What could cause Earth to be a periodic target?

Very good question!

My understanding is that comets have large elliptical orbits that take them close to the Sun. Most fall into the ecliptic or are veery close to it. This means that their paths cross the Earth's orbit.

As they pass, comets are blasted by the solar wind, which slowly strips off the material on the outer core of the comet and produces a visible tail of debris. These debris are stripped off and when the Earth crosses the path of those debris they are pulled into our upper atmosphere.

Examples of this are the Leonids and Persiod meteor showers that happen every year as the Earth sweeps through the remnants of the comets that left debris behind.

That is why it is a yearly event. Each year is different since the material is not static in its place in orbit. Some years the amount of meteors are stunning. Some years are much less active (those are the ones I seem to get!).

This is very true but I am more interested in what could cause objects of the order of 1 Km in diameter to stray from their normal orbits into one that could impact Earth.

One theory that I have heard is that there maybe a dark companion star to the sun that periodically upsets the orbits of or the asteroids sending them on a collision course with Earth.

Something else that the article talks about is that from the core samples that the Apollo astronauts brought back from the moon they have determined that the solar wind two billion years ago was not the same as it is today. What could possibly change the composition of the solar wind? Dose this mean that the sun is not a stable as we would like to think and hope?

Well, there are other objects that run perturbed orbits about the Sun. Not everything is near circular. There is a lot of debris about and interaction with larger objects or planets disturb their orbits. It is a complex game of celestial billiards, but very real.

I am not sure of a dark companion star. So far nothing has been detected, but if there is we should be able to determine that based on disturbances in orbits of other objects, which we are getting quite good at measuring. So, the universe and our solar system are not in a steady state and gravity changes things over time and on rare occasions we get an extraterrestrial visitor to light up the sky or worse.

Unfortunately, we don't have a really long history of tracking such objects. We only just started over the last several hundred years and not until the last few decades have we been able to really look at these objects with any detail. So we only have a snapshot of what is happening out there and there is a lot we don't yet know. We catalog these types of objects all the time and it typically catches us by surprise when we find a new one, which is usually pretty close to Earth by the time it is discovered.

One thing to consider, the frequency of these types of events is much lower now than it was in the past. When the solar system was young there was a lot of junk looking for a home. Just look at the surface of the Moon. Things have calmed down a lot since we have matured, but there is still a few rouge rocks running amok out there. Most are not a threat to anyone.

Yes, the Sun is changing. It has a finite life and I think it is now a middle aged star with about 4 to 5 billion good years left. I don't know what the exact composition was or is of the Sun, but it is slowly changing over time.

"One thing to consider, the frequency of these types of events is much lower now than it was in the past. When the solar system was young there was a lot of junk looking for a home. Just look at the surface of the Moon."

But the moon seems to be showing us that there are recurring periods when the number and size of the impacts is higher on a cyclic basis. If it were just a system that was running out of object that could impact then we would expect an exponential decay in the number and size of impacts. While there is a decay there seems to be a cyclic pattern imposed over the top of this fall off in impacts.

My question is what could cause and why would there be such a cyclic phenomenon?

A couple of reasons:

1) The mass of a comet changes - every time it gets close to the sun it sheds some of it's material, so over time, it's orbit is bound to change. That change is bound to effect the orbits of other objects. (Scientists have calculated that the orbit of an asteroid or comet headed to earth could be altered enough to miss the earth simply by sending a few ton probe near it. It really doesn't take a lot to change a trajectory.)

2) There is A LOT of stuff flying around out there. Stuff hits stuff all the time, pieces fly away and hit more stuff. Orbits change.

3) No orbit is constant. For instance, the moon is slowly moving away from us. Orbital dynamics are incredibly complicated - everything effects everything else.

4) The solar system is not a closed system. We are constantly being bombarded by particles from elsewhere in the universe. More chaos.

Hi Masu, just one question, is the moon immune from this cosmic hail? I'm sure I can see craters up there! Maybe that's a bad reason to go back.........

The word "stable" needs to be placed in context. On a short timescale, things are relatively stable. Tomorrow will be a bit like today, though there are some alarming indications over temperature and carbon dioxide rates of change at the moment. Over millenia, mankind has developed at a rate without parallel in other species. Over geological time, continents drift about and collide with each other, causing fundamental changes to landscape, flora and fauna. Over millions of years, large impacts have alledgedly caused mass extinctions. Over 13-14 billion years, all that is observed has been conceived, created, adjusted and decayed...

