Starship Asterisk*

dougettinger wrote: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 ?

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

dougettinger wrote: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 ( ? )

dougettinger wrote: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 ( ? )

Chris Peterson wrote:

dougettinger wrote: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).

dougettinger wrote: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 ?

dougettinger wrote:
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 ?

http://en.wikipedia.org/wiki/1866_Sisyphus wrote:
<<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.>>

http://en.wikipedia.org/wiki/Chicxulub_crater wrote:

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

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<<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.>>

http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event wrote:
<<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.>>

Chris Peterson wrote:

dougettinger wrote: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.

dougettinger wrote: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 ?

dougettinger wrote: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?

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.

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 (?)

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).

dougettinger wrote: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.

dougettinger wrote: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 ?