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From: Henry Spencer <>
Subject: Re: Best response to incoming planetary threat?
Date: Sun, 21 Jul 1996 20:54:47 GMT

In article <4seuh7$> writes:
>How many days to develop a nuclear weapon that could be flown into

All existing ICBM warheads are designed to be flown in space... briefly.
Revising the design to spend days or weeks in space might present some 
problems, although it would be difficult to assess details without access
to design information which is probably classified.

A much more fundamental problem is that available warheads are *small*.
The biggest ICBM warhead the US ever fielded was 9 megatons, on the old
Titan II... and since the Titans have been retired, those warheads have
been retired and quite likely are no longer available.  I don't know just
what is in second place, but I doubt that it's much over one megaton. 
Even in the Soviet Union, the really gigantic bombs probably never entered
operational service at all, and certainly are long gone now.  Big H-bombs
are inefficient -- they spend much of their energy re-re-re-re-pulverizing
the area immediately around ground zero -- and the military prefers a
pattern of smaller bombs for destroying large targets.

But for asteroid deflection, especially if time is sufficiently tight that
one gets only a few shots, you want the biggest possible bombs.  The MIT
student study in the late 1960s picked 100MT, not because it was optimal
for the job but because it appeared to be a good compromise between power
and mass, *and* because it seemed to be close enough to the state of the
art (in the mid-1960s) that it could probably be made available in a year 
or two with reasonable assurance that it would work.

If you wanted something on seriously short notice, you'd probably have to
settle for 9 megatons -- there's an aircraft variant of the Titan warhead,
meant for the B-52, which is probably still available.  If you had a bit
more time, the biggest bomb the US has actually tested was about 15
megatons (it was meant to be 5, but actually delivered 15 due to a small
oversight...), and a few of those could probably be put together without
needing to start from scratch.  This is pretty small for doing anything
about an incoming asteroid, I'm afraid. 

>(This is assuming the STS was used. I imagine it'd be easier just to
>use a modified ICBM-type missle, eh?)

No, the ICBMs are too small.  Considering that you want to launch the
biggest possible bomb well beyond escape velocity, nothing short of a
heavylift launcher will really work well; even Proton or the shuttle is
not really big enough.  The MIT study had the Saturn V available.  (For
those who want to play with the numbers, they estimated 40,000 pounds for
the bomb and its associated casing, safety systems, etc.)

If you wanted to do the best you could on seriously short notice, you are
probably best advised to forget the shuttle and tell the Russians that you
want to buy a Proton launch in a big hurry and will pay a large premium
for fast results. 

Actually, no, if you want to do the best you could on short notice, you'd
do that *and* get the shuttle launch crew moving fast *and* get the USAF
to tell LM to prepare a Titan IV in a big hurry.  The more bombs you can
launch in a hurry, the better.
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Henry Spencer <>
Subject: Re: Best response to incoming planetary threat?
Date: Sat, 27 Jul 1996 15:08:35 GMT

In article <> "Charles P. Tomes" <> writes:
>> Unfortunately, there's a serious problem here:  to effectively use
>> multiple warheads simultaneously against a small target, you need *very*
>> precise detonation timing, which those warheads aren't equipped for.
>> Otherwise the X-ray pulse from the first warhead kills the rest before
>> they detonate...
>Aren't they hardened to survive a relatively powerful pulse?

They're reasonably durable, but there are limits -- hardening makes the
electronics less vulnerable, but can't really protect against pulses that
are strong enough to do major physical damage.  Remember, the whole point
of this exercise is doing major physical damage to the surface of the
incoming rock, and the X-ray pulse carries much of the energy for that.
Granted, the bombs will be closer to the rock than to each other, but
that won't help *that* much.

>...If you time the detonations with the same kind
>of precision that is already demonstrated with GPS satellites, wouldn't the 
>delta-v of the individual warheads be enough to spread the bombs out?

The individual warheads don't have any delta-V, at least not in orthodox
systems -- the bus that carries them does all the deployment work.  Alas,
this would almost certainly have to be done at the last minute to achieve
the necessary accuracy, so there are severe limits on how much spacing can
be arranged.  (The problem is not relative positioning, but hitting the
rock, whose position won't be known nearly as accurately as that of the
usual ICBM target points.)

Also, here again we start talking about extensive redesign of the support
hardware (e.g., for detonation timing), which is not something that could
be done overnight.  Building a bigger single bomb might well be quicker.

In any case, even if we do all this, we're still talking about a total
of only a few megatons, which is marginal at best unless the rock is
not very big.  The basic problem remains:  existing off-the-shelf bombs
are just too small for this job.
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Henry Spencer <>
Subject: Re: Best response to incoming planetary threat?
Date: Fri, 26 Jul 1996 14:39:51 GMT

I wrote:
>...If you had a bit
>more time, the biggest bomb the US has actually tested was about 15
>megatons (it was meant to be 5, but actually delivered 15 due to a small

One or two people have asked for an expansion on this, and I suppose it
might be of more general interest (although rather peripheral to this
newsgroup -- please take followups elsewhere unless space-related)...

The "Castle" test series of 1954 tested the US's first combat fusion bombs
(the "Mike" test of 1952 -- the first fusion explosion -- was basically a
physics experiment and its bomb was not even vaguely portable).  First of
the lot was test "Bravo", testing a bomb design named "Shrimp", meant to
be five megatons.

Shrimp, like most of the Castle bombs, used lithium deuteride as its
fusion fuel.  It was, in fact, the very first lithium-based fusion bomb. 
Lithium-6 is the bomb isotope, but lithium isotope refining was only just
getting started, and Shrimp's lithium was only 40% Li6, the rest being the
unreactive Li7.  Now, you can get Li7 + n -> Li6 + 2n, but the physics
people thought the rate of this would be negligible.  They were wrong. 

The result was a 15MT explosion, which smashed instruments, terrified
observers, and spewed fallout far and wide.  The test crews spent the next
day or two decontaminating men and ships, and digging people out of
collapsed observation bunkers.  There were no deaths in the test force,
but the crew of a Japanese fishing boat, passing by outside the official
fallout danger zone, weren't so lucky -- they ended up in hospital with
radiation sickness, and one of them died. 

Several of the later Castle tests also ran away in the same manner, but
then people were prepared.  None was quite as big as Shrimp. 

All of this was very highly secret at the time, which is why official US
statements on the subject of the fishing-boat accident sound so contrived. 
As far as I know, the first open publication of the real story was in
Richard Rhodes's recent book "Dark Sun" (which I highly recommend).
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Josh Hopkins <>
Subject: Is asteroid deflection really that difficult?
Date: Sat, 01 Mar 1997 22:37:56 -0800

I admit to only skimming the recent discussions here on the value of an
asteroid defense system, but I haven't seen anyone address the
question I've been pondering: Is a viable asteroid defense system really
that difficult?  So, I was inspired to sit down and do some calculating.

As far as I know, the first really serious look at an asteroid defense
system was an MIT study in the late 70s (?).  I've never actually read
the study, but my recollection is that they assumed that the
Earth-crossing asteroid Icarus, which at the time was about to make a
relatively close approach to Earth, was instead about to impact.  My
impression is that a short reaction time (on the order of less than one
year notice) was one of the assumptions.  The design solution developed
during this study was to use one or more Saturn V rockets, carrying very
large nuclear bombs, to deflect Icarus.  I dredge up this history,
because it seems to have set the paradigm for many current assumptions
about asteroid defense.  A typical scenario is that a large dino-killer
class asteroid (say, 5km diameter and up) is discovered on an impact
course with Earth, and the intrepid humans have only a short time to stop

Now, this scenario is very dramatic and challenging, which may explain
why it shows up in documentaries, cheesy network movies, science fiction,
and also in some relatively serious studies by people who, it might be
argued, are looking for good excuses to build big bombs and rockets.  In
a case like this, we really would need those large nuclear weapons and
advanced space propulsion systems.  Big objects are hard to deflect.
They need to be deflected pretty hard if they are close by, and you may
need a very energetic trajectory to intercept an asteroid if you don't
have time to pick a favorable alignment.  Since it would take years and
billions of dollars to develop these new systems, and since the
assumption is that we would not have much time to react, some people
argue that we need to spend some money now, so we won't be caught flat
footed.  Conversely, others argue that the whole idea of asteroid defense
is silly, because we would have to spend lots of money to protect us
from something that probably isn't going to happen anyway.

