Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.
WASHINGTON (Reuters) - Computer simulations gave new life on Wednesday to a theory that has intrigued astronomers for years: the idea that one big collision between the Earth and a Mars-sized planet gave birth to the moon.
The so-called ``giant impact'' theory was first envisioned in the 1970s, but now scientists at the Southwest Research Institute and the University of California-Berkeley have put together a scenario that would account for the moon's creation as well as the fact that a day on Earth is 24 hours long.
``The previous models of the impact theory had identified impacts capable of producing the moon, but they were unable to account for all of these features of the Earth-moon system simultaneously,'' researcher Robin Camp said in a telephone interview from Boulder, Colorado.
``By showing that just one impact can do the job, what we're doing in effect is demonstrating a more probable scenario,'' she said.
The new research, presented in the current edition of the journal Nature, postulates an enormously energetic but oblique crash between Earth and a planet the size of Mars, which is about half Earth's size.
The energy unleashed by this collision some 4.5 billion years ago would have been enough to destroy the incoming planet and melt Earth all the way through, Canup said. There would also have been some vaporized rock debris kicked up from the crash, which would start orbiting Earth.
``Once the orbiting debris cooled, it's from that stuff that the moon then coalesced,'' Canup said. The whole process, from collision to formation of the moon, took less than 100 years, she said -- an almost inconceivably short time in planetary terms.
The glancing angle of the collision -- perhaps 40 degrees or so -- caused Earth to start spinning, Canup said, but much faster than it does now. In those early times, an Earth day would have lasted only five hours.
The moon was also thought to be much closer to Earth than it is now; in fact, the Earth and the moon continue to get more distant from each other by several inches (centimeters) a year, Canup said. As the moon moved away from Earth, Earth's rotation slowed, she said.
The planet that caromed into Earth is long gone, one of a dozen or more mini-planets in the process of formation that never quite made the grade in our solar system. Most of these mini-planets were about Mars' size, Canup said, providing plenty of candidates.
``Everything has really come together, because it looks like the type of impact you need to explain the Earth's mass and initial spin rate also tends to naturally place a sufficient amount of material into orbit to form a moon with the size of our moon,'' Canup said.
An image and animation of the big crash are available online at
http://www.swri.org/press/impact.htm .
fyi
I still like Immanuel Velikoskey's(sp?) from 'World's in Collision' theory .... at least it's entertaining.
The current issue of American Scientist likewise has an article touching on the Moon's formation. The main thrust is on the necessity (OK, hypothesis) for water to have arrived extraterrestrialy. There is also an article on the black smokers on the ocean floor and an article about the possible shapes of the universe.
Scientists spend so much time and money trying to prove all of these evolution theories.
Why can't they just believe, "In the beginning God, created the heavens and the earth."
The more interesting question is "what are we going to do with it?". It's time to go back and set up a permanent base/colony there and begin a space-based economic revolution.
Interesting!
Scientists spend so much time and money trying to prove all of these evolution theories. Why can't they just believe, "In the beginning God, created the heavens and the earth."
I think it's because our instruments of observation have increased dramatically since that explanation was written.
Because science isn't religion.
Scientists spend so much time and money trying to prove all of these evolution theories. Why can't they just believe, "In the beginning God, created the heavens and the earth."
I think it's because our instruments of observation have increased dramatically in power since that explanation was written.
Why can't they just believe, "In the beginning God, created the heavens and the earth."
Yeah, and luckily the cure for polio was right in the Bible too -- oops, I guess it is lucky that scientists don't fret too much what the mythical Bible has to say or not say on any particular subject. Good on them.
Scientists spend so much time and money trying to prove all of these evolution theories.
Why can't they just believe, "In the beginning God, created the heavens and the earth."
You sound like the type that would have told Orville and Wilbur Wright, "If God had intended for us to fly, he would have given us wings."