Timing is everything. As you know many objects return due to their orbit around the sun. Many objects return a few times within our lifetime and many not even within our lifetime. Depending on the duration of their unobstructed orbit and the orbit of other masses that may affect these objects, they retun. Remember not long ago, Mars was closer to us than it will ever be in the next 5000 years. Objects can be diverted from earth, or can be drawn in. It's all in the timing and frequency of return in relation to other masses and their orbits. It reminds me of while driving and sitting at a stop light, you put your blinker on. The guy in front of you has his blinker on as well. Both blinkers are at different frequencies. During this time your blinker may be on at the same time as the other in exact timing for a second. You can have a good beat on an object such as Hailey's comet, because of intellegent record keeping. There are others out there we just have no clue about that may have been a result of a collision 50,000 years ago. Some of the debris hit earth and some missed. A return of such objects depends on the timing of where other masses are in the frequency of their orbit whether we get hit or spared the next time.

"Hi Masu, just one question, is the moon immune from this cosmic hail? I'm sure I can see craters up there! Maybe that's a bad reason to go back"

This is something NASA have been concerned with and there are studies of the unlit portion of the moon going on right now. They are looking for the telltale flash of an object impacting the lunar surface and the data is not looking that good. Currently they believe the moon is hit by objects that could cause damage around 300 times a year.

However one of the things that can be learnt from the moon is the details of the cyclic pattern that I have mentioned. This could be extremely important information because if there is indeed a cycle it could tell us when we are likely to see the next extinction level event on Earth.

This alone is I believe a good enough reason to return to the Moon.

I don't believe there's any evidence that impacts are either cyclic, or period, in the engineering sense of these terms. Something that happens every 10,000,000 years is periodic. Some that happens every several million years is not. Just like tossing a coin will create what seems like patterns of heads and tails, so it is with any random phenomena.

However, one can certainly imagine how a random event - say the collision of two large objects - might set off a series of events to create a peak in impacts.

But, if there is really a cycle to these things, you don't have to look any further than the cycles of other objects in our local part of the galaxy. I believe I read of a star speeding past us being discovered just recently.

"The solar system is not a closed system. We are constantly being bombarded by particles from elsewhere in the universe. More chaos."

As far as I know every object that has hit Earth and had is velocity calculated has revealed that they are all traveling at less than the Sun's escape velocity. This means that they have all originated from within this system and have not wandered in from deep space.

Again some very valid points but still none of these could explain a cyclic pattern. What could trigger and why would there be a sudden increase in the number and severity of impacts?

Most objects in the solar system would have by now circled the Sun between 1 and 10 billion times. One would expect that by now anything on conflicting orbits should have been involved in a collision. At worst one would expect a steady exponential decrease in the size and number of impacts.

So what could upset the apple cart enough to cause an increase in the number and size of impacts?

Ah. Yes, I would expect some cyclic nature to the events. Still, the overall trend will decay in frequency over time. I am sure that the Moon as well as other objects in the solar system can help us learn more about the dynamics of the solar system and the forces that drive it.

I think that the nursery that you elude to is called the Ort Cloud. It is a region far out beyond Pluto where a lot of loose mass from the size of dust to small planetoids exist. It is possible that extra-solar and as well as solar objects interact to disturb that region in general and some objects in particular to send them on a path to the inner solar system. This appears to be the origin of many comets, too.

I am sure that NASA is concerned about the probability of impacts on the Lunar surface. They are certainly concerned about impacts in orbit, too. Neither have the atmospheric blanket we enjoy on the Earth to protect them. Here is a link to a neat site that cites that 100,000 tons of extraterrestrial matter falls to Earth each year! Obviously, most is in the form of dust.

http://www.oum.ox.ac.uk/learning/htmls/meteors.htm

The biggest threat in orbit is actually man made debris - like the bit of circuit board that blew a hole in the shuttle a couple of flights back.

On the moon, it's natural debris that's the issue. Between that and radiation, she's a harsh mistress for sure.

Where is the data that provides cyclic evidence? How much variation are we talking about period to period? What is the period?

Large scale impacts that are measurable are not common place and that means that the sample size is low, which means it is very hard to statistically show a trend.

My gut tells me not to read too much into this because there isn't a lot of data on this and what data there is lacks a lot of independent validation. I tend to think that we are still filling pieces of the jigsaw puzzle and it isn't yet clear if this is an elephant or a giraffe.

