Re: Possible evidence for Stone Age (Clovis) Cosmic Catastrophe?

Apparently on date Sat, 05 Nov 2005 18:52:18 -0800, Tom McDonald
<tmcdonald2672@xxxxxxxxxxxxxxxxxxx> said:

>nospam@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
>> Apparently on date Fri, 04 Nov 2005 16:09:13 -0800, Tom McDonald
<snip>
>>
>>> Here is a list of comets named after their human discoverers.
>>>You will often see comets named Linear X, or SOHO Y; but these,
>>>and some others, are the names of comet-hunting observatories,
>>>not individuals:
>>>
>>>http://www.comethunter.de/discoverers.txt
>>
>>
>> Yes I realise this. I also realise that most of these people are finding the
>> comets by looking at SOHO images of the sun - which appear online somewhere or
>> other. They find objects very near the sun in the images and these are,
>> presumably, comets.
>>
>> These are essentially not relevant to the risk of impact on earth, even less so
>> than the trillions of comets up out there which don't orbit into the inner
>> solar system. At least the latter have some prospect of being pitched down
>> sunwards at some future moment.
>
> I would prefer that you mark your snips. In this case, you cut
>the following bits in quotation marks without comment. (Of
>course, if you don't have any comment on a particular bit of a
>post, there is no need to address it. Courtesy and honesty
>suggest it's best to make note of it when you cut something
>unaddressed.)

Old school snipping, and to be honest, while I can find loads of netiquette
pages that give me no requirement to mark snips, I recognise there are probably
also pages that add one.

Suffice to say I have never felt the need to explicitly mark snips and not
doing so it not meant to be an insult or anything.

I didn't address your original text because the post was already pretty long,
and I figured I'd made my point and didn't want to overdo it. It clearly wasn't
an attempt to dodge the notion of "comets are common" as I am addressing that
head on with my "(relevant) comets are quite rare" assertions.

But I shall address it now.

> "These, of course, are in addition to the many comets known
>from long ago.

The problem with this is most of the comets that we know about today *are*
comets from long ago.

Many of the periodic comets in the list on
http://www.cometography.com/
have periods that are quite large and they are judged to have been cycling
around their orbits back into the past, although obviously it is hard to find
anything to prove this unless you can identify a specific comet from a specific
description.

If you follow the links to 1P, which is Halley's Comet, you can find the
historical references to it, which go back to Chinese and Babylonian
astronomers. We think the comet was present back then rather than some other
comet with the same "fit" to the descriptions, but there is some speculation.

Assuming this is the same one, we can infer that this same comet has appeared
to mankind 30 times, it's probable that some visits were remote, i.e. earth was
in a bad position to view the tail, but most were ok and would have been
regarded as a different comet each time up until Halley's day.

>Comets are not uncommon; and IIRC, they frequently
>have orbits relatively well aligned with the plane of the solar
>system. And, of course, the gravitation of Jupiter and Saturn can
>alter a comet's orbit so that it becomes an Earth orbit crosser."

Actually, this is not the case. Please look at the list of periodic comets and
notice the last figure in the table. This is the inclination. It describes the
angle between the plane of the orbit of the comet and the plane in which the
planets and asteroids basically reside.

You'll see that only a couple are close to the ecliptic, 1.3 and 1.4 degrees
respectively. None are inside one degree, and the rest are arbitrarily out to
180 degrees (which works the other way and is effectively a small inclination
with the orbit going the other way round the sun to everything else*). There is
a trend towards the ecliptic, I suppose because the usual reason a comet exists
in the first place is because it has been influenced by something that was, in
turn, influenced by a planet and these lie on the ecliptic.

However, you can see that a few degrees at least is involved for virtually all
these comets and, because of the way an orbit works, looking at the orbit edge
on, coming from high and right of the sun, the plane of the comet's orbit
crosses the plane of the ecliptic as the comet passes the centre of the sun as
observed from the side, dips below the ecliptic by some amount while passing
perihelion to the left, so to speak, and then rises on the return pass through
the centre of the sun on the other side, and goes above the ecliptic from that
moment on.

Here's a picture to clarify that: http://tinyurl.com/74rs7

(* - to give an example, Halley is inclined 172 degrees from the ecliptic.
Practically, this means the orbit is inclined 8 degrees from the ecliptic but
instead of counter clockwise from above the north pole, the comet orbits in a
clockwise direction. That's unusual, but not unique.)

The point is, you can look up the inclination and see that none of these comets
can hit any planets. 1.3 degree inclination to the ecliptic, applied to a
distance of 1 AU (the Earth to Sun distance), puts the comet with the least
inclination about sin(1.3) = 0.02 AU above our orbit.

This is in Astronomy terms, while 0.02 AUs sounds pretty close, it's huge in
practical terms, ten times the distance from earth to moon, or something like
300 diameters of the earth.

This is the comet with the least inclination, the rest miss by more.

On the other hand, if you count sun-grazers, where a blob of the sun is ejected
on an orbit that goes around it and returns to the surface of the sun on the
way back, as a comet, these will frequently have zero eccentricity because the
sun is rotating and it is easier to launch from the equator in the direction of
spin because on earth it gives you a free launch of a bonus 1000 mph of speed
even before you light the blue touch paper. On the sun, its rotation gives you
a launch velocity of 2 kilometers per second extra than at the poles. So it is
quite normal for stuff to be ejected from the solar equator in the direction of
spin and on the ecliptic.

