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Cambridge Conference Comments - February 26, 1998

CAMBRIDGE-CONFERENCE COMMENTS, 26 February 1998
------------------------------------------------

(1) WHY DAVID MORRISON IS UNHAPPY WITH RECENT PRESS REPORTS ABOUT 
    'INCREASED' METEORIC ACTIVITY
    David Morrison  

(2) INCREASED INTEREST, NOT INCREASED METEORIC ACTIVITY ACCOUNTS FOR 
    RECENT FIREBALL REPORTS
    Jeremy Tatum 

======================================
(1) WHY DAVID MORRISON IS UNHAPPY WITH RECENT PRESS REPORTS ABOUT 
    'INCREASED' METEORIC ACTIVITY

From: David Morrison  

NEO News (2/20/98)

Friends and students of NEOs:

There has been a lot of press interest in bright fireballs during the 
last year. It is good that people are aware of the presenrce of bright 
meteors, and some of this media coverage has helped to inform the 
public about NEOs and the impact hazard. However, some of the press 
reports have been sensationalistc and have drawn false parallels 
between relatively common meteors and rare events that can cause 
extensive damage and casualties. Following is an especially instructive 
example in which many individually correct facts are linked (perhaps 
unintenionally) to form a quite misleading conclusion. I have made 
comments (in upper case) within the text to note how this confusing and 
perhaps misleading result is achieved.

David Morrison

-------------------------------------------------------

More Fireballs Seen in the West

By MARTHA BELLISLE

c The Associated Press  AP-NY-02-15-98 1202EST

DENVER (AP) - The phone lines to Denver's Museum of Natural History have been
buzzing since a fireball streaked across the Colorado sky last month.

That flash of light, caught on a homeowner's security camera, was not an
isolated incident; it was followed by at least four more fireball sightings,
said Jack Murphy of the museum's geology department. He hopes to find pieces
of the celestial objects for the museum's collection.

VERY FEW BRIGHT METEORS PRODUCE METEORITES. THUS MURPHY MAY HOPE TO 
FIND SOME, BUT MOST LIKELY HE WILL NOT.

As new reports of sightings keep coming in, scientists are debating the
meteorites' origin and the meaning of the increased activity. There is more
at stake, they say, than where a piece of rock fell to the ground.

WHAT METEORITES? THE REPORT JUMPED FROM METEORS TO METEORITES AS IF 
THEY WERE EQUIVALENT.

``These little things are the little brothers and sisters of the bigger
ones,'' said Doug Revelle, a scientist at Los Alamos National Laboratory.
``The reason for the interest is eventually a big one is going to hit, a real
big one. And the question is: Can we protect ourselves?''

If a large meteor hit Earth, ``life as we know it would be very different,''
he said.

REVELLE'S COMMENTS ARE TRUE BUT IRRELEVANT AS INTERPOSED IN THE STORY HERE.
IT IS A LONG LONG WAY FROM A FEW BRIGHT METEORS (OBJECTS A FEW CENTIMETERS
ACROSS) TO A KILOMETER-SIZE IMPACT.

When a fireball fell into the Earth's atmosphere on Jan. 11, a Front Range
resident's home security camera documented the bright light and shadows along
with the sonic boom caused by the apparent meteorite, Revelle said.

Scientists will use the time between the flash and boom - 132 seconds - to
help determine where the meteorite touched down, assuming it didn't burn out
before landing.

AGAIN, THIS IS A DUBIOUS ASSUMPTION, SINCE VERY FEW METEORS "TOUCH DOWN" TO
PRODUCE METEORITES.

Then, at about noon on Jan. 27, a commercial airline pilot flying over
Wyoming spotted ``a ball of flame trailing smoke.''

``He reported he did get some turbulence from the object,'' Jim Patton,
operations supervisor for the Federal Aviation Administration's flight
service center in Casper told the Rawlins, Wyo., Daily Times. ``He saw the
debris and felt the shock wave from it.''

