camera, gave a good factual account of the event, 
but his comments were lost in the nonsense and half truths of a 
local operator of a public observatory. The story ended up
being about the $ value of the "meteorites". Of course, he may 
have been misquoted!

Certainly, scientists do not always agree on the details, or even the
fundamentals, but I am unaware of any serious questions regarding the
physics of small fireballs (centimetre to meter size. 
Electrophonic sounds may be one issue). It is thus depressing to 
have ill informed comments from people with no background in this 
field (planetarium directors, operators of public observatories 
and astrophysicists included).

Regarding the perception of a fireball by the human eye/mind, Jeremy is
actually incorrect in saying that there is no information on the actual 3D
motion of the fireball. The human mind has evolved to correctly 
interpret changing angular velocity as motion of an object 
relative to the observer. Usually this occurs in a rich visual 
environment with many other cues to size and distance. In the case 
of a fireball, these other cues are lacking (changing size, shape, 
intervening haze, obscuration of distant or by near objects etc.), 
which certainly results in indeterminacy of the distance, but in 
my experience, the observer usually correctly interprets the 
general direction of motion of a fireball. The linear deceleration 
of the fireball in the atmosphere certainly affects this 
judgement, but the changing angular velocity is a strong cue. But 
Jeremy is certainly correct in that the projected path across the sky 
give the essential data from which the real trajectory is determined.  
[A single photograph with timed interruptions CAN result in the 
derivation of the radiant (orientation of the real path), by assuming 
the early part of the path has near constant linear velocity and 
fitting the changing angular velocity to the angular distance covered.  
The distance cannot be derived from this technique, but plausible 
assumptions can be made.]

Regarding the problem of misperception of the distance of a fireball, this
is where the lack of cues leaves the mind floundering. Almost 
certainly, the human mind UNCONSCIOUSLY and directly interprets the 
event in terms of more familiar objects with the following result:

it was bright, therefore it is close
it has a high angular velocity, therefore it is close
it is large, therefore it is close

Once it is "seen" to be close, this sets one parameter that many others must
follow. If it appeared to be say, 300 metres away, and was seen at 30 
deg altitude, then it will appear to have a height of 150 metres.  If 
the distant hills are say 500 metres above the observer, this can 
result in the PERCEPTION that the fireball was below the height of the 
hills, despite being of higher angular altitude. It is unlikely that 
this perception once made can be altered, and one has to be very 
tactful in explaining this illusion. It can result in resentment at the 
arrogance of scientists who ignore what people actually see. [When 
sonics are heard some minutes later, the eye-witness is usually aware 
of a problem with the perceived distance.] Aurorae "seen" in front of 
hills, is presumably caused by the same psychological process. Another 
result of this misperceived distance is that the size of the object 
becomes fixed.  "It was the size of a dinner plate". Asking "The size 
of a dinner plate at what distance?" really doesn't help, as the 
angular size of the object becomes secondary to the (mis)perceived 3D
size, despite the angular size having primacy as the original sense data.
UFO reports caused by Venus "following" a car is the same overall
psychological process of misperceived distance with all its consequences.
Once the observer becomes fearful of the "UFO", a correct interpretation is
not likely to be possible and again there is resentment if the observed
phenomenon is questioned. The perceptual process most at play here is
called "constancy".

As a final point, I'd like to point out that meteoric fireballs are 
probably some 100 times or so more common than satellite re-entry 
fireballs. I'm only aware of three satellite re-entries having been 
photographed by fireball networks, whereas many hundreds of meteoric 
fireballs have been recorded. Re-entries are rarely much brighter than 
Venus and typically last for many tens of seconds, going from horizon 
to horizon usually with dozens of fragments trailing.  Whilst some 
meteoric fireballs may be of long duration with numerous fragments 
(like Peekskill), they would typically be very much brighter than 
Venus. A fireball lasting a few seconds is unlikely to ever be a 
satellite re-entry, regardless of the fact that re-entries are much 
less common.

Recent events suggest to me that too much importance is being placed on
visual observations of fireballs when there is little or no hard data to back 
up the claims. The extent of reporting in the media (including the 
internet) may indicate little more than hype or lack of background 
knowledge. As Jeremy says, it takes quite a bit of foot slogging (and $ 
of petrol) to make in situ measurements of visual sightings and usually 
for poorly defined results. With the photographed Pribram meteorite 
fall, Ceplecha mentions both systematic and large random errors in the 
visual observations. In the Eastern Australian Fireball Network, we are 
awaiting the next bright fireball, to compare the photographic real 
trajectory with that derived solely from visual sightings to assess the 
nature of these effects in visual observations. This is necessary in 
interpreting  eye-witness reports of satellite detected "superbolides".

Without a satellite detection, or good physical data, I fear that most
reported "impacts" will turn out to be hype. If this were to go on for
much longer, there will be widespread cynicism about the phenomenon.

Rob McNaught (rmn@aaocbn.aao.gov.au)

======================

From: Simon Mansfield 

Benny,
 
maybe the "increased activity" is mistaken for Iridium satellite 
flashes.
 
Simon