Well I'm of the type that says, "If God didn't want us to think, he wouldn't have given us brains."
ping
I'm confused a little. This is talking about all the planets flying around and destoying Earth I to make present Earth and moon. My question, why couldn't one of these planets, about half the size of the earth, get captured in the Earth's gravity?
The moon was also thought to be much closer to Earth than it is now; in fact, the Earth and the moon continue to get more distant from each other by several inches (centimeters) a year, Canup said. As the moon moved away from Earth, Earth's rotation slowed, she said.
I wonder if the global warming models consider this. It might explain why they work so well.
Find it hard to believe that an early earth, in a good stellar orbit, could undergo such a collision without either being eventually ejected from the solar system, plunged into the sun, or end up with a highly elliptical (and unstable) orbit. But from the Apollo rock data, Luna did not come from the same stuff earth came from, too much iridium(?) or something.
Answer this, O great men of science!
If the moon was torn from earth, why is the orbit circular and not elliptical?
If a Mars-size impactor hit earth, why only one moon? Why no rings of fragments like Saturn?
Why are the relative abundances of elements found on the Moon so dissimilar to Earth?
And, of course, why do some scientists still hold to the theory that the moon formed from dust the same time as the earth?
Sounds like guessing to me, not science.
Guessing is a big part of science. So what? Of course you then need to back your guesses up with proof before you can be taken seriously.
If the moon was torn from earth, why is the orbit circular and not elliptical?
The Moon's orbit is elliptical. Why didn't God make it a perfect circle? And to say the Moon was torn from the Earth isn't very accurate. The debris from the collison was thrown into a plane that orbited the Earth. Take a look at the graphic on the page that is linked in the article.
They've been reading Stichin again, haven't they?
Why didn't God make it a perfect circle?
I'll ask Him when I see Him.
Nice diffusion there, Moon. The question wasn't why the Moon's orbit wasn't elliptical because, by definition, all orbits that aren't perfect are elliptical. The question is why doesn't the orbit reflect the impact? If it did, the theory that the moon was formed at the same time as the earth wouldn't still be out there. But you knew what I meant, didn't you? (Should I give you the benefit of the doubt?)
At least one old-earth cre is way ahead of you there: http://www.reasons.org
all orbits that aren't perfect are elliptical
No. All Orbits are elliptical. Period!
I gave you the answer in my first reply -- the moon accreted from a disk of post collision debris. It doesn't have to be highly elliptical to reflect an impact.
Orbits around a single center of mass are elliptical (circular is a special case of elliptical). Multiple centers of mass can cause deformation of orbits.
But from the Apollo rock data, Luna did not come from the same stuff earth came from, too much iridium(?) or something.
Wrong again. The Apollo rocks support the premise that the moon is made up of the same "stuff" as the earth's crust. Isotopes of Oxygen (16O, 17O and 18O) occur in similar relative proportions in Lunar and terrestrial rocks.
Orbits around a single center of mass are elliptical (circular is a special case of elliptical).
Quite correct.
And the firmament. How thick is the firmament?
I was condescending in my last post to you. Certainly was not my intention. It came out wrong and I publicly apologize! :( Sorry.
Oh, several billion light years.
I don't believe it. When you add up the angular momentum of the Earth-Moon system, you find that its axis is nearly perpendicular to the ecliptic plane. This tells you two things: that the Mars-like body was travelling in the ecliptic plane, and that it struck the Earth with essentially no impact parameter with respect to the ecliptic (i.e., it couldn't come in too high or low). (In fact, it couldn't have had too much impact parameter in the left-right direction, which leads to an apparent fine-tuning problem, but we can ignore that because we're starting out with a Moon as a given.)
Here's the problem. If you look at the way planets form, they end up at specific radii (Bode's law) and with highly circular orbits. They do this as a consequence of the conservation of angular momentum. Things in highly elliptical orbits will eventually intersect with each other and stick to each other. On average, their radial velocities will cancel, but ultimately there will be one component of angular momentum that cannot be cancelled in this way: the component aligned with the initial angular momentum of the cloud you started with. By the time you have large bodies forming, they are in circular orbits, which do not intersect with each other.
