Apollo Asteroids

[dougettinger]

There is much talk about the chance of these subject asteroids striking Earth in the near future.

But, I do not know what is considered to be the size for an asteroid that may be fatal to most life on Earth or what size may be fatal to life living on a land mass the size of a continent ? Below what size is considered to be a minor incident on a global scale ?

Don't ask "Chicken Little" ?

[rstevenson]

You'll find some info in Wikipedia's Extinction event page.

Rob

[dougettinger]

Rob,
I can find only information about dates and sizes of craters but very little data about the possible sizes of asteroids that struck the Earth. I realize the velocity and angle of impact are also important. In other words, should we worry about an asteroid the size of a city block wiping out all of humanity ? Perhaps scientists have no respectable theories at this time. What size asteroid produces a certain size crater ?

Doug

[Chris Peterson]

Rob,
I can find only information about dates and sizes of craters but very little data about the possible sizes of asteroids that struck the Earth. I realize the velocity and angle of impact are also important. In other words, should we worry about an asteroid the size of a city block wiping out all of humanity ? Perhaps scientists have no respectable theories at this time. What size asteroid produces a certain size crater ?

Nominally, bodies smaller than a few hundred meters will produce craters not more than a few kilometers across, and have no significant effects on climate (city killers). Bodies on the order of a kilometer are likely to be regionally devastating (a million square kilometers) and have some impact on the climate (country or continent killers). Bodies several kilometers in diameter will have immediate short term effects around the world, will radically effect the global biosystem, and have major climatic impact for several years (dinosaur killers).

Ocean impacts are potentially devastating to coastal areas over a large part of the Earth due to tsunamis. Bodies on the order of 100 km are probably large enough to scrub the Earth of most non-oceanic animal life.

[dougettinger]

Thanks, Chris, for your very concise and "to the point" reply. That is what I was looking for. I do not believe that Wikipedia has absorbed these postulations yet. So any asteroid at 1 km or larger is a serious threat worth diverting much of our defense spending to NASA. Some cross-training of personnel will be necessary. How many Apollo asteroids are your people presently tracking that are 1 km or greater in size ?

Numerous large caves were found with large groupings of varied species of fossilized animals of the Younger Dryas Period. Apparently, these animal were seeking cover and did not care what other animals were present. Conclusion: Caves do not work during a serious catalysmic event.


Counting on all asteroid trackers and asteroid diversion techniques to keep us earthlings safe,

Doug

[Chris Peterson]

How many Apollo asteroids are your people presently tracking that are 1 km or greater in size ?

There is extremely little risk from any asteroids. All the ones large enough to do significant regional damage or worse are known, and are in safe orbits that are unlikely to pose problems for many thousands of years.

A bigger concern would be a long period comet, which might not be detected at all until just days before impact, or even in the best case, not more than a few years.

[dougettinger]

I did not wish to know which asteroids might strike Earth in the next 100 years - only the number known larger than 1 km.

Apparently, the known "longer period comets" supposely from the Oort cloud have sizable bodies in the 1 km range ( ? )

Doug

[Chris Peterson]

I did not wish to know which asteroids might strike Earth in the next 100 years - only the number known larger than 1 km.

Not specifically Apollo asteroids, but the number of bodies with Earth crossing orbits or the potential to develop such orbits, and with sizes greater than 1 km, now number about 900. It is estimated that another 100-200 such bodies remain to be discovered, and will all be found in the next few years.

Apparently, the known "longer period comets" supposely from the Oort cloud have sizable bodies in the 1 km range ( ? )

The size range of cometary bodies isn't well established... but certainly the suggestion that many are in the 1-10 km range is likely.

[dougettinger]

I would suspect with that many there could be an unplanned perturbation.

What is the maximum aphelia of these Apollo asteroids ?

The source of these Earth-orbit-crossing asteroids are collisional debris from previous impacts on Earth. Is this correct?

Is NASA planning for some type of diversion technique in case one such Apollo asteroid is on target to strike Earth? Probably their typical orbital circumferences are not much larger than Earth's thereby minimizing the time to react ( ? )

Completely exhausting all my queries,

Doug

[Chris Peterson]

I would suspect with that many there could be an unplanned perturbation.

No. The perturbation environment is well known. It takes hundreds of years for chaotic elements to add significant uncertainty.

What is the maximum aphelia of these Apollo asteroids ?