ALMOST CERTAINLY A MISINTERPRETATION BY THE AIRLINE PILOT. METEORS ARE
TYPICALLY A HUNDRED KM OR MORE AWAY, AND IT IS ALMOST INCONCEIVABLE THAT
ONE CAME BY WITHIN A FEW HUNDRED METERS OF THE PLANE AND CAUSED TURBULANCE
OR A SHOCK. I DON'T DOUBT THE TURBULENCE, JUST THE ASSOCIATION WITH THE
METEOR. THE UFO LITEATURE IS FULL OF FALSE CAUSE-EFFECT ASSOCIATIONS OF
THIS KIND.

Residents in Breckenridge, Colo., also reported seeing that daytime fireball.
Murphy said they believe the space rock was heading south to north and landed
just north of Hanna, Wyo.

WITNESSES OFTEN THINK A METEOR "LANDED" WHEN IN FACT IT WENT BEYOND THE
HORIZON OR BURNED OUT IN THE ATMOSPHERE.

That night, another fireball broke into the Earth's atmosphere.

Scientists believe that meteorite came down in southern Colorado or northern
New Mexico, Murphy said. People in Breckenridge spotted that fireball, too.

VERY UNLIKELY THAT RESPONSIBLE SCIENTISTS THINK IT "CAME DOWN"; SEE ABOVE.

``That one was seen traveling east to west,'' Murphy said. ``It has been a
long time since we've seen one moving like that.''

WHO IS "WE"?  METEORS GO IN PRETTY MUCH RANDOM DIRECTIONS. MAYBE MURPHY
HASN'T SEEN ONE RECENTLY GOING EAST TO WEST, BUT THERE IS NOTHING UNUSUAL
ABOUT THE DIRECTION.

Another meteorite was seen and heard at sunrise in eastern Colorado on Jan.
30. And Murphy is investigating a report that came in earlier this month.

So what's happening?

``I don't know,'' Murphy said. ``We can't attribute it to anything. But it is
unusual to have so much activity.''

ONE DOESN'T NEED TO ASSUME ANYTHING CAUSED THIS UNUSUAL ACTIVITY. IT IS
PROBABLY JUST A STATISTICAL FLUCTUATION. ONLY IF THE INCREASE WERE
OBSERVED WORLWIDE WOULD THERE BE A STORY HERE.

University of Denver astronomer Robert Stencel suggested that Earth may be
getting pelted with pieces of the Hale-Bopp comet. Early in January the Earth
passed through the part of space the comet had traveled.

``Comets are like kids with muddy boots,'' Stencel said. ``They leave a trail
of debris in their wakes.''

THIS ASSOCIATION WITH HALE-BOPP NOT A VERY CREDIBLE IDEA, AS LATER NOTED BY
OTHERS.

Meteorites from asteroids breaking out of the orbital belts between Mars and
Jupiter are made up of metals, mostly iron. A meteorite from a comet would
have a lighter element composition, Stencel said.

NOT TRUE; PROBABLY THE REPORTER MISUNDERSTOOD. ONLY A SMALL FRACTION OF
ASTEROIDAL METEORS ARE IRON, PROBABLY LESS THAN 2%.

Such space debris is rare and would be of great scientific value, he said.
Scientists will test the composition of the meteorites - if they can get
their hands on them.

VALUABLE YES, BUT RARE, NO.

Revelle said he's excited about the meteorite activity, but he can't account
for it.

NOR SHOULD THE REPORTER EXPECT HIM TO ACCOUNT FOR IT.

The reports describe a smoke trail following the fireballs - or bolides,
which are exploding meteors.

``The smoke trail is an indication that the object was quite big and
strong,'' Revelle said. ``Over the globe we see objects that are about a
meter across an average of only 12 times a year.''

THIS IS A SUBSTANTIAL UNDERESTIMATE. MORE THAN 100 PER YEAR WOULD BE
CLOSER TO THE CONSENSUS FLUX RATE FOR 1 METER OBJECTS.