If there is a measurable period then I would think gravity is the culprit. After all, Jupiter actually causes the Sun to wobble at a rate of 11 meters per second.

It could be a collective effort or there may be another body out there that induces instability in the Ort Cloud.

Or, maybe some alien just likes throwing rocks across the Solar system like a young boy skips stones on a lake.

The latter theory is pure speculation. J

No, the point is not that we're being hit by extra-solar objects but that solar objects may have held stable orbits a billion years, only to be perturbed by passing extra-solar objects, although we are constantly bombarded by extra-solar particles as well, but they tend to be actual particles, rather than objects.

Like I said, it doesn't take that much to nudge an orbit. We see comets disintegrate all the time - that changes the orbital dynamics of everything around them. A change of 1/100 of a degree to an object in the asteroid or Ort belts translates into a substantial difference in position over time.

What "one would expect" is irrelevant. It's what one observes that counts. And what we observe is that "anything on conflicting orbits should have been involved in a collision" is just not the case.

And the only mechanism I know of to make that not the case is the intricacies of orbital mechanics, and the decay and accretion of objects in space. There's much more in the details of it, of course - celestial mechanics makes my head spin.

Gravitational forces will drag any object towards any other object.

So it is very complicated to calculate the net effect of drag of the planets on other circulating objects.

Then there are millions of may be devastating comets and asteroids.

Three places where comets and asteroids come from:

First orbit is the asteroid belt between mars and jupiter where more than 4000 objects are traced and surveyed, none of these is estimated a danger for the next 100 years.

Second orbit is the kuiper belt outside plutos orbit where half a million objects of more than 20 miles diameter are estimated. 1 mile diameter would kill all of us on impact.

Much further out beyond the resolving power of todays telescopes is located the oorths cloud.

Comet Hale-Bopp came from this cloud. Comet Halley is periodically coming from the asteroid belt.

The total effect is that there are some big and many small objects coming in with enough energy to destroy a satellite (10 to 20 km/s) but I do not see any periodicity.

RHABE

What could possibly change the composition of the solar wind?

My guess would be that there is more helium now and less hydrogen (or ionized, more alpha particles and fewer protons).

Hi, Masu

We like to think in terms of regular, periodic orbits that are occasionally perturbed by collisions, near misses and the like. For many objects, that is true for short time spans. However, I believe that for our solar system, our local cluster, the Milky Way Galaxy, etc., chaotic behavior is dominant. This behavior as described by mathematical chaos theory can result in "periodic" cycles. These cycles of bombardment may over a finite period of time be periodic but clouded by other more irregular recurring events. I am no expert in this field, but I believe that viewing events from this point of view is educational. I find chaos theory to be interesting with wide application to many real-world situations.

"I believe that for our solar system, our local cluster, the Milky Way Galaxy, etc., chaotic behavior is dominant."

Maybe so initially. Wouldn't chaos become less dominant over time, as events start to stabilize within the milky way? Like someone said earlier, we are just a snap shot in the grand scheme of things. Is it possible that an asteroid passing close by could be the remnants of the debris field when the earth was hit before?

It's certainly possible. We won't know for sure until be go and look at a few more asteroids and comets.

There is debris from all over. Some of it is just leftovers from the formation of the planets. Some of it is the result of collisions - the moon itself is debris from a collision between the Earth and a Mars-sized object. It's entirely possible that Venus's backwards rotation is the result of a large planet to planet impact. I don't think we know enough yet to determine the origins of individual objects, and we can only make educated guesses about classes of objects.

In order for a collision to create space borne debris, the impact has to create debris with enough energy to escape the objects that hit. So that implies that most debris would be the result of collisions of smaller objects, or of two large bodies colliding rather than a small object colliding with a large one.

"Comet Halley is periodically coming from the asteroid belt."

This is not correct. Halley also hailed from the oort cloud.

"It's entirely possible that Venus's backwards rotation is the result of a large planet to planet impact."

Venus's rotation is retrograde, because she has no moon, and no magnetic field. It is possible that her rotation is how she arrived. Venus still has mass and gravity, so she is doing her own thing.

???

From NASA:

There is no physical law that states that when an explosion occurs all particles will be spinning in the same direction. Angular momentum (the physical measure of spinning) will be conserved, so if the original object was spinning (had angular momentum), the sum of the angular momentums of all the "shrapnel" will equal the original angular momentum, so more will be spinning in the original direction. But individual "shrapnel" can be spinning in the opposite direction.