No idea. Apollos are defined as having their semi-major axis greater than Earth's. There is no other limit on their aphelions.

The source of these Earth-orbit-crossing asteroids are collisional debris from previous impacts on Earth. Is this correct?

Definitely not. Other than a handful that might be extinct comets, all the Apollos are main belt objects perturbed into Earth crossing orbits. None came from the Earth.

Is NASA planning for some type of diversion technique in case one such Apollo asteroid is on target to strike Earth? Probably their typical orbital circumferences are not much larger than Earth's thereby minimizing the time to react ( ? )

There are no plans. There are theoretical approaches that have been considered. Apollo asteroids represent a very small risk, since most large ones have been identified (and all will be in the near future), and those which are known are also known to pose no impact risk for thousands of years (a couple have tiny risk windows in the next century).

[BMAONE23]

Here is an interesting graphic from Think Quest

It shows just how non planar our solar system is

[dougettinger]

I would suspect with that many there could be an unplanned perturbation.

No. The perturbation environment is well known. It takes hundreds of years for chaotic elements to add significant uncertainty.

What is the maximum aphelia of these Apollo asteroids ?

No idea. Apollos are defined as having their semi-major axis greater than Earth's. There is no other limit on their aphelions.

The source of these Earth-orbit-crossing asteroids are collisional debris from previous impacts on Earth. Is this correct?

Definitely not. Other than a handful that might be extinct comets, all the Apollos are main belt objects perturbed into Earth crossing orbits. None came from the Earth.

Is NASA planning for some type of diversion technique in case one such Apollo asteroid is on target to strike Earth? Probably their typical orbital circumferences are not much larger than Earth's thereby minimizing the time to react ( ? )

There are no plans. There are theoretical approaches that have been considered. Apollo asteroids represent a very small risk, since most large ones have been identified (and all will be in the near future), and those which are known are also known to pose no impact risk for thousands of years (a couple have tiny risk windows in the next century).

How do planetary scientists really know that Apollo objects are really main belt objects? Over the life of the solar system there have been uncountable collisions of objects with Earth and the Moon which should have caused numerous collisional debris to reach escape velocity and go into elliptical orbits which eventually return close to their origins. Why is not collisional debris considered the source of at least some Apollo objects ?

Doug

[Chris Peterson]

How do planetary scientists really know that Apollo objects are really main belt objects? Over the life of the solar system there have been uncountable collisions of objects with Earth and the Moon which should have caused numerous collisional debris to reach escape velocity and go into elliptical orbits which eventually return close to their origins. Why is not collisional debris considered the source of at least some Apollo objects ?

It is rare for material to be ejected from the Earth-Moon system by collisions with either body- particularly asteroid sized debris. It is doubtful that any such material is in solar orbit. The Apollo asteroids show the same types of reflectance spectra as other asteroid family objects, and orbit on the ecliptic- neither of which would be expected if they were comprised of lunar/terrestrial material. Both in terms of composition and dynamics, they are entirely consistent with asteroidal bodies.

[neufer]

There is much talk about the chance of these subject asteroids striking Earth in the near future.

But, I do not know what is considered to be the size for an asteroid that may be fatal to most life on Earth or what size may be fatal to life living on a land mass the size of a continent ? Below what size is considered to be a minor incident on a global scale ?

<<1866 Sisyphus is a binary Apollo asteroid which, at approximately 10 km in diameter, is the largest of the Earth-crossing asteroids. It is comparable in size to the Chicxulub object whose impact may have killed off the dinosaurs. This Apollo asteroid will reach apparent magnitude of 10.0 on November 26, 2071 when it will be a mere 0.116 A.U. from Earth (when it was discovered it peaked at magnitude 9.0 in November 25, 1972), being one of the brightest of its class. In Greek mythology Sisyphus (Greek: Σίσυφος, Sísyphos) was a king punished by being compelled to roll an immense boulder up a hill, only to watch it roll back down, and to repeat this action forever.>>

[b][color=#0000FF]Artist's impression of an asteroid striking Earth at the end of the Cretaceous[/color][/b]

---

<<The Chicxulub crater is a prehistoric impact crater buried underneath the Yucatán Peninsula in Mexico. The crater is more than 180 km in diameter, making the feature one of the largest confirmed impact structures on Earth. The impactor had an estimated diameter of 10 km and delivered an estimated energy equivalent of 100 teratons of TNT (4.2×10²³ J). By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×10¹⁷ J), making the Chicxulub impact 2 million times more powerful. Even the most energetic known volcanic eruption, which released approximately 240 gigatons of TNT (1×10²¹ J) and created the La Garita Caldera, was substantially less powerful than the Chicxulub impact.