Last Oct. 10, a meteorite crashed near West Texas and New Mexico; then on
Dec. 9, a large fireball crashed near Greenland, and on Dec. 13, a meteorite
was seen across hundreds of miles, from Minnesota and Wisconsin south into
Iowa and northern Missouri.

NOT TRUE. BOTH THE TEXAS AND GREENLAND EVENTS WERE INITIALLY REPORTED 
AS CRASHES, BUT THERE IS NO EVIDENCE OF A METEORITE HITTING THE GROUND 
IN EITHER CASE. THIS IS A GOOD EXAMPLE OF HOW ONE EXAGERATED PRESS 
STORY SPAWNS ANOTHER.

Revelle said history may give us some insight into the meaning of the
increase in fireball activity.

About 60 million years ago, an asteroid crashed into the Earth and kicked up
enough dust to blot out the sun. Some scientists believe this resulted in the
death of more than 80 percent of all animals and led to the extinction of
dinosaurs.

"SOME SCIENTISTS BELIEVE" IS AMAZINGLY CONSERVATIVE. BETTER WOULD BE 
"MOST SCIENTISTS ARE CONVINCED". IT IS INTERESTING HOW THE REPORTER 
EXAGERATES IN DEALING THE WITH METEOR/METEORITE ISSUES BUT BECOMES VERY 
CONSERVATIVE WHEN MENTIONING EXTINCTIONS.

``These events seem to occur every 60 million years, give or take 10
million,'' he said. ``We're about three million years short of 60 million.

STRANGE STATEMENT; I DOUBT ANY SCIENTISTS THINK THESE LARGE IMPACTS ARE
STRICTLY PERIODIC.  THUS A CLEVER PHASE BECOMES MISLEADING IN THIS CONTEXT.

``In order to defend the Earth from a large meteor, we would need to know
about it while it was months away to deflect it,'' he said. ``If we knew
about it when it was weeks away, it would be too late.''

NOTE THAT USE OF THE TERM "METEOR" FOR A BIG EXTINCTION-CAUSING COMET OR
ASTEROID IMPACT MAKES THE MASS EXTINCTIONS SEEM LIKE A LOGICAL PART OF THE
METEOR STORY.
 
==========================
From: David Morrison 

Benny:
 
If you chose to distribute to the CC list my commentary on the 
Fireballs over Colorado story, you might also like to include this note 
that was sent to me. Thanks.
 
Dave
 
--------------

(2) INCREASED INTEREST, NOT INCREASED METEORIC ACTIVITY ACCOUNTS FOR 
    RECENT FIREBALL REPORTS
 
Date:         Fri, 20 Feb 98 15:42:54 PST
From: UNIVERSE@UVVM.UVIC.CA
Subject:      Fireballs, etc
To: dmorrison@mail.arc.nasa.gov

Dear David:

The Bellisle article and your comments were interesting.

I have investigated a number of fireballs over Vancouver Island in 
recent years. In particular I investigated 1995 Dec 22 and 1996 Dec 17 
in detail, driving thousands of km to interview witnesses in situ and 
making measurements with compass and clinometer, and getting witnesses 
to re-enact their movements to estimate the time between sight and 
sound, and analysing seismograph data and so on. Witesses have included 
captains of ships and pilots of aircraft, as well as ground-based 
witnesses. In both cases it took more than a year between appearance of 
the fireball and completion of the analysis and submission for 
publication, and in both cases the conclusion was the same, namely:  
There is insufficient evidence to say whether or not an impact 
occurred, or, if it did, where, within 250 km, the impact point is 
likely to be.

Yet I keep reading reports of fireballs followed by "scientists" within 
24 hours saying where the meteorite landed, as though it were no effort 
at all to reach such a conclusion. A typical example was the Greenland 
event. Within days people from the Niels Bohr Institute were saying 
where the meteorite landed, on the basis of reports from four trawlers 
(I have found without exception that reports from ships and from 
aircraft are totally useless as far as getting accurate angles are 
concerned), a videoed image of a reflection from a car hood, 
seismograph records (which certainly were not of an impact, if indeed 
they had anything to do with the fireball) and clouds detected by a 
satellite and having nothing whatever to do with the fireball.