The problem I have with this scenario is that it isn't very realistic.  I
would argue that a viable deflection system is actually not all that
difficult.  If we can, in fact, deflect asteroids with existing rockets
and technology, then we don't need to spend money on deflection unless
and until we detect a real threat.  Moreover, we can focus on an asteroid
detection system (which is cheap and quite useful even if there are no
near term impacts to predict) without politicizing the whole thing with a
debate over whether this would mushroom into a big, expensive hardware
development program.

Two references I would suggest for this discussion are:
"Deflection and Fragmentation of Near-Earth Asteroids," by Thomas Ahrens
and Alan Harris.  You can find this in the Dec 3, 1992 issue of Nature,
starting on page 429.  It discusses methods of asteroid deflection.

"An International Program to Protect the Earth from Impact Catastrophe:
Initial Steps," by David Morrison.  This one is in Acta Astronautica, Vol
30 pp 11-16.  It discusses the risks of various sizes of impactor.
There may well be better papers on the subject - I just happen to have
these in my files.

There are two problems with the traditional scenario:
1) There are relatively few really big Earth-crossing asteroids, and
because of their size they are the least likely to be sneaking up on us.
Smaller asteroids, while somewhat less dangerous, are much more numerous
and more difficult to detect.  Instead of a 5-10 km asteroid, a 0.5-2 km
asteroid is a much more likely target for deflection.  Asteroids even
smaller than this are still more numerous and harder to detect, but also
much less dangerous.

2) Most asteroids are not discovered when they are approaching close to
the Earth, but when they are rather farther away.  Sure, there are
several cases of asteroids that weren't discovered until they were closer
than the Moon.  However, the cases I am aware of were all 10 m class
asteroids.  These are quite difficult to detect from long distances, but
are essentially harmless.  We can probably expect to detect an
threatening asteroid more than a decade before it would actually hit us,
especially if we make some effort to do so.

These are very important differences.  Smaller asteroids are much easier
to move.  If you have notice several orbits in advance, a successful
deflection is possible with a much smaller velocity change.  And finally,
you can intercept an asteroid with a much less energetic trajectory if
you can pick a convenient time to do so.  All of this means that smaller
explosions and smaller rockets should be sufficient to stop most

I won't repeat the math here, but the article in Nature concludes that if
you can deflect an asteroid several years (i.e. several orbits) before it
would otherwise threaten Earth, a Delta-V on the order of 0.01 m/s is
probably sufficient to cause the asteroid to miss.  This compares quite
favorable to the roughly 0.5 m/s you would need in the traditional (i.e.
less than one orbit warning) scenario.  The Nature article also concludes
that a nuclear explosion of  10-100 kilotons yield is sufficient to
impart the necessary impulse to an asteroid in the 1km diameter class.
A 10 km asteroid would require a fraction of a gigaton to deflect, even
with sufficient advanced warning.

So, how hard is it to deliver a warhead of, say, 50 kton to an
uncooperative asteroid?  I don't follow strategic weapons very closely,
so I don't have any appropriate references on existing warheads.
However, I ran across a Lockheed Martin web page on the Peacekeeper
missile at
It states that the throw weight of the missile is about 3500 kg, and the
payload is "10 Mark 21's".  Now, I don't know what the yield of a Mark 21
is, or how much of that weight is the "physics package" and how much is
the reentry aeroshell and other unnecessary hardware.  However, this
appears to me to suggest that the relevant part of a 50-100 kton nuclear
device can be built for about 300 kg or less.

In addition to a warhead, we need a spacecraft bus which can operate for
months or a few years in space, and deliver this warhead to the asteroid.
In particular, we need to be able to detonate the device a few hundred
meters from the asteroid, on the appropriate side.  Precision on the
order of several tens of meters is useful for maximizing the effect of
the blast.  (For an explanation of this, please see the Nature article.)
It happens that NASA has already launched a lightweight spacecraft capable
of carrying a heavy payload to an interplanetary target.  The spacecraft
which is carrying the Mars Pathfinder entry probe masses about 250kg wet
(i.e., fully fueled).  It has power, attitude control, medium rate
telemetry, and enough propellant for trajectory corrections, but not
for any significant changes to the orbit.  Another example is the Deep
Space 1 probe for New Millennium.  While it isn't designed to carry a
large external payload, it does have cameras and a high data rate telecom
system, which would be useful for precise targeting.  It also has a
significant capability for deep space propulsion (albeit slowly), thanks
to the integral ion-thruster.  Its wet mass is about 350 kg, although I'm
not sure whether pressurized Xenon should be referred to as "wet."  There's
every reason to believe that we will soon be building these kinds of
spacecraft even lighter, with even more capabilities.

So, it appears that a spacecraft designed to deflect an asteroid can
probably mass about 600 kg, including the nuclear device.  How big a
launcher do we really need to launch this?  According to Isakowitz,
(referenced more thoroughly in the space launch services FAQ) the Atlas
IIAS can launch a 600kg payload onto an interplanetary trajectory with a
C3 of about 50km^2/sec^2.  Without any effort at optimizing, I've
calculated that adding a Star 37 solid kick motor can increase that to
around 75km^2/sec^2.  So, how fast is this?  Well, it won't let you scream
around the solar system, ignoring conventional behavior in orbital
mechanics the way that the early Pluto Fast Flyby trajectories would.  It
is, however, none too shabby.  For comparison, the Near Earth Asteroid
Rendezvous mission was launched with a C3 of about 26 km^2/sec^2.  While
NEAR will need some additional energy to eventually rendezvous with its
target asteroid, that energy is sufficient to allow it to intercept the
Main Belt asteroid Mathilde on its way, sometime in June.  If  75km^2/sec^2
doesn't seem high enough, I think further improvements in the kick motor
selection and trajectory architecture can increase the performance.
Moreover, more powerful launch vehicles like Atlas IIAR and Sea Launch
will soon be available at similar prices to the Atlas IIAS.  Therefore, we
should be able to intercept even relatively uncooperative asteroids without
resorting to heavy lift vehicles, as long as we have several years'

Now, I'm not suggesting that any particular combination of current hardware
should be used, because I am skeptical of  back of the envelope "plug and
play" engineering.  The specific hardware listed above is just an example
of existing capabilities.  I'm particularly not suggesting that anybody go
out and start integrating this stuff unless a legitimate asteroid threat is
detected.  However, it seems pretty clear to me that if we ever need to, we
can respond to most (not all, but most) potential impact threats with
existing capabilities.  Just as an estimate, it might take us two years
from start to launch, and might cost us a few hundred million dollars per
shot, including development.  It should not require any dramatically
advanced technology, and it definitely shouldn't require us to start
developing anything now.

Josh Hopkins

From: (Jordin Kare)
Subject: Re: Ground spotting adequate (was Re: Asteroid Defense Network)
Date: 19 Apr 1997 06:02:32 GMT

In article <>, (Bill
Higgins) wrote:

> In article <>, writes:
> >>>>
> >>>>Any plans for designing/puting up a DEW Line like defensive
> >>>>networked of satellites and maybe even some sort of anti-missle
> >>>>network as well. Maybe Star Wars can be reinvented but for world
> >>>>defense?
> [...]
> > Rather than using brute force, if we set up observation posts (preferably
> >space-based) and determine likely hostile bodies in advance, then we can use
> >more subtle methods to change the orbit.
> To a great extent, ground-based observations are adequate to the task,
> and much cheaper. (You can redefine the task to require space-based
> observatories if you like; I'm talking about global-catastrophe
> asteroids around 100 meters radius or larger;  you may wish to track
> smaller ones.)

Having taken part in a couple of conferences on the topic and witnessed
some impressive arguments (e.g., Tom
Gehrels screaming from the audience at a speaker who proposed a
space-based asteroid watch) herewith some comments:

Ground-based searches have a fundamental limitation in that they cannot look
close to the Sun.  Most Earth-crossing asteroids spend most of their time
within 90 degrees of the Sun as seen from Earth, and some
(the Atens?  it's been a while)
spend virtually all their time closer to the Sun than the Earth, and thus
never get even 90 degrees from the Sun (until they hit us :-( )

Space based searches are thus potentially superior to ground-based
searches, as they do not have atmospheric scattering to contend with
and can look relatively close to the Sun with full sensitivity.
Space-based searches can also operate in the thermal IR, which
increases the search sensitivity and allows working even closer to the Sun.
There's a gentleman at Los Alamos who has modelled this extensively
and there are many orbits in which asteroids would simply will not be seen by a
ground-based search unless/until they hit us.