The impact would have caused some of the largest megatsunamis in Earth's history, reaching thousands of meters high. A cloud of super-heated dust, ash and steam would have spread from the crater, as the impactor burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, broiling the Earth's surface and possibly igniting global wildfires; meanwhile, colossal shock waves would have triggered global earthquakes and volcanic eruptions. The emission of dust and particles could have covered the entire surface of the Earth for several years, possibly a decade, creating a harsh environment for living things. The shock production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. Over a longer period, sunlight would have been blocked from reaching the surface of the earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain.

In February 2008, a team of researchers led by Sean Gulick at the University of Texas used seismic images of the crater to determine that the impactor landed in deeper water than was previously assumed. They argued that this would have resulted in increased sulfate aerosols in the atmosphere. According to the press release, that "could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain)."

There is an ongoing dispute whether the impact was the sole cause of the Cretaceous–Paleogene extinction event. There is evidence that there was an interval of about 300 ka from the impact to the mass extinction.>>

<<The Cretaceous–Paleogene extinction event, often referred to as the Cretaceous–Tertiary extinction event, occurred approximately 65.5 million years ago (Ma) at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant species, most notably dinosaurs, in a geologically short period of time. Also known as the K–T extinction event, it is associated with a geological signature known variously as the K–T boundary, the Cretaceous–Paleogene boundary or the K–Pg boundary, usually a thin band of sedimentation visible wherever rocks of this age are exposed. Numerous groups of organisms became extinct during the K-Pg extinction event, most notably the non-avian dinosaurs. Non-avian dinosaur fossils are found only below the K–T boundary, indicating that they became extinct during the boundary event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian clades passed through the boundary with few extinctions, evolving and thriving well past the event.

Scientists hypothesize that the K–Pg extinctions were caused by one or more catastrophic events, including at least one asteroid impact (especially the one that created the Chicxulub crater) or increased volcanic activity. Several impact craters and massive volcanic activity, such as that in the Deccan Traps, have been dated to the approximate time of the extinction event. These events would have released massive amounts of dust and ash into the atmosphere, reducing surface sunlight, hindering photosynthesis, and severely disrupting Earth's biosphere. Many researchers believe the extinction was more gradual, resulting from the sea level and climate changes already occurring during the late Cretaceous, and aggravated by impact events or increased volcanic activity.>>

[dougettinger]

Neufer, I believe the K-T extinction event was a combination of impacts occurring close together that included massive volcanic activity such as the Deccan Traps that are dated close to the time of the impacts. Hopefully, 1866 Sisyphus misses Earth by 10 million or more miles. Is the orbit of a binary asteroid more difficult to calculate? Or is the smaller member ignored in any calculation ?

Doug

[dougettinger]

How do planetary scientists really know that Apollo objects are really main belt objects? Over the life of the solar system there have been uncountable collisions of objects with Earth and the Moon which should have caused numerous collisional debris to reach escape velocity and go into elliptical orbits which eventually return close to their origins. Why is not collisional debris considered the source of at least some Apollo objects ?

It is rare for material to be ejected from the Earth-Moon system by collisions with either body- particularly asteroid sized debris. It is doubtful that any such material is in solar orbit. The Apollo asteroids show the same types of reflectance spectra as other asteroid family objects, and orbit on the ecliptic- neither of which would be expected if they were comprised of lunar/terrestrial material. Both in terms of composition and dynamics, they are entirely consistent with asteroidal bodies.

I am puzzled just a little. If an object escapes the Earth's gravity after a larger impactor strikes Earth, then typically what trajectory does it take in our solar system? A Martian meteorite was found on Earth. This collosional object obviously orbited the Sun numerous if not millions of times before falling on Earth.

Some illustrated Apollo asteroid orbits appear to be rather circular and traveling near the Earth's orbital region. Can scientists actually create a computer model that perturbs an asteroid from the main belt that finally becomes stable in an Apollo orbit?

Doug

[Chris Peterson]

I believe the K-T extinction event was a combination of impacts occurring close together that included massive volcanic activity such as the Deccan Traps that are dated close to the time of the impacts.

