Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
for a long-period comet crossing the Earth's orbit is 2.2 to 2.5 x 10-9 per perihelion passage. The mean impact velocity is approximately 52 km sec-1 but the most probable impact energy is characterized by a velocity of 56 to 58 km/sec. The estimated current impact rate for cometary nuclei large enough to create 10 km diameter (or larger) craters on the Earth is between 5 x 10-7 and 2.8 x 10-6 per year, with a bed estimated value of 1.0 x 10-6 per year. Nuclei large enough to initiate global climatic disturbances strike the Earth on average every 16 Myr. The impact frequency may be increased substantially for brief periods of time during cometary showers, initiated by major perturbations of the Oort cloud. Improved technologies are needed to detect approaching long-period comets at large heliocentric distances so as to increase the warning time for potential impactors. - Original Message - From: Chris Peterson [EMAIL PROTECTED] To: meteorite-list@meteoritecentral.com Sent: Friday, December 08, 2006 12:02 AM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Comet Tempel-Tuttle, the parent body of the Leonids, is in a low-inclination, retrograde orbit. We encounter the debris at 71 km/s, and our own orbital speed is 29.6 km/s. Subtract that out and you get the orbital speed for Leonid meteoroids: ~41.4 km/s. The solar escape velocity at the Earth is 42.1 km/s. That's why the Leonids are as fast as any periodic meteors can be- faster meteoroids would leave the Solar System. Of course, a sporadic meteor could be produced by a body that would escape the Solar System if it didn't encounter the Earth- either because it originated outside the Solar System, or because it picked up enough energy through momentum transfer during some sort of slingshot around another body. I don't know if anybody has worked out the likelihood of that happening- very, very rare I'm sure. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list Any questions? Get answers on any topic at www.Answers.yahoo.com. Try it now. __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
I was discussing the probability of encountering a meteor with a velocity greater than the Sun's escape velocity at the Earth. The likelihood of that happening should be much lower than the likelihood of simply encountering a stray object kicked out of the Oort Cloud. There are only a few ways an object can pick up additional energy by momentum transfer, and an infinite number of ways it can avoid doing so. And I wasn't particularly thinking of ruin-your-day sorts of events; even very rare high speed meteoroids should follow a power law size distribution, so a pebble should be much more common than a boulder (even if neither has happened more than a handful of times since the formation of the Solar System). Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com - Original Message - From: Sterling K. Webb [EMAIL PROTECTED] To: Meteorite List meteorite-list@meteoritecentral.com Cc: Chris Peterson [EMAIL PROTECTED] Sent: Friday, December 08, 2006 12:59 AM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Hi, All, Chris said: I don't know if anybody has worked out the likelihood of that happening- very, very rare... I called my oddsmaker in Vegas (or was it Vega), and here's what he said... The problem is essentially the same as the likelihood of being smacked by a one-time long period comet; it falls in from the back of nowhere , slingshots around the Sun, and zaps back out. It's completely random; it could come from any direction -- the Oort Cloud is a sphere. So, imagine that the radius of the orbit of the Earth defines an inner sphere surrounding the Sun, through which the object will have to pass in order to swing around the Sun and back out. The surface area of that sphere is about two billion times the cross section of the Earth itself, so the odds of being hit by the incoming comet is one in two billion, and the odds of being hit by the outgoing comet is one in two billion. Overall, the odds are about one in a billion for both coming and going. There is a good sized (10 kilometer diameter and up)* long period comet almost every year, so we will get comet-whacked every billion years or so. [* Comet Hale-Bopp was 40 MILES in diameter.] On average... Little long period comets (1 kilometer to 10 kilometers diameter) are 5-10 times more common, so expect a medium comet whack every (couple of) 100,000,000 years or so. Of course, being gob-smacked by a long period comet is just about the worst. I hate when that happens. The comet is going at the solar system escape velocity (almost); the Earth is going at its orbital velocity. What the vector total of those two? Answer: Too much. The kinetic energy goes up by the square of the velocity, so maybe 4 to 6 times the energy of the impact of an asteroid of the same mass. That's going to leave a mark, as they say. Just to prove that the Universe isn't a sporting proposition, a long period comet coming from the Oort Cloud isn't likely to brighten enough to be detected by visual comet finders until it's near the orbit of Jupiter, which would give us about 2-3 weeks of warning of an incoming encounter -- hardly enough time to get drunk, have a last fling, and say your prayers. Of an outgoing encounter, we'd have 4-5 weeks of warning time. That's some improvement but not much. Not, for example, enough time to move several billion people to the side of the planet away from the impact point. Hmm. How many frequent flyer miles you got? You feel like a long vacation? Of course, if the comet was just from Far Kuiper County, with a period of 3000-4000 years, we'd have months (instead of weeks) to get ready. You'll be ready