On the other hand, space-based systems are certainly more expensive
than ground-based over the next few years, so it's not unreasonable to
take the position that a space-based search should wait until a
full-fledged ground-based asteroid watch is in place; measuring Earth-crossers
found per dollar probably favors this.  On the third hand, though,
satellites are "sexier" than ground-based systems, and even a relatively
expensive satellite may be easier to get funded than a ground-based
search, especially if it can also do other unique science.

With regard to using subtle versus violent deflection, large asteroids
can almost certainly be cataloged well in advance of impacts (subject to
the near-sun observations noted above) and deflected "gently"
Smaller rocks (10 - 100 m) may not be detectable, especially from the
ground, at
long enough range to catalog them all -- but they're also going to be
hard to see on final approach, and will thus require "instant"
response (e.g. <1 hour) to intercept; they may be more easily fragmented
than deflected.  (Fragmenting bigger rocks is probably a bad idea --
too much chance of ending up with lots of still-dangerous pieces or
of doing serious thermal/environmental damage when the debris hits the
atmosphere even if you reduce them
to powder).  Small rocks, however, don't threaten global catastrophe
and may thus be less urgent to defend against (although
a 200-foot tidal wave on every Atlantic coastline seems like a
big enough catastrophe to me...)

Comets, however, are potentially "first pass deadly" on the global
scale.  They
can arrive from the outer Solar system or even
interstellar space with no warning.  If these comets don't contain
lots of volatiles, they are currently more-or-less indetectable
except at close range (<1 a.u.).  They also move faster relative to
the Earth and thus have much more energy per unit mass than
asteroids, so a smaller object can cause more damage.  They can
also arrive from any angle, including far from the ecliptic or
very close to the Sun.

So no long-term search/gentle deflection system can completely protect the
Earth.  The argument is over the relative odds of an asteroid impact
vs. a comet impact, generally believed to be somewhere between 50-50
and 10-1 in favor of asteroids.

So do you spend a little money and give yourself 50% (or maybe 90%)
protection?  Or do you spend much more and give yourself (somewhat)
better odds.  You pays your money and you takes your chances....

Jordin (rock-et scientist) Kare

Jordin T. Kare

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Newsgroups: sci.astro,sci.misc,sci.physics,sci.skeptic,,
Subject: Re: Tunguska impact, anti-mattter, black-hole, ocean surge
Date: 16 Apr 1997 08:55:59 GMT wrote:
: The object was almost certainly a small comet which vaporized before
: impacting the Earth.  However the shock wave *did* strike the Earth,
: flattening the trees, etc.

A comet is too fragile to get to the depths that the Tunguska
fireball got to (about 8 kilometers altitude).  It has been shown
that the object was likely a stoney asteroid about 50 meters in
diameter.  Comets break up too high while stronger nickel-iron
asteroids survive to hit the surface.

: Hesperos, M.P.


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: Henry Spencer <>
Subject: Re: Effect of Asteroids On Trajectories
Date: Sat, 20 Apr 1996 05:34:17 GMT

In article <> writes:
>  ...I could imagine 
>  that if the distribution of all boulders > 1 m were known some  
>  conclusions might be drawn from an analysis thereof?

Yes, but distributions can generally be established by well-planned
sampling.  Exhaustive enumeration is not needed, and indeed is scarcely
ever cost-effective.

>  Are there 
>  no scientific benefits from tracking all the tiny objects in
>  Earth orbit?

(I assume you mean "near-Earth orbit"?)

Basically none.  A representative sample will get you just about everything
that's worth knowing.

>  As I understand it nobody has established as yet whether the 
>  Tunguska meteorite was a small asteroid or a comet. I also don't
>  recall any estimates of its size before impact.

Actually, both are (tentatively) fairly well known now, based on some work
done at Goddard a few years ago.  The known damage indicates a 10-20MT
explosion about 8km up.  Comets break up well before then; even the
tougher short-period comets don't get below maybe 20km.  Chondritic
asteroids last longer and are an outside possibility, but they're still
fragile enough to break up a bit too early.  Nickel-iron asteroids of the
right size reach the surface, and we know there was no crater at Tunguska. 
Stony asteroids are the only thing that fit the bill:  they would break up
at 8-10km.  A stony asteroid 50-100m across (depending on density) is the
only really good fit.
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Newsgroups: sci.astro,sci.misc,sci.physics,sci.skeptic,,
Subject: Re: Tunguska impact, anti-mattter, black-hole, ocean surge
Date: 17 Apr 1997 16:12:45 GMT

Dan Evens ( wrote:
: Any relation to Wayne?

: David R. Throop wrote:
: > I read a theory years ago that the Tunguska impact (1908) was caused
: > by either a piece of anti-matter or a black hole hitting the earth,
: > passing right through, and coming out the other side.

: Nobody is particularly sure what hit. There are plenty of
: theories but little hard evidence. An expedition a few years
: after the impact found the site, but nothing really conclusive.
: They dug holes, somewhat halfheartedly since it was a swamp,
: took some pics, then went home.

Quite the contrary.  Although the meteor itself has not yet been
found (though I've heard continued plans which may yet yield its
vaporized remnants), models of the entry dynamics makes it rather
clear that the object was most likely a stony asteroid, not a
comet or a stronger nickel-iron body.  The hard evidence is in the
flattened trees, eyewitness accounts, and other peripheral data
acquired around the globe at the time of the impact such as
barometric readings.

: All that is really sure is that it was a very large explosion.
: Exactlly how large (as large as a megaton? doubtful but maybe)
: is not well established either.

The size of the explosion is reasonably well known and was
around 15 Megatons equivalent of TNT which corresponds to about
a 50 meter stony asteroid.  The explosive yield was determined
by examining the barometric readings, and analysis of the blast
damage in the region, and has been recalibrated thanks to the
nuclear explosions, especially the airblasts of hydrogen bombs
of comparable size to the Tunguska blast.

: > But the 'boiling seas' report would fit with a more recent theory,
: > that a boloid impact would give rise to a shockwave that would travel
: > through the earth and focus anti-polarly.

I doubt this report can be correlated with the Tunguska explosion
itself.  Were there any similar observations coincident with large
atmospheric nuclear explosions?  I suspect the Tunguska explosion
and its nuclear cousins are far too small to provide any significant
antipodal side affects, but if correlations can be found with the
similar sized nuclear explosions, then I'd like to hear about them.

: The focus would be pretty piss-poor. Think about the radically
: different kinds of stuff on different paths. Surface waves,
: for example, would be travelling radically different combinatrions
: of land, water depth, weather conditions, etc. Even the waves
: that went straight through the Earth would have to go through
: different distances of ocean depth etc. to get to the actual
: ship. Even if it was a perfectly homogeneous sphere, spheres
: don't focus things very well. It certainly would
: not amount to any kind of "boiling seas."

Yeah, that about sums up my concerns, although I can imagine the
possibility of antipodal focusing of energy - there's just far too
little of it to be convincing.  The barometric evidence, however,
shows the atmospheric shock wave traveling around the globe twice.
Now, when you get into the impact of a multi-kilometer sized body,
like that which hit at the K-T boundary, I can easily imagine
geological side affects during the creation of a 200 kilometer

: Dan Evens


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

Date: 7 Aug 92 01:29:30 GMT
From: Jim Scotti x2717 <>
Subject: Meteor Soaks Datona FL

In article <> (Rick Emerson) writes:
> (Greg F Walz Chojnacki) writes:
>> From article <1502@tnc.UUCP>, by m0102@tnc.UUCP (FRANK NEY):
>> > 
>> > -----I quote-----
>> > A giant wave that drenched Datona FL and caused a lot of damage
>> > in July turns out to have probably been caused by a 1 meter 
>> > meteor!  
>> >  
>> IS there any source on this <alleged> meteor event other than a TV news repor
>> Greg
>Yes.  A very definative report from a guy in a boat who saw a flash around
>the same time the wave hit.  I guess that wraps up that issue.
>If a 1M rock dropped in at speeds roughly on the order of kilometers per
>second, there'd be a darn sight more than a splash.