The consensus is substantial that the K-T extinction was the result of a single impact, which formed the Chicxulub crater. There are several small impact craters that are dated to the same time, so the asteroid involved may have produced multiple impacts... but only the Chicxulub impact was large enough to have a major global effect. Many people think that volcanism associated with the Deccan Traps was contributory, or slowed recovery, but few consider volcanism to have been a major factor in the K-T extinction event.

Hopefully, 1866 Sisyphus misses Earth by 10 million or more miles.

It only needs to miss by a few hundred miles. It's the "miss" part that matters!

Is the orbit of a binary asteroid more difficult to calculate? Or is the smaller member ignored in any calculation ?

The calculation is based on the center of mass (barycenter) of the pair, and is exactly the same for single asteroids or binaries. Neither member is ignored.

[dougettinger]

A near miss of a plotted few hundred miles by a 10 km asteroid will cause a general ruckus; such as large populations traveling to mountains tops to pray or die; or NASA sending a diversionary apparatus to meet it in interplanetary space.

Thanks for telling me about the barycenter calculation. I still want to know how an Apollo asteroid finds a nearly circular orbit near Earth from it supposely original residence in the main belt of asteroids. Do scientists actually have computer models to confirm their changing of orbits?

Doug

Doug

[Chris Peterson]

I am puzzled just a little. If an object escapes the Earth's gravity after a larger impactor strikes Earth, then typically what trajectory does it take in our solar system?

It would probably be in an orbit with a radius similar to that of Earth's (possibly an eccentric Earth-crossing orbit) and an inclination unlikely to be zero. A small body (and impacts are probably unable to lift anything greater than a few meters across into orbit) with a perihelion inside the orbit of Mars is not in a stable orbit, and will not exist more than a few million years before being perturbed out of the Solar System, or into the Sun or a planet. As there have been no large planetary impacts in this time, it is unlikely that any orbital debris from the Earth is in orbit.

A Martian meteorite was found on Earth. This collosional object obviously orbited the Sun numerous if not millions of times before falling on Earth.

There are about 100 Martian meteorites, which appear to be associated with at least four Martian impact events, all within the last 10-20 million years. It is quite a bit easier to eject material from Mars, but ejected material is still small (meteoroids, not asteroids). Martian meteoroids orbit for several hundred thousand years to several million years before being perturbed into the Earth, or more likely, into the Sun.

Some illustrated Apollo asteroid orbits appear to be rather circular and traveling near the Earth's orbital region. Can scientists actually create a computer model that perturbs an asteroid from the main belt that finally becomes stable in an Apollo orbit?

Sure- that's the only model that explains Apollo asteroids. I'm not aware of any hypothesis that suggests a different origin.

[dougettinger]

I personally do not have any hypothesis for Apollo asteroids unless they were left over from some giant impact of a minor planet with early Earth.

So scientists believe that all collisional debris created by the event of the Giant Impact Hypothesis that created the Moon has been completely swept clean in the Earth's orbital region (?) So it is postulated that absolutely, no large debris could be left behind over these billions of years like the asteroids in the main belt were left behind (?) Hmmmm.

I really, really would like to talk to one of these computer modelers and have them explain their starting conditions and each subsequent condition that kept perturbing an asteroid from the main belt closer and closer into an Apollo orbit. I am just completely overwhelmed by their confidence in their model. Chris, you probably do some modeling yourself, but not of this magnitude (span of time).

Doug

[Chris Peterson]

I personally do not have any hypothesis for Apollo asteroids unless they were left over from some giant impact of a minor planet with early Earth.

That is not possible given the short lifetime of bodies in Apollo orbits- only a few million years.

So scientists believe that all collisional debris created by the event of the Giant Impact Hypothesis that created the Moon has been completely swept clean in the Earth's orbital region (?)

Yes.

So it is postulated that absolutely, no large debris could be left behind over these billions of years like the asteroids in the main belt were left behind (?)

Material in the main belt wasn't "left behind" (except in the sense that the planets were). It exists in stable orbits. Unstable regions of the belt have long since been cleaned out. No large debris could really be left from the time the Moon was formed because there isn't any stable place for it to end up.

I really, really would like to talk to one of these computer modelers and have them explain their starting conditions and each subsequent condition that kept perturbing an asteroid from the main belt closer and closer into an Apollo orbit. I am just completely overwhelmed by their confidence in their model.