in 4-5 months, won't you? Since the Leonids are retrograde and the Earth prograde, the encounter velocity is the vector sum of the two, but the angle of incidence between the Earth and the Leonid stream varies from year to year; when it's 180 degrees, or face-on, the encounter velocity is the oft-quoted 71,000+ mps. At lesser angles, it's somewhat less but still hefty. Nice that they're mostly just pea gravel and sand sized bits; very pretty and they don't leave marks. Sterling K. Webb __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Hi all - --- Chris Peterson [EMAIL PROTECTED] wrote: even very rare high speed meteoroids should follow a power law size distribution, so a pebble should be much more common than a boulder I think that meteorids as currently defined includes both comet bits and asteroid bits. If that is so, then their size distribution would not follow a power law, but rather would be the sum of two power law distributions. I couldn't understand what you were trying to say in tne next part of this, but then it's still early in the morning and I haven't even finished my coffee yet. good hunting, Ed Need a quick answer? Get one in minutes from people who know. Ask your question on www.Answers.yahoo.com __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Hi all - Ahah - things are becoming clearer - Perhaps this explains to some degree the apparent lack of cometary meteorites - their speeds are too high to survive entry - of course, the other alternative is that the experts in meteoritics have simply mis-identified cometary meteorites - my guess is that some of the iron-silicites come from cometary nucleus's (nuclei?), but then who knows? That's one of the things that makes meteoritics so interesting - it's still a developing science, and a fundamental one at that. I wonder if the term cuttings will ever replace Bessey specks in advertisements? good hunting, Ed --- Chris Peterson [EMAIL PROTECTED] wrote: Comet Tempel-Tuttle, the parent body of the Leonids, is in a low-inclination, retrograde orbit. We encounter the debris at 71 km/s, and our own orbital speed is 29.6 km/s. Subtract that out and you get the orbital speed for Leonid meteoroids: ~41.4 km/s. The solar escape velocity at the Earth is 42.1 km/s. That's why the Leonids are as fast as any periodic meteors can be- faster meteoroids would leave the Solar System. Of course, a sporadic meteor could be produced by a body that would escape the Solar System if it didn't encounter the Earth- either because it originated outside the Solar System, or because it picked up enough energy through momentum transfer during some sort of slingshot around another body. I don't know if anybody has worked out the likelihood of that happening- very, very rare I'm sure. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com - Original Message - From: Sterling K. Webb [EMAIL PROTECTED] To: Chris Peterson [EMAIL PROTECTED]; meteorite-list@meteoritecentral.com Sent: Thursday, December 07, 2006 8:30 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Hi, Visual, Chris, List For the benefit of Listees following the question of how slow a meteoroid can be... The orbital velocity for any body is maximally the escape velocity divided by the square root of 2, or 70.707070707...%. Can we just call that 71%? Escape velocity is 11,263.04 meters per second. So, the highest orbital velocity is 7964.17 meters per second. That's the orbital velocity at the lowest possible orbit, skimming over the surface. The orbital velocity gets less and less the higher the orbit, so that geo- synchronous orbital velocity is positively pokey, around 3000 meters per second. You have to go faster than that just to get there, then slow down to stay there. Crazy stuff, that gravity. The only orbit that can decay is one close enough to the top of the atmosphere to be slowed into re-entry. But (big but), the only way an object from somewhere not of this earth can get to the top of our atmosphere is to fall there, in the course of which fall, it will acquire additional velocity, up to escape velocity. Escape velocity is like taxes, in that there just doesn't seem to be any way to wiggle out. By the time an object gets to the top of the atmosphere, it will have acquired all of escape velocity except that which it would (try to) pick up in the last 50 miles. By even the Earth's escape velocity of 22,263 mps is quite slow compared to the approach of most meteoroids. Leonids are among the fastest (70,000 mps) in approach velocity (theirs and ours). Most objects from the asteroid zone are going to intercept Earth at twice our escape velocity or more. The slow fireball is a rarity, but the one most likely to get something to the ground. The statistics of meteorites (on the ground) are misleading: irons are much rarer than their proportion on our collections. It's just that they can withstand re-entry so much better than rocks and that they can persist longer in an Earth environment than mere rocks do. In re-entry, irons are better than rocks; slow rocks are better than fast ones; big rocks are better than little ones. A meteorite in the hand is better than 1000 in freefall. Sterling K. Webb __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list Any questions? Get answers on any topic at www.Answers.yahoo.com. Try it now. __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Hi, Ed said: Think of it as the ultimate test of human intelligence. Will we pass? I don't know. In 1752, another author, Voltaire, wrote a story about a giant alien tourist from Sirius, Micromegas, and his companion from Saturn, who tour the solar system and visit the Earth. The Saturnian believes that the Earth cannot be inhabited: In truth, what chiefly makes