A couple of comments:  First, from the numbers of 10 meter asteroids 
(or perhaps I should call them meteoroids) which have been discovered 
in the last two years by Spacewatch, and from the number of bright 
fireballs, we can estimate the number of impacts per year that should 
be expected from objects less than 10 meters in diameter.  We expect 
that about 10 objects of about 10 meters or larger should impact the 
Earth each year.  Most of them will burn up or fragment into smaller
pieces high in the atmosphere probably resulting in a shower of smaller
objects which are occasionally seen to land.  We can also estimate that
about 5000 objects of 1 meter or larger impact each year, or about 14 each
day!  In other words, impacts of these sized objects are a common occurance
and almost always results in simply a bright fireball and occasionally
in falls of meteorites.  Second, if a small object were to actually
survive to hit the surface of the Earth with cosmic velocity intact, a 
1 meter object would probably make a crater about 10-20 meters in diameter.  
A 10 meter object, impacting at around 20 km/sec would make a crater 
around 200 meters in diameter.  It is unlikely however, that such an 
object would maintain its cosmic velocity.  Rather than impacting at 
20 km/sec, it would more likely be decelerated to perhaps only 5 km/sec 
if it survived intact.  A 1 meter object deposits about 0.6 kilotons of 
TNT equivalent into the atmosphere/ground, while a 10 meter object deposits 
about 60 kilotons of TNT equivalent into the atmosphere/ground.  An object 
10 meters in size surviving to impact at 5 km/sec would deliver about 
4 kilotons of TNT equivalent energy and I suspect might make the kind of 
wave seen here.  I would guesstimate that maybe 1 in 100 objects of this 
size are strong enough to survive the dynamical stress of atmospheric 
entry to reach the surface of the Earth more or less intact.

Sorry this got so long, but I would conclude that the probability of
seeing impacts of objects larger than 1 meter diameter is very high and
that it would require an object of at least a few meters to have caused
a wave as seen in this case.  A 1 meter object would likely not strike
the surface with enough velocity to cause "a darn sight more than a 
splash", though it would make a nice little splash.

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

Date: 8 Oct 92 15:39:41 GMT
From: Jim Scotti x2717 <>
Subject: Impact in AD 2000?

In article <> roberts@CMR.NCSL.NIST.GOV (John Roberts) writes:

>-From: (Nick Szabo)
>-Subject: Re: Toutatis impact in 2000 AD? (was Re: Help !)
>-Date: 5 Oct 92 06:55:07 GMT
>-This translates to (2e1/1e6)*(5e9) or 10,000 dead people, actuarily 
>-speaking, or 0.2 per 100,000 population.  The death rate from
>-airline crashes is 0.04 per 100,000 people.
>  {Some stuff deleted....}
>I notice you kill just about everybody on the planet, no matter where the
>impact. An impact of a two-mile-diameter asteroid would be pretty severe,
>but I'm not sure it would be quite *that* bad. Isn't that only about a
>tenth of the mass of the alleged "dinosaur killer" asteroid?

We believe that the threshold for devistating global effects by an 
asteroid impact occures with asteroids of about 1/2 to 1 kilometer 
diameter.  The most damaging effects for humans are due to dust and
debris flung into the atmosphere, blocking sunshine and affecting the
global climate.  It is akin to the hypothesized "Nuclear Winter".  By
the time an asteroid is two miles in diamter (about 3 kilometers), you
are well above what we believe would cause such a disaster.  Even the
affects of something as small as about 50 meters has caused noticeable
global affects.  Namely, the Tunguska impact event in 1908 caused 
widespread local devistation and the atmospheric shock was measured
as far away as Great Britain.  Also, the sunsets were enhanced for
several days following the event.  Recently, large volcanic eruptions
have spewed large amounts of dust into the air, causing remarkably
enhanced sunsets for several months as well as perhaps some global
weather changes.  These recent volcanic eruptions are small compared
to the impact of small asteroids larger than a few hundred meters in

>The analysis you refer to is apparently just a worst-case scenario - it
>wasn't meant to reflect the actual uncertainty in the trajectory. I believe
>a later post gives the uncertainty as a fraction of an Earth radius.

Yes, this analysis was (more or less) a worst-case scenario.  But this
scenario can happen with much smaller asteroids than you might expect.
Devistating local affects can happen with asteroids (and comets) as small
as 50-100 meters in diameter, and from the recent discovery rates of 
such small objects by Spacewatch, we can estimate the probability of
such an impact.  We think that 100 meter objects (which, by the way,
are at about the size threshold that we expect to survive more often
that not to impact on the surface of Earth rather than break up or
explode higher in the atmosphere) should impact the Earth about once
every century.  I don't think it is a coincidence that the Tunguska
event happened within the last 84 years.  We may have had at least one 
close call since.  A small object (size estimates ranged from 5 meters 
to about 50 meters - I'd guess it was probably about 20 meters diameter) 
skimmed through the atmosphere in 1972 over the western US and Canada, 
missing an impact by a miniscule 10-20 kilometers or so (if it had been 
that much lower, it would have likely been slowed enough by the thicker
atmosphere to have come down instead of bounced back out).

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

Date: 31 Oct 92 07:55:47 GMT
From: Jim Scotti x2717 <>
Subject: "Earth Gains a Retinue of Mini-Asteroids"
Newsgroups: sci.astro,

In article <> (Paul Dietz) writes:
>Science (10/16/92, page 403), reports that Gehrels and colleagues in
>the Spacewatch program have detected 8 very near earth asteroids over
>the last two years with sizes from 5 to 100 meters.  This implies that
>at any time, there are as many as 50 mini-asteroids passing between
>the Earth and Moon every day.  This figure is some 100 times larger
>than had been inferred from observations of the number of larger
>Since the Tunguska event is thought to have been due to a 40 meter
>body, and such events were calculated to occur once every 2 to 3
>centuries, something is screwy here.

Not really screwy.  The number of 50 meter objects is enhanced by
about 10 times and the Tunguska type events probably happen once
or a few times per century.  Remember, 3 out of 4 enter over water
and may be less likely to be detected.  Also, perhaps a large
fraction of them disintegrate higher in the atmoshpere, causing
a much smaller disturbance in the lower atmosphere than that of
the Tunguska event.  Also, the best guess I've heard for the size
of the Tunguska progenitor is between 50 and 100 meters.  The 10
meter sized objects are enhanced by a factor of 100 and the trend
from the larger objects to the smallest is a gradual transition
that starts at around 100 meters size.

>The orbits of the bodies are unexpectedly similar to Earth's.
>Two have orbits more like earth's than any known body; one was that
>asteroid that was mistaken for a spent rocket body.

Also known as 1991 VG.
>These couldn't be Frank's minicomets, could they?  His putative
>objects are supposedly in prograde, earth-like orbits, to reduce the
>impact velocity enough to avoid observational constraints.

Frank's minicomets were estimated to be 10-30 meters in size.  The
number of objects in this size that he estimated to account for
what was probably detector noise was at least a million times the
the number extrapolated from the larger Near Earth asteroid population
and therefore is a population at least 10,000 times more numerous than 
what has now been found by Spacewatch.  In short, with our sensitivity,
we should see at least 1000 of Frank's minicomets each NIGHT!!!!!!  I
think the Spacewatch survey has now effectively disproven Frank's
hypothesis used to explain his so called "atmospheric holes".

Incidentally, an earlier CCD developed by Spacewatch in the
early 1980's (an old RCA 320x512 chip) was used by a colleague
of Frank's from JPL to survey for the minicomets directly.  This
investigator claimed to have detected at least one such object
on two consecutive images.  Without consulting the Spacewtach
crew, he announced his "discovery".  When we were finally able
to look at his images, we concluded that what he was looking 
at was detector noise!