This means that if there is something the size of Mars, it probably didn't form in the solar system, because if it did, it wouldn't be in a highly elliptical orbit. Assuming it formed in a circular orbit, nothing in the solar system could kick it into an elliptical orbit, because all of the other large bodies are also in circular orbits, which won't intersect with the Mars-like body. It had to be an interloper.
But look: the interloper's momentum had to lie right along the ecliptic plane, else it would have had a significant velocity transverse to the ecliptic plane, which would have imparted a tilt to the Earth-Moon system far in excess of what we observe. What are the odds of that?
Furthermore, if the interloper's path was displaced too far above or below the ecliptic plane, you again have a problem with misaligned angular momentum. The tolerances are really close. I crunched some numbers on this a few years ago, and the tolerance on the impact parameter was something like 10 miles, on angular momentum alignment alone. I wish I had time to reproduce it. I'd lay it out for you all to scrutinize.
So I strongly doubt this model, on the grounds of fine-tuning. That's not to say that a technically workable solution doesn't exist.
Hey! This kinda invalidates that 1960's movie "The Crack In The Earth"! When I was a kid, I always thought that was a cool way for the earth to get a second moon. This of course was before I took a physics class. :) LOL!!!
I suspect God goes about part of it mechanistically.
How did we write this stuff into our shared computers?
Theories of Formation for the Moon
Crack in the World. (I may be a physicist by trade, but I'm first and foremost an expert in bad science fiction.) I watched that movie with an experimentalist's eye, and said, "Is that right? Let's test that hypothesis!"
Kewl! I thought it was a pretty nifty movie what I was 9. :) Haven't seen it for years though!
Let's test that hypothesis!"
It would have to be some pressure to eject that much mass at 17,000 miles per hour or so. LOL!
Didn't you chide my coziness with such "fine-tuning", only the other day? Or was that just the quantum variety?
Refresh my memory...what were we talking about?
So I strongly doubt this model, on the grounds of fine-tuning.
Of course God is pretty much of a crack shot wouldn't ya say? ;-)
Got it. You mean this post, where I said:
Fine-tuning problems have a way of disappearing with improved understanding. I have a feeling that many of the unresolved fine-tuning problems will ultimately turn out to be cases of our legs being juuuust long enough to reach the ground.
By that I meant that the canonical fine-tuning problems of particle physics and cosmology will, I expect, turn out to be reflections of underlying (but as yet unknown) principles that force things into agreement. We will find that they could not have been otherwise.
In the case of the Earth-Moon system, nothing forces the angular momenta to be aligned if indeed the Moon was formed by a collision. It would almost certainly have been otherwise. That's a real fine-tuning problem. If instead the Moon formed in place, coalescing out of the primordial cloud the way the Earth formed in orbit about the Sun, then that would tend to force them into alignment, exemplifying the point I was making in the quoted post.
"In the beginning God, created the heavens and the earth."
Oh but God did create the Heavens and the Earth. And he set the Universe in motion and cause the galaxies to spin about themselves as they do now. And in His infinite wisdom he sent a Mars-sized planet in motion in such a way as to collide with an infant Earth. In so doing He set about to create the Moon and cause the Earth to spin about it's axis as it does now. No, not after the fact as it were. But before all time began, before there even was a Universe....
No my friend, science is only beginning to explain some of life's great mysteries, and in so doing, gives us a greater understanding of our little corner of this wonderous and infinite place we call the Universe. The more we understand, the more we realize what we don't know, and this is the beauty and wisdom of His plan, so it is that we begin to realize how much we really need Him in our lives.
IMHO, science and Faith are completely intertwined, the two are complementary and inseperable.