The investigation of a fireball is a long drawn-out affair that takes 
weeks to do, and with very uncertain results. Indeed it is a serious 
question as to whether the effort of interviewing eye-witnesses is 
really worthwhile at all. The idea that one can telephone a couple of 
people and go out the next day and pick up the meteorite over the next 
hill is nonsense.

Fireballs do seem to come in groups. A long gap with no fireballs, and 
then a flurry of several within weeks. This could just be a part of the 
Poisson distribution, but it's actually an expression of public 
interest. The same phenomenon is well-known in birdwatching: one rarity 
generates others in the following days. One well-publicized fireball, 
and then everyone is seeing them (often just bright shooting stars).

The time between flash and boom is important, but it is important to
note that the boom is not necessarily (or even likely) caused by an
impact. There is a supersonic boom as the meteoroid comes in at 
supersonic speed, and often there is a terminal burst if the object 
explosively disintegrates. Both of these phenomena generate acoustic 
waves, and they take place tens of km above the ground. The 
interpretation is not straightforward. Even in the unlikely absence of 
wind, the sound does not travel in a straight line, because the sound 
speed varies with temperature (height) in the atmosphere. If the 
temperature lapse rate in linear, the sound paths are cycloidal arcs.
I shall be submitting a paper shortly to Meteoritics on the propagation
of sound from fireballs.

The turbulence experienced by the airline pilot clearly had nothing
whatever to do with the fireball, which, as you say, would have been
100 of km away.  It is almost universal that witnesses think that the
object is very close.

Re the report that the object was travelling from N to S - another very 
common mistaken assumption. If the witness was facing west and saw the 
object move from right to left, that's all that can be reported. It 
does NOT mean that it was going from N to S. In the absence of any 
radial velocity information, all the witness is seeing is the 
transverse component of the path. The object could have been
going towards the south +/- 89 degrees either way!

You are being very kind in saying that the suggestion (by an astronomer!)
that we are being pelted by Hale-Bopp is "not a very credible idea"! I 
would have said that it was preposterous for an astronomer to suggest 
any such thing! In a similar vein, some astronomers actually suggested
to the press that the 1996 December 17 fireball over Vancouver Island 
might have been associated with the Geminid shower - and a re-entering
satellite that appeared here in November was associated with the Leonids!
These were SCIENTISTS who were saying such things - one cannot blame the
newspaper reporters. Again, the Bellisle article said: "Scientists 
believe the meteorite came down..." and you commented that it was "very 
unlikely that responsible scientists think it came down". Not quite 
fair to the reporter. After all, she did not make any claim that the 
scientists who believe it came down were "responsible" scientists. She 
would just have been reporting what scientists told her; not 
necessarily responsible ones!

Another point on which the reporter may have been on slightly firmer
ground that you credit her for is her "some scientists believe" in the 
K/T asteroid. I think your "most scientists are convinced" maybe errs a 
bit in the other direction! Incidentally Alan Hildebrand thinks it was 
a comet, because the Ir abundance at the K/T boundary agrees with 
cometary Ir abundance. Ir has never actually been detected in a comet, 
of course, but Alan's a good friend, so I shan't push that minor 
detail...

Anyway, it was an interesting article, with some misconceptions and 
exaggerations, but good to see that there is some public interest in 
the subject. It is also of interest, I think, that it is not only 
newspaper reporters (who are working to a deadline day after day on
a hundred different topics) who sometimes get things not quite right,
but scientists who ought to know better.  A lot of the stuff about the 
Greenland event, for example, must have been given to the press by 
"scientists" on flimsy evidence almost immediately after the event.

I enjoy your NEO NEWSes.

Jeremy Tatum