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

Date: 4 Nov 92 20:28:22 GMT
From: Jim Scotti x2717 <>
Subject: "Earth gains a retinue of mini-asteroids"

In article <> ("Patricia Reiff (713")) writes:
>In a recent SD, (Phil G. Fraering) writes:
>>>These couldn't be Frank's minicomets, could they?  His putative
>>>objects are supposedly in prograde, earth-like orbits, to reduce the
>>>impact velocity enough to avoid observational constraints.
>>I'd bet Frank will claim that these bodies represent the "larger"
>>members of his mini-comet population, but I don't know.
>I can affirm that Lou Frank considers these results to be confirmation of 
>the "small-comet" hypothesis, and his calculations show that the fluxes are 
>approximately correct.  
>  {Some stuff deleted...}

On the contrary, in his earlier work, Frank made some estimates for 
how many small comets are necessary to account for his detection rate.  
The rate of impact on the Earth of his cometesimals is about 10 million 
per year.  The rate of impact of our 10 meter objects is on the order of 
about 10 per year.  I believe his estimates of size placed his objects 
in the 10-30 meter class.  He would therefore expect about a million 
times as many objects as we are seeing or we should detect on the order 
of 30000 of his cometesimals EACH NIGHT!  I think we quite effectively 
disprove his hypothesis on the existance of cometesimals as he has 
proposed them.  (Note:  My estimate of 30000 per night differs from
my estimate in an earlier post since I made the earlier estimate from
my memory of his work.  I looked up his estimate before revising it and
I was apparently over conservative in my memory, having said earlier 
that we should detect about 1000 per night.)

I'd like to see his latest estimates for the number and size of his
small comets required to account for his detection rates.  He must
have downsized them considerably to account for the 6 order of magnitudes
required to reconcile the difference between his estimates and the
Spacewatch discovery rates.

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

Date: 4 Nov 92 20:47:44 GMT
From: Jim Scotti x2717 <>
Subject: "Earth Gains a Retinue of Mini-Asteroids"

In article <> writes:
{Actually, I ( wrote this:}
>> Not really screwy.  The number of 50 meter objects is enhanced by
>> about 10 times and the Tunguska type events probably happen once
>> or a few times per century.  Remember, 3 out of 4 enter over water
>> and may be less likely to be detected.  Also, perhaps a large
>I wonder... Could this be the explanation for the non-radioactive  
>mushroom cloud seen by airline pilots in the Pacific one day in the  
>late 70's - early 80's? It was never connected to any source that I  
>am aware of.

I vaguely remember hearing something about this.  The rate of small
asteroid impact could very well account for such an event.  I doubt
we can conclusively identify it as an asteroid impact, but the
probability of such an event happening is quite high.

>Some theorized it was caused by an undersea volcanic explosion, but  
>no one succeeded in associating it with one. It was not a nuclear  
>explosion, although some at first suggested that. It is unlikely to  
>have been a non-nuclear explosion, ie no ships disappeared and I'm  
>not sure I see a motive for an experimental blast in that part of the  

In the absence of further evidence, we'll just have to add small 
asteroid impacts as a possibility.

>Just a thought, although I would not expect a mushroom cloud from  
>such a comet strike. I'm not sure I can even see a mechanism for  
>creating one from a Tunguska class strike.

The mechanism is quite understandable.  You have an object moving
at hypersonic velocity as it enters the atmosphere.  A stony or
stony iron object could easily survive into the low atmosphere where
the aerodynamical stress catastrophically ruptures it and it explodes
just like a bomb.  The estimated impact energies of 10-100 meter
objects traveling at typical velocities is measured in the kilotons
on the small end to 10s of megatons on the large end and that kinetic
energy has to go somewhere!  Smaller objects fracture high in the
atmosphere and appear as bright bolides which leave trails and 
fragments along the way.  If the object is strong enough, it might
survive largely intact with most of its kinetic energy and might
create an impact crater such as was made about 50,000 years ago when
the Diablo Canyon Meteor crater was formed in Arizona.  That crater
is thought to have been formed by the impact of a stony iron object
only about 30 meters in diameter.  Objects strong enough to survive
atmospheric entry are fortunately rare.

By the way, the Tuguska event was observed by residents of the 
region and their description of the explosion matchs that of a nuclear
bomb blast quite closely.

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

Date: 7 Nov 92 01:16:56 GMT
From: Jim Scotti x2717 <>
Subject: "Earth Gains a Retinue of Mini-Asteroids"
Newsgroups: sci.astro,

In article <> (Dan Tilque) writes:
> (Jim Scotti x2717) writes:
>> (Paul Dietz) writes:
>>>Since the Tunguska event is thought to have been due to a 40 meter
>>>body, and such events were calculated to occur once every 2 to 3
>>>centuries, something is screwy here.
>>Not really screwy.  The number of 50 meter objects is enhanced by
>>about 10 times and the Tunguska type events probably happen once
>>or a few times per century.  
>This sparked a memory, but unfortunately not a detailed one.
>There was a second (but less powerful) Siberian meteor-explosion
>sometime after Tunguska.  I seem to remember that it was in either the
>20's or the 40's, but the name of it totally eludes me.
>Anyone know about this?

Yup, I do.  You are remembering the meteorite fall called "Sikhote-Alin"
which happened in 1947 in Siberia.  The fall created about 200 small
craters of the non-explosive type created by the fall of large objects 
traveling relatively slowly, perhaps at terminal velocity.  The largest 
crater was about 26.5 meters in diameter.  I think that about 50 tons of 
material was recovered and the progenitor was estimated as being about 
200 tons before atmospheric entry.  I suppose the object would have been 
around 5 meters diameter before entry and was probably a stoney iron.

This size object probably hits the Earth around 10 times per year and
I suppose about 1% of them are stoney iron, so a fall of this type
probably happens on the order of once a decade.
>Dan Tilque    --

Jim Scotti 
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA

From: Steinn Sigurdsson <>
Newsgroups: sci.astro
Subject: Re: Asteroid margin of error.
Date: 19 Mar 1998 10:22:38 +0000 (Peu a peu) writes:

> Michael Dworetsky <> wrote:

> >> The differences are due to use of different data for the two
> calculations.  The mathematical methods are essentially identical. <<

> NASA/JPL's web site at ( states that the
> "probability of Earth impact is zero with AND WITHOUT the 1990 pre-discovery
> observations".  It goes on to say that WITHOUT the 1990 data the "most likely"
> miss distance is  54,000 mi with a minimum miss distance of 18,000 mi (a 99%
> probability error bound).   That's about an 80% difference in the most likely
> distance and more than a factor of 6 (!) difference in the error boundary,
> based on the same data.  So, obviously, the mathematical methods are NOT
> essentially identical.   Something is amiss.  Either Marsden and the people who
> have confirmed his 30000 mile miss distance are using the wrong method or
> NASA/JPL is (in which case the asteroid might still have a non-zero probability
> of impact) or both groups are!

Ok. When you have an orbit and want to calculate a
close approach, the plane in which the close approach
takes place has a formal "error ellipse" which is the spread
in distances that the random error due to observational limitations
gives you.
It is an error _ellipse_ rather than an error circle, because
observations of transverse motion in the sky are relatively
precise, whereas it is hard to get good information about
motion along the line of sight.  However, most of the time
error ellipses in the Earth target plane are moderately
squashed circles; so, if you know the area of the error ellipse,
then the spread in distance errors is roughly the radius
of the circle with the same area as your error ellipse.

For XF11, the orbit is moderate eccentricity and semi-major axis
not too far from 1 AU;  as it happens, this orbits error ellipse
is highly elongated in the target plane AND the vector towards
closer approach to Earth is close to the minor axis of the ellipse.
ie the formal error in position is large in one direction and
small in another direction - the small error (as revealed by
a longer formal analysis and confirmed by Monte Carlo simulation)
is in close approaches to Earth - hence the JPLs group (correct)
assertion that a full analysis of the original data gives
negligible probability of Earth impact - the position at close
approach is uncertain by more than the separation, but most of the
error is in a direction orthogonal to the direction towards the Earth.

Now, try explaining that to a journalist in one sentence and
getting it into the 30 second soundbite actually used...

Marsden and the IAUC people did not do anything wrong;
their primary responsibility is to alert the community
to potentially interesting events that need rapid followup,
Earth crossers and PHOs are most certainly part of that job
and the IAUC and information sheet provided contained the
information they should and could have give the timeliness
of the release.

Next step will be to figure if XF11 will hit in the future,
if there's no large change in orbit elements, then it looks
to have ~ 50% chance of hitting in the next one-few million years,
though I have not done a long term orbit integration to check that
and don't think there's enough data yet to try one.

From: (Henry Spencer)
Subject: Re: Near Earth Objects
Date: Sat, 6 Jun 1998 15:54:00 GMT

In article <>, Cary Martynuik  <> wrote:
>> Surely the Tunguska object was larger than 15 metres? I have heard
>> estimates of around 100 metres...
>	About the Tunguska object.... I have heard that it may have been a
>meteor, but I have also heard that it may very well have been a small
>comet! Any ideas?