I don't believe it. When you add up the angular momentum of the Earth-Moon system, you find that its axis is nearly perpendicular to the ecliptic plane. This tells you two things: that the Mars-like body was travelling in the ecliptic plane, and that it struck the Earth with essentially no impact parameter with respect to the ecliptic (i.e., it couldn't come in too high or low). (In fact, it couldn't have had too much impact parameter in the left-right direction, which leads to an apparent fine-tuning problem, but we can ignore that because we're starting out with a Moon as a given.)
Here's a paragraph from an article written by Robin Canup in 1999:
For nearly a decade, the giant impact theory was heavily critiqued. The idea that the moon was the result of a particular large impact event was considered too arbitrary, and did not fit in well with the existing view of a quiescent planet formation process. In 1984, a conference devoted to lunar origin prompted critical comparison of the existing theories. The giant impact theory emerged from this conference with nearly consensus support, enhanced by new models of planet formation that suggested large impacts might indeed be common events in the end stages of terrestrial planet formation. Such models demonstrated that the relatively quiescent stage of planetary growth continued only until young planets grew to sizes ranging from lunar to Mars-sized, and that the final stages of growth were characterized by collisions among tens to hundreds of these large, planet-sized bodies. In the course of the many impacts apparently required to yield the final four terrestrial planets, it did not then seem so unreasonable that one of the impacts would be of the type required to yield the moon.
Correct. Circular, or near circular, is the rule, provided there are other bodies in orbit. NOVA did a fascinating explanation of why the rings of Saturn tend to reinforce each other, they talked about how random objects act as "sheperds", keeping the rings ruly, so to speak.
I was "taunting with the license of ink." Uh...bits.
My greater fascination with laziness allowed the math of this bamboozle me a few years ago. I still find the fate of the "third" body unsatisfying.
Thank you for the qualitative explanation. I'll attempt to quantify it tomorrow, when I'm sober.
Here's an excerpt from an article regarding the inclination of the Moon's orbit:
The moon's orbit can be traced backwards in time to reveal that when the moon formed near the Earth, its orbit was inclined by approximately 10 degrees relative to the Earth's equator. Most other planetary satellites in the solar system have orbital inclinations smaller than 1 or 2 degrees. The cause of the moon's large orbital tilt has long been a mystery."The inclination problem had been one of the last remaining obstacles for the impact hypothesis of moon formation," says SwRI Institute Scientist Dr. William R. Ward. The widely favored "giant impact theory" proposes that a Mars-sized body collided with Earth 4.5 billion years ago, creating a hot disk of debris from which the moon accumulated. Previous models of the moon's formation from such a disk predict that the lunar orbit should have been nearly aligned with the Earth's equator, with only about a 1 degree tilt.
The new theory, published in the February 17 issue of Nature, proposes that the moon acquired its large tilt soon after it formed because of a gravitational interaction with debris left over from the impact event. Modeling results presented in the paper, authored by Ward and SwRI planetary scientist Dr. Robin M. Canup, show that the moon could have acquired its 10 degree tilt as a consequence of the moon-forming impact.
NOVA did a fascinating explanation of why the rings of Saturn tend to reinforce each other, they talked about how random objects act as "sheperds", keeping the rings ruly, so to speak.
The Shepherd Moons. Enya wrote a song about them.
Good catch, Moonman!
I've always enjoyed the writings of Isaac Azimov in both Science and science-fiction.
He was perplexed by the idea that our 'Moon' simply didn't belong here! 'It's too big', 'It's too close', 'It's not rotating right'!
I don't have any opinions about it. But I love reading about it. Thanks.....FRegards
"Why can't they just believe, "In the beginning God, created the heavens and the earth."
Actually, according to Genesis, God did not create the universe or this planet we call Earth. Heaven is the name God gave to the "firmament", a barrier of some kind God placed amidst the pre-existing waters, and "Earth" is the name God gave the dry land exposed by the receding terrestrial waters - the pre-existing waters God called "Seas".