In the case of Venus and Uranus, the most reasonable theory is that a massive collision while they were forming caused them to spin opposite (in the case of Venus) or at right angles (in the case of Uranus) to the other planets and the Sun.

http://helios.gsfc.nasa.gov/qa_plan.html#spinning

You might be interested in this news article:

http://www.space.com/scienceastronomy/070130_st_neptune_trojans.html

Answerer 1.: Upsetting the periodic table.

I liked the old view of the universe as a clockworks mechanism. We know that is not entirely true, but it served to satisfy ignorance for centuries. In some respects the universe does seem to display a mechanical nature. Everything working harmoniously in the peaceful sky. I like to think of the universe at the beginning like a pool table rack of balls, thrown into chaos by the impact of the Que. All of the universe is still in motion and will be, long after we are gone,

My wife has a different assessment of the universe. She says the earth is like a small, fungus covered pea, traveling through space at 22,500 miles an hour while rotating at a thousand miles an hour, in a orbit around a bomb that has been continually exploding all our life. What threat could there be from anything falling out of the sky?

Check out this Wikipedia link.

http://en.wikipedia.org/wiki/Chaos_theory

It has a brief explanation and some useful links on the subject of chaos theory. There is a picture of a Lorenz system in which an object is in a stable orbit around an attractor (in the picture, a pair of attractors) but no two circuits are exactly identical. While it may not look like our solar system, it might resemble a planet in a binary star system.

My point is that while our orbits in the solar system, galaxy, cluster, and universe might appear regular, no two orbits are exactly the same, leading me to believe that chaos theory applies as it is described by mathematics. I think the colliding billiard ball concept is accurate and a subset of the overall chaos.

TLGEngrCo and bhankiii as well as everyone else in this discussion make good arguments for each position. This is a great and fascinating topic. There is not nearly enough time to pursue this to the extent it deserves.

TLGEngrCo asks: "Wouldn't chaos become less dominant over time, as events start to stabilize within the milky way?"

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Not necessarily. Chaotic systems in general do not tend toward stability over time, even if they appear to be deterministic on short time scales. Someone mentioned Lorentz orbitals, for example. Although Lorentz orbitals tend overall to remain within an "envelope" no two orbitals are the same - ever. Another example of faux stability can be seen in bifurcation fractals. Here you have a single thread suddenly exploding into uncountable individual paths at the merest change in parameters.

You can bet orbital mechanics tend toward chaotic behavior when many objects - large and small - are partners in the same dance. The icy denizens of the Oort Cloud are no exception. Eventually a few of these will find themselves on a collision course with Earth or the Moon.

I tend to worry far less about a threat from the sky when I consider that the greater threat to our existence is Mankind itself.

-e

Masu -

I've heard two theories for the (apparently) periodic bombardment:

1) Perhaps the Sun has a companion star in a very elongated orbit. When it comes close to the Sun, it disturbs the Oort cloud & causes a barrage of comets. This "Death Star" theory has been raised by researchers studying mass extinctions in the fossil record. PROBLEM: If a companion had such an elongated orbit, it would have a high chance of interaction with other stars - disturbing the period of the orbit or possibly even stripping the star away from the Sun altogether.

2) Perhaps the Sun's orbit in the Milky Way is at an angle to the plane of the galaxy. If so, its orbit would take it above and below the plane, crossing the plane twice per orbit. The galactic plane is more densely populated with stars; hence, when the Sun crosses the plane, it is more likely to encounter another star that disturbs the Oort cloud. PROBLEM: The galactic plane is still pretty sparse - the odds are poor that the effect would be noticeably periodic.

A third possibility is that the bombardment isn't really as periodic as it seems. The data are sketchy and open to some interpretation, and we're dealing with pretty wide error bars!

From what I've read, the jury is still out on this one. Astronomers have lots of fun discussing this stuff, but very few are ready to place bets on any one theory.

"The data are sketchy and open to some interpretation, and we're dealing with pretty wide error bars!

From what I've read, the jury is still out on this one. Astronomers have lots of fun discussing this stuff, but very few are ready to place bets on any one theory."

You are absolutely correct and there may not be a pattern at all but as you also said it a good topic to start of an interesting discussion and that is exactly why I thought the CR4 crowd would appreciated it. Trying to ascertain any pattern in a attempt to predict the probability of us needing to find a way to deal with an extinction level type asteroid is also a pretty good argument for returning to the Moon.