The best estimate I'm aware of is Christopher Chyba's work a few years ago
(which may possibly have been improved on since -- it's not an area I
really keep up on).  He concluded that the Tunguska object was almost
certainly a rock about 60m across, possibly somewhat larger if it arrived
at an unusually low velocity.

>	 The one thing that I could never understand was why did this object,
>whatever it was, never impacted the Earth, but instead exploded above it?

This is what makes it almost certainly a rock -- more precisely, a stony
asteroid -- rather than a comet or carbonaceous-chondrite asteroid or
nickel-iron asteroid.

An object of this size that is mechanically strong, like a nickel-iron
asteroid, reaches the ground mostly intact and not slowed down much, at
which point all Hell breaks loose and you get Barringer Crater.  But a
weaker object doesn't get that far.

(People have the impression that stone is strong.  It's not.  Oh, it can
carry heavy loads in compression, but take that massive stone obelisk,
solid and strong-looking, and turn it on its side, with one end supported
and the other end hanging, and it will immediately crack and break.  Stone
has essentially no tensile strength -- *pull* on it and it will snap.  A
stone object more than a few meters long has to be supported carefully and
uniformly or it will break under its own weight.  It took thousands of
years of technological development to produce things like the great
cathedrals, large open structures in which no component is ever in
tension.  A piece of rock 60m across is a very fragile object.)

What happens to a weaker object is that as drag forces build up, it starts
to crack and break.  That increases its surface area, and increases the
drag forces (plus, the atmosphere is rapidly getting thicker as the thing
descends), so it breaks up more, and the pieces start to spread out to the
sides.  That increases the surface area still more...  This process may
start slowly but then goes exponential, reducing the object abruptly to a
pile of small chunks, with such huge surface area that they essentially
come to a halt very suddenly.  All the object's kinetic energy is released
in one sudden burst, and a lot of it ends up as heat, almost completely
vaporizing the remains of the object.  BOOM!

Much larger or much smaller objects have different fates.  An object only
a few centimeters across has very little mass behind each square cm of
frontal area, so it decelerates essentially to a halt before drag forces
build up to the point of structural failure, and then it's just a falling
stone (or whatever).  Meter-sized rocks will still break up, but won't
entirely vaporize, which is why major meteorite falls tend to be localized
showers of small rocks rather than single big ones.

On the other end, when the object starts to be several hundred meters
across, the breakup process is too slow -- the sheer inertia of the huge
masses dominates the situation -- and it reaches the ground before it has
time to explode.

Tunguska was roughly a 15MT blast at an altitude of roughly 8km.  That is
too low for a comet; the breakup of even a short-period comet (somewhat
depleted in the most volatile materials) would go exponential and explode
at 20-25km, and a long-period comet would do it at 30km or so.  It's even
too low for a carbonaceous chondrite, which would explode at maybe 15km.
A stony asteroid of the right mass would explode at 8-10km, a good fit to
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

From: (Henry Spencer)
Subject: Re: Near Earth Objects
Date: Mon, 8 Jun 1998 17:06:02 GMT

In article <>,
Allen Thomson <> wrote:
>>A stony asteroid of the right mass would explode at 8-10km, a good fit to
>Coincidentally, 8 km isn't too far off (a bit high, but not badly
>so) the optimum height of burst for a 15 MT bomb to maximize
>the area on the ground affected by destructive overpressures.
>(Question: I'd assume overpressure is the dominant damage mechanism
>for an airburst meteor like Tunguska. True?)

There is quite a bit of radiant heat as well, but I don't know the
percentages offhand.  If memory serves, at Tunguska there was an outer
area of forest mostly affected by blast, but an inner area where heat
effects were also visible.
Being the last man on the Moon is a |  Henry Spencer
very dubious honor. -- Gene Cernan  |      (aka

From: (Henry Spencer)
Subject: Re: Economically important planets/moons/asteroids?
Date: Fri, 9 Jul 1999 14:20:23 GMT

In article <>,
Dennis Wingo  <> wrote:
>> What would be a good way of searching for Earth-Sun Trojans and other Near
>> Earth Objects?  A space-based telescope with some sort of sun shield?
>I like putting one at Earth/Sun L1 and looking sunward as proposed by
>I think Drummond in the resources of near earth Space.

Drummond et al don't seriously propose this -- they say that it would be
a good strategy "except for one glaring problem".  That is, the presence
of the Sun.

Because of the opposition effect -- almost all objects reflect rather more
light at angles of almost exactly 180deg -- it's almost invariably easier
to detect NEAs when your line of sight is right along the illumination.
This is why most NEA searches look in the anti-sunward direction, outward
from Earth.

In theory, you can get the same effect by looking very close to the Sun.
The asteroids on the far side of the Sun get the same brightening effect,
although because they are farther away, you need a better telescope.  The
payoff is that *because* of the greater distance, a given field of view
searches far larger volumes of space.

The problem is that keeping sunlight out of the telescope is very, very
difficult.  Sunshades are not a magic answer, because light scatters off
the sunshade surfaces and edges!

Unfortunately, there is no good compromise here.  Looking near but not
*too* near the Sun is the worst of all possible search strategies -- there
is a strong *minimum* in NEA visibility at about 45deg to the Sun, where
the asteroids are far away and opposition effect does not help much.

Possibly the best strategy in the long run is substantial telescopes in
orbits fairly near the Sun, looking outward.  That gives the benefits of
distance -- searching large volumes of sky with a modest field of view --
while avoiding the difficulties of having the Sun very near the telescope
field of view.

In the short term, much the most cost-effective way of searching for NEAs
in general is more and better telescopes *on the ground*.  This isn't sexy
or challenging, but it is much the most cost-effective method, far more so
than any sort of space-based system.

The two classes of asteroids that such telescopes don't find effectively
are Earth Trojans and other objects in Earth-resonant orbits, and
asteroids with orbits entirely within Earth's.  Both types come within the
anti-sunward field of view seldom or never.

For orbits within Earth's, but still close to Earth's, possibly the best
approach is observing at about 90deg to the Sun.  There is a weak peak in
NEA visibility there, because asteroids with near-Earth orbits spend a lot
of time in that region of sky.

For Earth-resonant orbits, a space-based telescope does have advantages,
because it can work *closer* to the Sun than an Earth-based one, and so
can search a larger fraction of Earth's orbit.  But in general, for
finding this class of object there is just no good substitute for putting
a telescope in an orbit somewhat inside Earth's.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Subject: Re: asteroid
Date: 28 Feb 2000 03:06:48 GMT

Wiseguy ( wrote:
: How big asteroid it has to be before it goes to earth?

Asteroids about 100 meters in diameter are big enough to start to survive
to hit the Earth's surface.  Objects smaller than that will burn up in
the atmosphere or catastrophically disrupt.  Objects between a few meters
and about 30-50 meters disrupt at high altitudes and often result in
strewn fields of meteorites.  Objects between that and 100 meters survive
to altitudes as low as a few kilometers and can cause substantial blast
damage on the surface below where they explode (catastrophic disruption
takes a fraction of a second resulting in the majority of the objects
kinetic energy being dissipated in a very small area - looking every bit
like a nuclear explosion with multi-megaton yields).  In 1908, an object
about 50 meters in diameter exploded at about 10 km altitude over
Tunguska in Siberia with an explosive force of around 10-20 megatons
equivalent of TNT.  Those events are probably century timescale events.

Objects smaller than a meter or so most likely burns up in the atmosphere
and rarely reach the ground intact, but if they do, they fall as small
meteorites, usually fragments of the original object.

As for the rest of the discussion, objects impacting at 10K year
intervals are probably a 200-300 meters in diameter.  Whether they cause
significant damage depends on where they hit.  If they hit land, they'll
cause craters about 2-5 kilometers in diameter and devastate a region
maybe 50 times larger in diameter than the crater.  If they hit in the
ocean (more likely), they may cause substantial damage on nearby
continental coastlines due to tsunami.  The rates of such impacts are
becoming very well determined, for sizes larger than 100 meters probably
good to a factor of a few.  The damage caused by such impacts is much
less well known, but work has been done in the subject and the best
source of information is a book edited by Tom Gehrels titled "Hazards Due
to Comets and Asteroids", University of Arizona Press, 1994.