This theory refers to the late stage of planetary formation (at which point it was down to the last few dozen Mars-sized planetesimals). Coalescence and orbit circularization were in progress, but not yet complete.
I'm afraid that you're arguing in a circle -- saying that because the collisions were finished with, a theory that assumes another collision can't be correct.
If this all happened within a century, why are there NO rocks dating back to 4.5 billion years? On both the Earth and the Moon, the oldest rocks are ~ 4.1 billion years old. The "maria", the vast plains or "seas" of molten magma covering much of the Moon facing us began pouring out around 4.1 billion years ago. Why does the side facing us have these "maria" when the other side is a heavily cratered highland? Why does the side facing us continue to face us? If the Moon had an ocean, the water would cover the side facing us while the opposite side would be above water.
The Moon was a witness to the collision between the proto-Earth and whatever impacted it. That's why the side facing us shows evidence - the maria - of being blasted. That was the side facing us when debris from the impact slammed into the Moon. The gravitational center of the Moon is closer to the side facing us too because the heavier elements coming from the proto-Earth and the impactor became embedded in the near side. Of course, I might be wrong ;-)
The moon's orbit can be traced backwards in time to reveal that when the moon formed near the Earth, its orbit was inclined by approximately 10 degrees relative to the Earth's equator.
If the Moon's orbital plane used to have a larger degree of tilt--in other words, if there's some process pulling the Moon's orbital plane towards the ecliptic--then that takes care of any "naturalness" problem right there. I don't know what that process is (solar tides?), but if there is one then I feel much better about the impact hypothesis.
Most other planetary satellites in the solar system have orbital inclinations smaller than 1 or 2 degrees.
But a significant fraction of the planetary systems considered as a whole have angular momenta that are badly out of whack. Consider Uranus, off by about 90 degrees, or Venus, off by almost 180. Collision theories might be better applied to them, although my "circular orbit" argument still applies. Uranus I could believe, since there's more room out there for a Mars-sized body to form without interacting with anything.
I'm afraid that you're arguing in a circle -- saying that because the collisions were finished with, a theory that assumes another collision can't be correct.
No, not at all. I naively expect that orbits will be circularized solar-system-wide (except at the outer reaches) well before any large bodies form. The forming planetesimals will ultimately collide with the things travelling along the same circle, of course, and will suck in materials from the accretion disk that have orbits some significant radial distance away, thanks to their tidal force.
In order to have a collision, it is necessary to kick one of these planetesimals into a highly elliptical path, so that it will intersect with other planetesimals. But if that were possible for any given planetesimal, why wouldn't the materials that went into the planetisimal already have been kicked out, before the thing formed? Some of that material existed within easier "reach" of other planetesimals, but it was obviously safe enough to survive then.
It is possible that there is some sort of stochastic resonance behavior ejecting the planetesimals. Before they form, the tides are weak everywhere. After they form, the tidal force from any one planetesimal isn't strong enough to kick any other out of its orbit, but every now and then several of them end up in conjunction, and the combined tidal force is enough to dislodge one of them.
Uranus I could believe, since there's more room out there for a Mars-sized body to form without interacting with anything.
The problem here is that Uranus is a bit too massive to be knocked so far out of kilter by a Mars-sized planetismal.
One thing this theory doesn't take into account is the seasonal tilting of the earth. How would such a large impact effect that?
Mike
Zecharia Sitchin, where are you?
I naively expect that orbits will be circularized solar-system-wide (except at the outer reaches) well before any large bodies form.
What about the Ort?
The problem here is that Uranus is a bit too massive to be knocked so far out of kilter by a Mars-sized planetismal.
That's nothing that couldn't in principle be cured by a larger relative velocity and a larger impact parameter, just as long as it wouldn't disrupt the planet completely.