"Wouldn't chaos become less dominant over time, as events start to stabilize within the milky way?"

I asked this question thinking about the age of our own solar system and it's origins. The birth of what we know exists has reached a point of it's predictable dance around the sun. Over the last 3000 years, we have gained knowlege, but not faster than over the last 500 years, although our solar system has existed eons of time longer. Is there an object out there large enough that could disrupt this dance? It would have to be extremely large. When Jupiter got hit by meteors, it was such a huge event to witness. In the end, is this seen as the workings of stability, or chaos within our solar system?

Your post was excellent in summing up what I believe to be the case.

"Chaotic systems in general do not tend toward stability over time, even if they appear to be deterministic on short time scales."

This is what we are experiencing right now. Let's face it, we are not even a snap shot when speaking of cellestial time.

Sorry that was me above, I wasn't logged in.

Closer to home, Saturn's shepherd moons exhibit a specific example of chaotic behavior.

In more general terms, heteroclinic bifurcation can be seen in systems consisting of as few as three gravitationally-bound bodies, and suggests that such behavior can send objects into new, stable orbits around one of the bodies or can force such objects out of stable orbits or send them into new ones. This presents a strong case for the periodic visitation of the inner Solar System by objects which normally reside in the vicinity of the Oort Cloud or even for nearer objects, such as comets that would not otherwise cross the path of Earth's orbit.

In biology, gypsy-moth populations exhibit chaotic bifurcations every 8-10 years, resulting in a pseudo-periodic population explosion of these moths. The same pseudo-periodicity can be seen in locust populations.

In each system, periodicity is observed, but in none of the cases is the periodicity completely predictable. In the case of our Solar System, no distant, dark-star companion is necessary to force an object from the outer reaches of the Solar System onto a doomsday path toward Earth. This mechanism is already in place, and so it's only a matter of time.

-e

heteroclinic bifurcation Yes exactly, that is a great link, it sums up what we have been talking about.

Regarding chaotic behavior with Saturn's moons, that is a billiard game for sure along the rings, but on a much smaller scale. I can't say I was surprised, Saturn's poling seems to go through what Jupiter and Earth have in common, Auroras. Sounds like short circuiting on the poles. One of Jupiter's moons lit up the atmoshere of Jupiter with 1,000,000 amps. of electricity. The same effect could be disrupting her magnetism and her moon's position relative to her own every 28 days. Very interesting.

The chaotic behavior of Saturn's shepherd moons is the result of the gravitational transfer of momentum as one moon passes the other, in combination with the two moons' slightly elliptical and intersecting orbits. Although a current ring between a satellite and its parent may perturb the satellite's orbit to a small degree, the effect must certainly be many orders of magnitude less than the contribution from gravitational interaction alone.

The current ring you mention exists between Io and Jupiter and is indeed on the order of millions of amps, as you say. I suspect Io's frequent volcanic eruptions inject large amounts of sulfur ions into the volume surrounding its orbit, and these ions may play a significant part in the existence of the current ring. However, Io's orbital period is only 1.8 days. Saturn's shepherd moons do not interact with their parent in this manner.

-e

That clip is definitely a keeper. Thanks for posting this, TLGEngrCo.

-e

Sounds like short circuiting on the poles. One of Jupiter's moons lit up the atmosphere of Jupiter with 1,000,000 amps. of electricity. The same effect could be disrupting her magnetism and her moon's position relative to her own every 28 days.

I think my ex-wife had this very same problem.

Pack My Suitcase. Dear.

Maybe now would be an auspicious time to start your own thread: "What Could've Caused Bhankiii To Be A Periodic Target?"

This is a perfect example of "All my ex's live in Texas."

One of my Ex's moved out-of-state, taking my two boys with her, and so no, all my Ex's no longer live in Texas.

Btw, I lived on Plum Island, MA for about six years. My daughter and one son were born in Stoneham. My other son is a native Texan.

-e

My mother graduated from Stoneham High School class of 1956. Some of my wife's siblings still live in San Antonio. I always liked Texas and found myself there quite a bit when I was living in Shreveprot La. I would jump on my bike, hit I-20 on the way to Dallas. When I reached the Texas line, I'd enjoyed driving on actual asphalt instead of poorly maintained concrete in La.

Small world, yes?

Spent the weekend in S.A. And speaking of Ex's (previous post), my Ex's cousin was the architect for S.A.'s Marriott River Center.

-e

Small world, yes?

My first wife was from S.A. As were her father and his family! Spooky.