me think there is no inhabitant of this sphere, is that I cannot suppose any sensible being would wish to live here. Well, said Micromegas, perhaps the beings who inhabit it do not possess good sense. Sterling K. Webb -- Text of Micromegas: http://wondersmith.com/scifi/micro.htm -- - Original Message - From: E.P. Grondine [EMAIL PROTECTED] To: meteorite-list@meteoritecentral.com Sent: Friday, December 08, 2006 8:29 AM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Hi all - I just wrote a book on man and impact. It's called Man and Impact in the Americas, and it's available through amazon. I'm tired now, so I'll keep this short. The experts numbers for impact appear to be off by about factor of ten, in the impactors' favor, not man's. Over the last 6,000,000 years, we've come close to extinction several times. Impact rate estimates have been crippled for about 30 years, largely due to confusion spread by Dr. David Morrison over the role of comets in impact. While Morrison did pioneer ground breaking work with Shoemaker some years back, since then his use of the power he gained from that work has been to the detriment of the field, and the detriment of us all. We can deal with this now, with the technologies we have in hand, but only if we make a concerted effort. Think of it as the ultimate test of human intelligence. Will we pass? I don't know. I'm going to get some more coffee and cigarettes. good hunting, Ed __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
In a message dated 12/7/2006 12:02:46 PM Eastern Standard Time, [EMAIL PROTECTED] writes: Reentering space junk is slow, and is usually reported as green. Chris I'm guessing that 'space junk' is slower because it was in orbit, and as the orbit decayed it entered the atmosphere as a shallow angle. Then, as the atmosphere grew thicker, it slowed gradually. All of the green fireballs I've seen during my years of hiking and camping out west were close to the ground. The much smaller and more numerous ones further away always appeared white. __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Objects in orbit around the Earth reenter close to Earth's escape velocity, which sets the lower limit for anything entering our atmosphere (the upper limit is set by the escape velocity of the Sun at the Earth- it's unlikely that anything we encounter would be faster than that). And for the most part, as you note, reentering objects are usually in flat trajectories, so they burn much longer, and are likely to slow down enough to stop burning before vaporizing. The Air Force has a group whose mission is to recover fallen junk. I'm not sure what you mean by close to the ground- anything you saw was probably more than 20 miles high, with 50 being more likely. There's no way to tell by eye how high a fireball actually is. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com - Original Message - From: [EMAIL PROTECTED] To: meteorite-list@meteoritecentral.com Sent: Thursday, December 07, 2006 12:17 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 I'm guessing that 'space junk' is slower because it was in orbit, and as the orbit decayed it entered the atmosphere as a shallow angle. Then, as the atmosphere grew thicker, it slowed gradually. All of the green fireballs I've seen during my years of hiking and camping out west were close to the ground. The much smaller and more numerous ones further away always appeared white. __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Hi, Visual, Chris, List For the benefit of Listees following the question of how slow a meteoroid can be... The orbital velocity for any body is maximally the escape velocity divided by the square root of 2, or 70.707070707...%. Can we just call that 71%? Escape velocity is 11,263.04 meters per second. So, the highest orbital velocity is 7964.17 meters per second. That's the orbital velocity at the lowest possible orbit, skimming over the surface. The orbital velocity gets less and less the higher the orbit, so that geo- synchronous orbital velocity is positively pokey, around 3000 meters per second. You have to go faster than that just to get there, then slow down to stay there. Crazy stuff, that gravity. The only orbit that can decay is one close enough to the top of the atmosphere to be slowed into re-entry. But (big but), the only way an object from somewhere not of this earth can get to the top of our atmosphere is to fall there, in the course of which fall, it will acquire additional velocity, up to escape velocity. Escape velocity is like taxes, in that there just doesn't seem to be any way to wiggle out. By the time an object gets to the top of the atmosphere, it will have acquired all of escape velocity except that which it would (try to) pick up in the last 50 miles. By even the Earth's escape velocity of 22,263 mps is quite slow compared to the approach of most meteoroids. Leonids are among the fastest (70,000 mps) in approach velocity (theirs and ours). Most objects from the asteroid zone are going to intercept Earth at twice our escape velocity or more. The slow fireball is a rarity, but the one most likely to get something to the ground. The statistics of meteorites (on the ground) are misleading: irons are much rarer than their proportion on our collections. It's just that they can withstand re-entry so much better than rocks and that they can persist longer in an Earth environment than mere rocks do. In re-entry, irons are better than rocks; slow rocks are better than fast ones; big rocks are better than little ones. A meteorite in the hand is better than 1000 in freefall. Sterling K. Webb - Original Message - From: Chris Peterson [EMAIL PROTECTED] To: meteorite-list@meteoritecentral.com Sent: Thursday, December 07, 2006 5:48 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Objects in orbit around the Earth reenter close to Earth's escape velocity, which sets the lower limit for anything entering our atmosphere (the upper limit is set by the escape velocity of the Sun at the Earth- it's unlikely that anything we encounter would be faster than that). And for the most part, as you note, reentering objects are usually in flat trajectories, so they burn much longer, and are likely to slow down enough to stop burning before vaporizing. The Air Force has a group whose mission is to recover fallen junk. I'm not sure what you mean by close to the ground- anything you saw was probably more than 20 miles high, with 50 being more likely. There's no way to tell by eye how high a fireball actually is. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com - Original Message - From: [EMAIL PROTECTED] To: meteorite-list@meteoritecentral.com Sent: Thursday, December 07, 2006 12:17 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 I'm guessing that 'space junk' is slower because it was in orbit, and as the orbit decayed it entered the atmosphere as a shallow angle. Then, as the atmosphere grew thicker, it slowed gradually. All of the green fireballs I've seen during my years of hiking and camping out west were close to the ground. The much smaller and more numerous ones further away always appeared white. __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
Comet Tempel-Tuttle, the parent body of the Leonids, is in a low-inclination, retrograde orbit. We encounter the debris at 71 km/s, and our own orbital speed is 29.6 km/s. Subtract that out and you get the orbital speed for Leonid meteoroids: ~41.4 km/s. The solar escape velocity at the Earth is 42.1 km/s. That's why the Leonids are as fast as any periodic meteors can be- faster meteoroids would leave the Solar System. Of course, a sporadic meteor could be produced by a body that would escape the Solar System if it didn't encounter the Earth- either because it originated outside the Solar System, or because it picked up enough energy through momentum transfer during some sort of slingshot around another body. I don't know if anybody has worked out the likelihood of that happening- very, very rare I'm sure. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com - Original Message - From: Sterling K. Webb [EMAIL PROTECTED] To: Chris Peterson [EMAIL PROTECTED]; meteorite-list@meteoritecentral.com Sent: Thursday, December 07, 2006 8:30 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 Hi, Visual, Chris, List For the benefit of Listees following the question of how slow a meteoroid can be... The orbital velocity for any body is maximally the escape velocity divided by the square root of 2, or 70.707070707...%. Can we just call that 71%? Escape velocity is 11,263.04 meters per second. So, the highest orbital velocity is 7964.17 meters per second. That's the orbital velocity at the lowest possible orbit, skimming over the surface. The orbital velocity gets less and less the higher the orbit, so that geo- synchronous orbital velocity is positively pokey, around 3000 meters per second. You have to go faster than that just to get there, then slow down to stay there. Crazy stuff, that gravity. The only orbit that can decay is one close enough to the top of the atmosphere to be slowed into re-entry. But (big but), the only way an object from somewhere not of this earth can get to the top of our atmosphere is to fall there, in the course of which fall, it will acquire additional velocity, up to escape velocity. Escape velocity is like taxes, in that there just doesn't seem to be any way to wiggle out. By the time an object gets to the top of the atmosphere, it will have acquired all of escape velocity except that which it would (try to) pick up in the last 50 miles. By even the Earth's escape velocity of 22,263 mps is quite slow compared to the approach of most meteoroids. Leonids are among the fastest (70,000 mps) in approach velocity (theirs and ours). Most objects from the asteroid zone are going to intercept Earth at twice our escape velocity or more. The slow fireball is a rarity, but the one most likely to get something to the ground. The statistics of meteorites (on the ground) are misleading: irons are much rarer than their proportion on our collections. It's just that they can withstand re-entry so much better than rocks and that they can persist longer in an Earth environment than mere rocks do. In re-entry, irons are better than rocks; slow rocks are better than fast ones; big rocks are better than little ones. A meteorite in the hand is better than 1000 in freefall. Sterling K. Webb __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
] Meteorite-list Digest, Vol 36, Issue 28 Comet Tempel-Tuttle, the parent body of the Leonids, is in a low-inclination, retrograde orbit. We encounter the debris at 71 km/s, and our own orbital speed is 29.6 km/s. Subtract that out and you get the orbital speed for Leonid meteoroids: ~41.4 km/s. The solar escape velocity at the Earth is 42.1 km/s. That's why the Leonids are as fast as any periodic meteors can be- faster meteoroids would leave the Solar System. Of course, a sporadic meteor could be produced by a body that would escape the Solar System if it didn't encounter the Earth- either because it originated outside the Solar System, or because it picked up enough energy through momentum transfer during some sort of slingshot around another body. I don't know if anybody has worked out the likelihood of that happening- very, very rare I'm sure. Chris * Chris L Peterson Cloudbait Observatory http://www.cloudbait.com __ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