More large impacts would also do the trick. Several Marses would be a bit much to ask, but perhaps it's been knocked about by several large but peripatetic Kuiper belt objects (such as Pluto, Charon, Varuna, etc.). Who knows how many of those there used to be?
If the moon was torn from earth, why is the orbit circular and not elliptical?
I single chunk knocked into orbit would be highly elliptical. However even there, if its path took it close to the earth in the early years, it would have strong tidal effects that may circularize it.
However, earth materials don't have sufficient strength to remain a single chunk in the face of such a powerful collision. The ejecta would certainly be small debris. Each individual piece of debris would have its own orbital parameters -- its own elliptical path. These myriad objects would mix among themselves in their orbits, each either colliding or gravitationally effecting each other in transit. The ultimate result would be a great averaging of the elliptical parameters. The bulk would become circularlized.
What about the Ort?
Density and timescale tell the tale. Many of the Oort cloud objects do exist in highly elliptical orbits, but first, the density is too low out there to make collisions likely (as conclusively demonstrated by the fact that it never collapsed into an accretion disk), and second, while these objects do intersect with the dense and circular inner solar system, their orbital periods are so long that they very rarely have an opportunity to collide with the things that are in the circular orbits. They have much smaller velocities at aphelion than at perihelion (Kepler's 2nd law); consequently, they spend essentially all of their lives out in the interstellar void.
If this analysis is right, then closer in, where the density is larger and the periods are shorter, you would expect the comets to form larger bodies, closer to the ecliptic plane, and in more nearly circular orbits. That is in fact what we see. Pluto is but the largest of an entire class of icy planetesimals, known as Kuiper Belt Objects.
Archiv Der Mathetik's latest issue has an interesting article showing how stochastic resonance leads to periodic behavior. The example uses the iceages.
Of more intellectual (but not so practical) interest is an article in the latest issue of Acta Mathematica Hungarica which shows that the smallest number q, 1 < q < 2, such that 1 has a unique expansion base q (with 1 and 0 as digits) is the number written in binary as 1.110100.... The k'th digit after the fraction point is 1 if the sum of the binary digits of k is odd and 0 if the sum of the binary digits of k is even.
In grade school they told us many things that are still theoretical. One of these wonderful things was that the moon came from a bulge in the earth and that the Pacific Ocean basin is the hole left after the material that is now the moon departed. They also told us we had 20 years of oil reserves. Gradeschool was in the 50s in my case, and the class argued over these things very energetically.
several billion light years
It is taught that the firmament is 5 centimeters thick, but very strong.
Physicist, I've always felt that the Oort Cloud "explanation" for comets was shakey at best. Do you agree? Maybe I don't understand.
But a significant fraction of the planetary systems considered as a whole have angular momenta that are badly out of whack. Consider Uranus, off by about 90 degrees, or Venus, off by almost 180. Collision theories might be better applied to them, although my "circular orbit" argument still applies. Uranus I could believe, since there's more room out there for a Mars-sized body to form without interacting with anything.
Here's a paper dealing with the formation of terrestrial planets:
We perform three-dimensional N-body integrations of the final stages of terrestrial planet formation. We report the results of ten simulations beginning with 22-50 initial planetary embryos spanning the range 0.5 - 1.5 AU, each with an initial mass of 0.04 - 0.13 Me. Collisions are treated as inelastic mergers. We follow the evolution of each system for two hundred million years at which time a few terrestrial type planets remain. On average, our simulations produced 2 planets larger than 0.5 Me in the terrestrial region (1 simulation with one m > 0.5 Me planet, 8 simulations with two m > 0.5 Me planets and 1 simulation with three m > 0.5 Me planets). These Earth-like planets have eccentricities and orbital spacing considerably larger than the terrestrial planets of comparable mass (e.g. Earth and Venus). We also examine the angular momentum contributions of each collision to the final spin angular momentum of a planet, with an emphasis on the type of impact which is believed to have triggered the formation of the Earth's Moon. There was an average of 2 impacts per simulation that contributed more angular momentum to a planet than is currently present in the Earth/Moon system. We determine the spin angular momentum states of the growing planets by summing the contributions from each collisional encounter. Our results show that the spin angular momentum states of the final planets are generally the result of contributions made by the last few large impacts. Our results suggest that the current angular momentum of the Earth/Moon system may be the result of more than one large impact rather than a single impact. Further, upon suffering their first collision, the planetary embryos in our simulations are spinning rapidly throughout the final accretion of the planets, suggesting the proto-Earth may have been rotating rapidly prior to the Moon-forming impact event.