So if these are included as typical comets, we can say that the vast majority
of comets are not inclined to the ecliptic, but we are then including a large
majority that should not be relevant to the discussion as they don't seem to
ever form an earth-crossing orbit, and must always return to their original
position and speed, which requires them to plunge back into the Sun on the way,
so they don't have the chance to keep going round until they hit.

I suppose I should add that the arbitrary nature of the inclination of the
majority of comets is more or less the entire reason for regarding the Oort
region as a Cloud rather than a belt: http://tinyurl.com/bteqp

> [Tom adds: we can discuss what 'many' and 'frequently' mean in
>this context. It would be useful to consider the effect of
>gravitation on the orbital elements of a comet, or even an
>asteroid. One thing I learned by chasing down your info on SOHO
>{for which I thank you; that was fun!} was that large comets can,
>and often do, break into bits; which bits over time may, through
>their slightly differing orbits, and thereby differing
>gravitational regimes, come to have orbits not predicted based on
>the known orbital mechanics of the parent comet.]

In fact, an orbit is independent of the mass of the orbiter, for all our
current intents and purposes. If you split a comet into two parts, each part
having more or less the same velocity vector, both parts will continue to orbit
side by side just as if they were one piece. You could attach a string between
them and find it still connected the next time the bodies come through.

This does happen in practice, each year we have various shooting star shows,
such as the Taurids. What happens is that earth's orbit intersects with an
elliptical orbit with nearly no inclination, which carries lots of pieces of
rock. The pieces of rock on this orbit are travelling around the orbit in a
sort of continual stream, each having almost the same orbit, regardless of
their mass, and as earth passes through, these bits of rock hit it and are
consumed in the atmosphere, appearing as shooting stars heading from a
particular place in the night sky, i.e. the direction of Taurus.

If you look up at Taurus tonight, wait a while and you will see a shooting
star, usually travelling across the sky away from Taurus. The peak passed last
night, in fact, although I wasn't about to bother because it being Guy Fawkes
here in the UK most of the "shooting stars" had a more terrestrial origin.

What'll happen with a debris field, and they're usually there because a larger
object has been a bit too close to a gravity well* and been torn to bits on the
way past, the debris gradually spreads out, mainly in terms of where it is on
the orbit relative to other bits, and to a slower extent, spreading out into a
broader range of orbits over a larger area. This is something that takes a long
time, by the way.

It's a fair bet that the origin of the material making the Taurid meteor
showers is a comet or similar that came to a crumbly end while passing one of
the planets fairly closely, not necessarily earth because as the orbit is an
eccentric, elliptical one in the ecliptic, it crosses the orbits of all the
planets, not just earth. My money would be on Jupiter being the source of the
gravity that mushed whatever the Taurids are a remnant of.

(Incidentally, if you do go out to see for yourself, this is not a very
impressive meteor shower and you may have to wait quite a while to see one or
two shooting stars. There are far more impressive ones at other times.)

* - the reason a gravity well can split objects up, is because of tidal forces.
All elements of the object experience the gravity and follow the same path, but
the bits nearest the source have a slightly stronger force and so, when the
difference in forces exceeds the adhesive forces, the object breaks up and each
bit follows a slightly diverging path from then on.

> "This is also not taking into account the asteroids that
>also are continuing to be discovered. More than a couple have
>been discovered very shortly before they had a near-miss with
>Earth. I've seen a couple that were announced on-line, as there
>wasn't time to get them into _Sky and Telescope_ or _Astronomy_,
>or the other monthlies."

Asteroids are far more likely to hit earth, they're extremely common and most
of them start off in the asteroid belt, lined up on the ecliptic and will, if
disturbed by Jupiter, quite easily end up on an orbit with near zero
inclination and also Earth crossing, which will mean eventually they will hit
the Earth.

These are mostly small objects of no great importance, but there can and will
be larger objects that will hit the earth causing damage that will range from
minor to disastrous.

I'm not afraid of comets, they're rare, it may be that a comet did the
Dinosaurs and it's as reasonable that it was a meteor and no comets otherwise
have struck the earth*. Meteors are not rare and they are the real threat from
the sky. Although, to put this in context, various Skywatch type organisations
have checked various categories of rock and concluded no actual threats are
present at this time. This is just luck, though, there is no doubt that
meteorites do hit the earth and some of them have been extremely destructive.
It's even possible significant impacts have been historically recorded.

(* - classical comets, this is. I've no doubt that all manner of things were
going on during the formation of the solar system and the water had to come
from somewhere, etc. Big, tailed, comets, in archaeological time, though, no.)

> [Tom adds: it doesn't make a lot of difference to the Earth and
>those on it whether the catastrophic impactor is an asteroid, a
>comet, or a comet-bit-turned-asteroid. Thus, while the discussion
>of Earth-crossing comets is interesting, it is incomplete when
>discussing cosmic catastrophes on Earth. A little nudge from
>another planet, or even the last fly-by of Earth, and several
>recent asteroids could have played merry hell with us.]

I agree, but the debate is all about comets because Eric (as I recall) was
puzzled about how a supernova can eject comets - which they can't and it is
only journalists getting the wrong end of a stick that brings the subject up.


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