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: (Henry Spencer)
Subject: Re: Nukes & NEOs
Date: Sun, 10 Sep 2000 21:27:28 GMT

In article <8pcgi1$l52$>,  <> wrote:
>...Which would be better, building a Shuttle-C/Energa LV class HLLV (how long
>would it take to develop such a vehicle in an emergence?) to send one
>massive nuclear strike or using existing ELVs Titan IVs, Ariane 5s &
>Protons with improved upper/final stages extra strap on boosters and send
>a series of small nukes all along its orbit gradually nudge it out
>of harms way?

Depends on how long "some time, but not a lot" is.

If it's ten years, hasty development of a heavier launcher -- taking on
the order of five years for design, development, and some quick testing --
is probably in order.  Indeed, you'd probably want to do two of them, say
a superheavy Delta and an Energia revival, to be sure one succeeds.

If it's three years, possibly you could jury-rig *one or two* copies of an
improvised heavylift launcher, either by cannibalizing shuttle orbiters or
by using the existing Energia hardware still in storage.  This is not
nearly as good, especially since putting things together would take some
time and so the intercepts would be fairly last-minute.

If it's one year, there really is no time for anything but existing
launchers -- no extra stages, no added strap-ons, just off-the-shelf
designs, because there isn't time to verify the practicality of anything
that's a big enough improvement to make a real difference, and launch rate
starts to become a major constraint, so it's better to focus resources on
launching as many rockets as possible.

Even with the longer warning times, you'd get started with a few smaller
bombs using existing launchers.  Partly, there is a lot of leverage in
starting early, when a small deflection has more time to build up into a
large miss.  (Even with last-minute intercepts, by the way, the emphasis
is on deflection, because it's much easier than destruction.)  Partly, it
would be useful to get a first look at how the asteroid responds to being
bombed -- it might influence the strategy later.  In particular, if the
"rubble pile" theory is correct, you cannot really move the thing in one
piece, and it's very important to get the small stuff dispersed as early
as possible, to eliminate it as a factor and to clear it away so you can
see the big rocks.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Subject: Re: Would a asteroid scare be a good thing?
Date: 11 Sep 2000 15:51:21 GMT

Paul Blay ( wrote:


: Just because lots (a majority?) of people believe something is not, in itself,
: sufficient reason to disbelieve it ;-)

Well, if the majority of _experts_ in the field believe something, I would
be inclined to believe them.  Of course, the experts can be wrong, but more
often than not, they are experts for good reasons.  Experts with an agenda,
on the other hand certainly bias the argument.  I've heard good arguments
on both sides of the issue being discussed here and I'm not an expert, so
I won't take sides.....  I think some people have underestimated the amount
of variability in the Earth's "normal" climate over time, though....

: In a (probably doomed) attempt to get this marginally more on topic I note
: that a recent program suggests that the 'little ice age' in the dark ages
: was possibly caused by large amounts of dust in the atmosphere from numerous
: asteroid / comet fragments impacting Earth.  (Same principle as short term
: climate change after Mt Etna (sp?) erupted.

: If this is true, and if climate change continues to take place in the hotter
: direction* then in 15 to 20 years time we could be discussing intentionally
: diverting asteroids to hit uninhabited areas of Earth in order to take the
: heat off.

Wait, I am an expert on THIS TOPIC!  Having looked for these kinds of
objects (and found many of them myself!), there is clearly the potential
for this kind of affect causing or triggering climatological side affects,
but is there a definite linkage?  Perhaps.  There could well have been
significant volcanic activity at this time as well - I don't know if anyone
has checked for a linkage there as well.  While asteroid and comet impacts
and the dust loading of a large comet breakup in the inner solar system may
have happened, and may have caused this little ice age, there are some
experts who will argue that it won't work.  It seems to me that the
amounts of dust required for this affect to have happened is unbelievable.
It seems to me that you would have to have an actual impact with the debris
generated and injected into the atmosphere to have any chance to cause
those huge affects.  Looking at the Hazards Due to Comets and Asteroids
paper by Toon et al., an impactor on the order of a half kilometer would
be required for optical depths of sub-micron dust to reach the the level
of Mt. Pinatubo.  It seems that the affects being discussed here is
significantly larger than that.  Half Kilometer impactors occur at
intervals on the order of 100,000 years, so there's about a 1 in 50
chance of such an impact in the last 2000 years.  Lack of a crater is not
an argument against this, since 3/4 of the impacts occur in ocean (but
then a large tsunami might be something that would be remembered - you
might think that great floods could have been the story about this kind
of event, but that's the wrong era).

The argument of the "cosmic catastrophists" on the other hand, argue that
the progenitor of comet Encke which orbits the sun every 3.3 years was
not only a very large comet and an Earth-crossing one at that, but as it
broke up, it caused a large debris complex which was swept up as meteor
showers and large fireballs to dust load the Earth's atmosphere.  It seems
to me that to get enough dust into the atmosphere, you'd have to have
spectacular meteor storms over the interval in question - not the subtle
ones we're used to, but meteor storms stronger than the Leonids of 1966
for the days surrounding the Earth's passage near the orbit plane of the
object - every year or a much stronger pulse of dust during one very
large storm.   Again, it seems that someone would have written about
such an event - especially the Chinese who were pretty good observers
and recorders of events 1500 years ago.  They also suggest that there
would be 10s or 100s of Tunguska class impacts - at least one or two of
those should have happened close enough to surviving humans to have been
recorded - the entry fireball would be spectacular over large areas, even
if you missed the actual explosion.  Tunguska itself wasn't large enough
to cause a measurable change in the climate (though there was a nice
show of atmospheric phenomena).

There are plenty of questions left to address before we can conclude
that the global climatological events 1500 years ago can be linked to
an asteroidal or cometary origin.  Certainly some of the climatological
and extinction events were caused by impacts, but I doubt all of them
were.  I think Prof. Mike Baillie, the Dendrochronologist who is at the
heart of this story, has definitely found a global climatological event.
The cause of that global event, however, is still open to question.

BTW, I still think that even asteroid impacts are off topic for - at least until an asteroid deflection has been
executed, then we can start talking about it historically....  Of
course if we can use Apollo Lunar data to provide evidence of a
dust shower 1500 years ago....


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: (Henry Spencer)
Subject: Re: Would a asteroid scare be a good thing?
Date: Tue, 12 Sep 2000 02:31:27 GMT

In article <8piv1p$j9g$>,
Jim Scotti <jscotti@LPL.Arizona.EDU> wrote:
>...Lack of a crater is not
>an argument against this, since 3/4 of the impacts occur in ocean (but
>then a large tsunami might be something that would be remembered...

Maybe, and maybe not...  If memory serves, there is recent tsunami debris
at impressive altitudes on some of the Hawaiian islands, with no known
specific tsunamis to match.  Particularly in non-literate cultures, these
things can get lost very easily when society is badly disrupted.

(And fairly quickly too...  Some of the American Indian cultures which
existed before Europeans arrived -- cultures attested to by archeological
remains and the accounts of the very earliest explorers -- were not only
gone but *forgotten* by the time serious European settlement of those
areas began, only a couple of centuries later.  Where the De Soto
expedition had found Aztec-like city-states with large-scale agriculture
and organized government and religion, the settlers found primitives
living in huts in wilderness, with no memory that their recent ancestors
had lived very differently.)
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Subject: Re: Would a asteroid scare be a good thing?
Date: 12 Sep 2000 18:19:39 GMT

Henry Spencer ( wrote:
: In article <8piv1p$j9g$>,
: Jim Scotti <jscotti@LPL.Arizona.EDU> wrote:
: >...Lack of a crater is not
: >an argument against this, since 3/4 of the impacts occur in ocean (but
: >then a large tsunami might be something that would be remembered...

: Maybe, and maybe not...  If memory serves, there is recent tsunami debris
: at impressive altitudes on some of the Hawaiian islands, with no known
: specific tsunamis to match.  Particularly in non-literate cultures, these
: things can get lost very easily when society is badly disrupted.

Very true.  I thought most (maybe even all???) of these tsunami debris
emplacements in the Hawaiian Islands were consistent or even correlated
with the large ocean floor debris deposits from mega-landslides off of
adjacent islands?  The Hawaiian islands are incredibly unstable on
100,000 year timescales....