Physicist, I've always felt that the Oort Cloud "explanation" for comets was shakey at best.
What's shaky? The formation of the comets? The formation of the Oort cloud? The existence of the Oort cloud? I'm not sure what you're asking.
The Oort Cloud is a circulating band (or sphere?) of comets around/outside the solar system, as I undersand it. How did it get there?/Where did it come from? Balls of ice (i.e. water) and dirt in deep space? Seems artificial and highly unlikely to me -- much like a "make do" explanation. But maybe I don't understand it.
Very cool. That sounds like it would be really fun to play with. Maybe they could make it into a screen saver.
We report the results of ten simulations beginning with 22-50 initial planetary embryos spanning the range 0.5 - 1.5 AU, each with an initial mass of 0.04 - 0.13 Me.
They're starting with the assumption that the protoplanetary accretion disk will fragment in this way prior to the existence of large planets. (That's fine, of course; the simulation can be used as a test of that assumption.) I don't know how they're arranging their planetesimals, but the starting orbital radii are probably random, which I'm sure is unrealistic, but you have to start somewhere. I don't doubt that such an arrangement would be unstable. These conditions probably favor collisions more than in the real world.
These Earth-like planets have eccentricities and orbital spacing considerably larger than the terrestrial planets of comparable mass (e.g. Earth and Venus).
Red flag there.
There was an average of 2 impacts per simulation that contributed more angular momentum to a planet than is currently present in the Earth/Moon system.
Perhaps that's not surprising; the Earth/Moon system does have quite a bit of angular momentum on account of the long moment arm. The large magnitude of the angular momentum is not really a problem, in that it's no easier to explain in one model than in another.
I wonder how many collisions they typically had per simulation, and how many planetesimals typically survived each run.
How did it get there?/Where did it come from? Balls of ice (i.e. water) and dirt in deep space? Seems artificial and highly unlikely to me -- much like a "make do" explanation.
There are two kinds of comets: short period comets and long period comets. The short period comets, such as comet Halley, have orbits that lie very nearly in the ecliptic plane. The long period comets, such as Kohoutek or Hale-Bopp, typically have orbits that do not.
Kepler's second law tells us that an object in a highly elliptical orbit will move very quickly near the Sun, and very slowly farther out. This means that each long period comet spends only a tiny fraction of its life in the inner solar system. The same is true of the short period comets, but of course the fraction is larger. For every short period comet we see, there is a much larger number of others we don't see, and they will be distributed in a belt (the Kuiper belt) around the solar system, because they lie in the ecliptic plane. For every long period comet we see, there will be a much MUCH larger number of others we don't see, and they will be distributed in a spherical halo around the solar system (the Oort cloud).
Every year there are short period and long period comets that come tooling through the inner solar system. Most of them you can't see with the naked eye, but they are there. In this way we can measure the population of both the Oort cloud and the Kuiper belt, and the distributions of the eccentricities helps us to put lower bounds on the population of comets that are in less elliptical orbits (i.e., the ones that never make it into the inner solar system where we can see them). The Oort cloud and the Kuiper belt really are out there.
Where did the comets come from? I don't know, myself. How did the Oort cloud form? I don't know that, either. But we do know that these balls of ice and dirt are out there, because we see them passing through.