These are not complex models. They are simple numerical simulations of basic Newtonian orbital mechanics. The models require no more confidence than the basic physics. Please understand that the actual system is chaotic- no numerical model can demonstrate that any particular body got into its present orbit by a deterministic set of gravitational interactions. All the models show is that perfectly reasonable physics can explain the positions of most bodies today, and the models can also conclusively exclude certain possibilities (such as Apollos in orbits more than a few million years old).

Chris, you probably do some modeling yourself, but not of this magnitude (span of time).

I don't do this kind of modeling. The only orbits I numerically propagate are meteoroid orbits, and only for a few days or weeks to determine the starting orbit for meteor parent bodies, where the Earth and Moon are the only perturbers.

[dougettinger]

dougettinger wrote:I personally do not have any hypothesis for Apollo asteroids unless they were left over from some giant impact of a minor planet with early Earth.

Chris wrote: "That is not possible given the short lifetime of bodies in Apollo orbits- only a few million years."

The lifetime of these bodies is simply determined by orbital dynamics. Is this correct? The idea is something like the following: These bodies over the life of the solar system would simply be eliminated by falling into the Sun, falling into the inner planets, or be perturbed into much larger elliptical orbits according to numerical models. The present Apollo asteriods could only come from the closest reservoir of asteroids, the main belt, in the near past. Then the question arises: What perturbed these asteroids two or more million years ago from the main belt ?

Do any of these Apollo asteriods have similar surface spectra to other asteroids in the main belt ?

So does numerical modeling also confirm that Jupiter's gravity field stabilizes the orbital regions of the main belt? And this is why these asteroids were left behing 4.6 billion years ago. Has this modeling also been applied to the minor belts of asteroids between the outer planets ? I am very curious to know how in some cases asteroids can only exist for a few million years and in other cases the asteroids can last as long as the age of the solar system.

Doug

[Chris Peterson]

The lifetime of these bodies is simply determined by orbital dynamics. Is this correct? The idea is something like the following: These bodies over the life of the solar system would simply be eliminated by falling into the Sun, falling into the inner planets, or be perturbed into much larger elliptical orbits according to numerical models. The present Apollo asteriods could only come from the closest reservoir of asteroids, the main belt, in the near past. Then the question arises: What perturbed these asteroids two or more million years ago from the main belt ?

Resonances with the gas giants, primarily Jupiter. There is zero doubt that this happens: a basic numerical simulation of the Solar System demonstrates that minor bodies interact this way.

Do any of these Apollo asteriods have similar surface spectra to other asteroids in the main belt ?

Yes. They all do. In terms of composition, Apollos are the same as other main belt objects.

So does numerical modeling also confirm that Jupiter's gravity field stabilizes the orbital regions of the main belt? And this is why these asteroids were left behing 4.6 billion years ago. Has this modeling also been applied to the minor belts of asteroids between the outer planets ? I am very curious to know how in some cases asteroids can only exist for a few million years and in other cases the asteroids can last as long as the age of the solar system.

Resonances create both zones of stability and zones of instability. But even stable orbits are only partially so- perturbations occasionally move bodies out of stable zones and into unstable ones. That is a major, perhaps primary source of meteorites which strike the Earth.

[dougettinger]

Chris, thank you for your patience and your answers. I promise that I have only only more question about asteroids. There are supposely computer simulations of accretion models for the inner planets and embryos for the Jupiter and Saturn. Supposely, these models have some agreement with the lifetimes of observable proto-planetary disks. Why is the main belt that possesses a Titius-Bode orbit the exception for having a completed accreted planet with its orbital region swept clean ?

Doug

[Chris Peterson]

Chris, thank you for your patience and your answers. I promise that I have only only more question about asteroids. There are supposely computer simulations of accretion models for the inner planets and embryos for the Jupiter and Saturn. Supposely, these models have some agreement with the lifetimes of observable proto-planetary disks. Why is the main belt that possesses a Titius-Bode orbit the exception for having a completed accreted planet with its orbital region swept clean ?

There is little to suggest that Titius-Bode orbits are anything other than coincidental. I don't place much weight on the idea that planetary orbits are determined by any such rule. It seems perfectly plausible to me (and supported by simulation) that the region of the asteroid belt was too perturbed by Jupiter (or the proto-Jupiter) during the few million years when the protoplanetary disc was dense enough for accretion to occur for a planet to come together there. Today we see the complex ordering of stable and unstable zones in that region created by orbital resonances.