: (And fairly quickly too...  Some of the American Indian cultures which
: existed before Europeans arrived -- cultures attested to by archeological
: remains and the accounts of the very earliest explorers -- were not only
: gone but *forgotten* by the time serious European settlement of those
: areas began, only a couple of centuries later.  Where the De Soto
: expedition had found Aztec-like city-states with large-scale agriculture
: and organized government and religion, the settlers found primitives
: living in huts in wilderness, with no memory that their recent ancestors
: had lived very differently.)

One nations history is another nations archaeology....  I've explored
many of the ruin sites in the SW, especially in Arizona, and many of
those sites have occupation by unknown peoples as recently as the 13th
or 14th century AD!  Yet all we have is the archeological evidence of
their existence and only weak links to tribes that exist today.

: Microsoft shouldn't be broken up.       |  Henry Spencer
: It should be shut down.  -- Phil Agre   |      (aka

I recently ran across the book "DOS for Dummies" and thought to myself:
"Ah, DOS for Dummies, because only Dummies use DOS!"


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Subject: Re: warming (was Re: Would a asteroid scare be a good thing?)
Date: 16 Sep 2000 00:51:24 GMT

Leonard Robinson ( wrote:

: The issue here is whether or not an asteroid scare would be a good thing for
: Earth. And the analogy was of the artificial global warming problem.

: Basically, I remember when it was announced that there was a possibility
: that an asteroid would hit the NYC area sometime in 2029. It was "debunked"
: (officially) not too long thereafter, but the knowledge involved may have
: been classified at the appropriate level.

There has never been a prediction so specific as to call out the NYC area,
even during the "1997 XF11 affair" which is the one you refer to.  The
biggest estimate of an impact somewhere on Earth was 1 in 1000 and that
was made using a far too oversimplified model for the uncertainty
ellipsoid before the few researchers who have proper models had run their
programs to get an accurate handle on the location of the uncertainty
ellipsoid with respect to the Earth in 2028 when the asteroid will make
a close approach.  If their models had been perfected (as they were in
the days following the original announcement), they would have almost
immediately have been able to say that there was ABSOLUTELY NO CHANCE of
an impact in 2028, though there was a small chance of a "keyhole" return
at a later date - iff it passed in just the right place in 2028, it would
be perturbed into a path that would later impact Earth a few orbits
later, but that particular possibility has since been ruled out.  The
knowledge was never classified.  The "debunking" as you call it was
simply the addition of additional pre-discovery positions found after
the announcement on images taken in 1990 into the orbit solution which
moved the close approach distance farther from Earth in 2028.

: We don't know whether or not the asteroid will hit any more than the issue
: of artificial global warming is accurate. The climate models are so many
: crystal balls consisting of frosted glass. The math, though, is more
: accurate re the possibility of the asteroid hit.

We now know for certain that 1997 XF11 and the 3 or 4 other objects
which have since been called out will not hit Earth in the predictable
future.  ZERO chance.  There are, however, mabye 500 kilometer or larger
objects that are still unknown and until we find those, there will be
a question mark.

We are approaching the 50% level in knowledge of the asteroids larger
than 1 kilometer in diameter, which is where we beleive that global
civilization threatening side affects are certain to occur - within
about 10 years, we should know where more than 90% of those asteroids
are and at that point, any impacts within about 100 years time in the
future will be easily predicted if present.  We'll then be working on
completion to much smaller sizes, say, 500 meters and eventually, though
there may be millions of tiny objects, we should be able to have at
least an early warning system ready to identify objects that are inbound
and a week out so that an object which might produce local damage (like
a Tunguska or Meteor Crater sized object) can be anticipated and an
area of Earth cleared out.

: Even if the asteroid does hit, preparations will be made by those in the
: know. Which will be limited to a select few. As for the rest of us, we will
: be left with reassuring statements designed to lull us and prepare us for
: the collective slaughter. "Eat, drink, and be merry, for tomorrow ye die."

Obviously you don't know what the Near Earth Asteroid (NEA) community
is like.  Secrets are not likely to be maintained, as the discovery of
all the NEAs is not carried out in secret - it's a very public process
with circulars issued after an object has been on a "NEO Confirmation"
web Page for several days, once enough observations have been made to
compute a first orbit for the object.  Then, the object is followed
up in order to improve the objects orbit.  The process is entirely
out in the open.  By the time you would have enough observations to make
a reliable impact prediction, the object will have been known for
literally years!  The orbit computing capability is widespread and
available to hundreds of individuals, though dominated by maybe a dozen
or so who do most of the work.

Most likely, especially with the upgrades to the existing and planned
surveys, all of the dangerous objects will be catalogued within maybe
20 years at most and at that point, we'll either know if one is going
to hit Earth or not for the next century or so.  Most likely, there will
not be a predictable impactor found, but if there is, it is most likely
decades or even centuries off in the future.  The real question mark are
the long period comets which come out of the distant outer solar system,
but they are only maybe 10% of the impactor flux.  Warning times for long
period comets would be less than a year or two - weeks or months for the
smaller ones - they are the only objects that I would worry about enough
to consider having a garage full of nuclear weapons available for a
deflection program.  Most other necessary deflections will be much more
liesurely events with decades of warning most likely.  Small objects are
not worth concerning about - we can treat them like hurricanes and


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: jscotti@LPL.Arizona.EDU (Jim Scotti)
Subject: Re: warming (was Re: Would a asteroid scare be a good thing?)
Date: 22 Sep 2000 04:18:48 GMT

Leonard Robinson ( wrote:

: In re the mathematics, peer review does tend to confirm your data. Point
: taken. I, unfortunately, have matured in a culture which tends to state that
: any data of this variety would be suppressed at high levels of the
: Government, in order to keep the hoi polloi out of the "loop". The Internet,
: fortunately, has begun the process of open discussion and free flow of
: information. And with it, the end of classification for the sheer sake
: thereof -- the end of "empire-building".
: Let us hope, though, that with knowledge, we shall be informed, and take the
: necessary decisions. One of which is settling Mars, the Moon, etc., as
: alternate habitats for Earth in case the Asteroid hits.

I don't see how one could keep the potential impactor out of the public -
at the least, the knowledge of an impending close approach would have to
be very public, even if you could somehow keep the actual impact
calculations to a limited group.

What I worry about (especially with the just announced UK initiatives
which are aimed at completion to smaller sizes than NASA has aimed for
recently) is that there are going to be many many false alarms that will
show impacts to be indistiguishable on the 1 in a 100,000 level from
"only" a near miss.  Already, we're seeing about 1 such event per year or
so and with detection rates accellerating (another factor of 2 in the next
year or 2 and many more times within the next decade), the public will be
inundated and each of these events are liable to hang around until enough
observations are in hand to reduce the chances below 1 in a million of an
impact.  The public does not easily grasp the concept of 1 in a million
versus 1 in a thousand or 1 in 10 even, as recent press hubbub has

BTW, asteroid defense on Mars or the Moon will be much more difficult
given the thinner or nonexistant atmospheres of those worlds.  Where
Earth's atmosphere protects us from objects smaller than 100 meters, the
Moon stands naked to those sized bodies, at least until we can terraform
it.....  Of course, that doesn't stop me from wanting to go to the Moon!


Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA       

From: "Paul F. Dietz" <>
Subject: Re: 2001: And in the plus column .... ?
Date: Sun, 21 Jan 2001 12:48:04 -0600

OM wrote:

> ...The evidence is started to lean towards a synthesis of the major
> theories all happening at the same rough time. Works something like
> this:

   [ theory deleted ]

The problem with this theory is the lack of evidence over the
simple theory that the impact alone was enough to kill them.
In particular, there is no good evidence from the fossil record
that the dinosaurs were in decline before the impact.  There
aren't that many dinosaur fossils to begin with, and only one
site (Hell Creek) where the population in the very end of
the Cretaceous has been studied in depth (with the result
that no significant change could be detected before the

Fossil evidence is also complicated by local changes that
can mimic extinctions.  Ammonites, for example, were thought
to have become extinct a few million years before the K/T
boundary, from fossil evidence in Spain.  But it was later
found that the decline in fossils there just reflected a
local ecological change; beds in France showed ammonites
right up to the K/T boundary.

I think the 'impact alone didn't do it' theories are just
sour grapes from a field with an anti-extraterrestrial bias.
IMO, there's also been an unwillingness among paleontologists
to admit the limitations of fossil evidence, especially in
settling questions over short timescales.


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