The speed of incomming asteroids?

yevaud · Jun 29, 2006

Who knows, you may be right about the attitude thing. I don't know if you noticed or not, but I have been pretty hands-off with the Science Forums the last few months, in part because I don't want people to feel that they're being micro-managed by me. If they run ok with me just watching them and not interfering, well and good (and they have been running ok). Anyways, I'm not an elitist. I just really disagree with Wayne's long lecture for the very reasons stated. I have long since taken the route of posting concepts that follow the law of the lowest common denominator, realizing what I'd said about the range of people who come here. In short, I've been trying to be anything *but* elitist about it. Science for the common folk, as it were. I don't require being corrected at every turn by someone newly here. And I do understand that Wayne was trying to show me something he felt I needed to know, but that's a bit much, don't you think? He hasn't any idea of what I know or what I studied, he merely assumed. He was quibbling about percentage points wrt a perfectly valid rule of thumb. So be it, I suppose. I hope he returns so I can explain myself to him, I really do. As far as you being a peckerhead (<img src="/images/icons/wink.gif" />) you aren't. No more than the rest of us can be, here and there. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

MeteorWayne · Jul 1, 2006

OK, this may be my final post (or not) so let's start from the top. I saw a forum titled Ask the Astronomer, so I checked it out. I naively assumed it was a place for people to come, ask questions, and get accurate answers from "The Astronomer". I happened to see a question that was in an area in which I have expertise, so I answered it. The person who asked the question (Temporal Mechanic) loved my answer, especially since it had no vectors

Boris1961 and Spacester liked my answer. THEN, Yevaud came in to refute my accurate information with "To my recollection, this is the final rule on this...." Since the person who asked the question had his answer, why was this necessary? Yevaud, it was you who made the demeaning, sarcastic comment "I'll inform the publishers of my college Astrophysics text...." Yevaud, you asked my to explain to you how the rule was incorrect, and I did. I did not lecture, I answered your question. I thought this forum was to ask and answer questions. To quote from your last post "Anyways, I'm not an elitist. I just really disagree with Wayne's long lecture for the very reasons stated. I have long since taken the route of posting concepts that follow the law of the lowest common denominator, realizing what I'd said about the range of people who come here. In short, I've been trying to be anything *but* elitist about it. Science for the common folk, as it were. I don't require being corrected at every turn by someone newly here. " I disagree with your concept that things should be dumbed down to the lowest common denominator. Someone asked a question and I answered it, he (or she, don't want to assume) found the answer satisfying. I disagree that children and amateurs should not be given the facts, and I disagree that they are the only people in an Ask the Astronomer forum. That's not why I came here. I came here to ask questions, and if I <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

MeteorWayne · Jul 1, 2006

Regarding Hartmann's book, my county library does not have it (they have 4 others by him that I will check out) so I'll have to widen my search to find it. But I will ..... Meteor Wayne <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

MeteorWayne · Jul 1, 2006

Wonderful, we have this book in out astronomy club's library, so I have it out to read! More Later Meteor Wayne <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

yevaud · Jul 1, 2006

Hmm. Well. First, since this was the main bone of contention between us, I should first address it. This is a fairly common and well understood rule of thumb in Astronomy; so basic that there is little but casual reference to it on the Internet. But there are some that closely address it. Here’s a thread from Phil Plait, the “Bad Astronomer.” He is critiquing the movie “Asteroid.” Please note the highlighted (orange) excerpts: Good: At 60,000 miles per hour, that rock will go through the atmosphere in a few seconds unless it is at a very shallow angle. At an angle that small it would skip off the atmosphere like a rock on water. This brings up another point. When small meteors hit the ground, the impact velocity is actually very small!. Only large (like, hundreds of meters across) meteors will actually hit the ground with a large enough force to cause an explosion. Small ones slow down drastically when they hit the atmosphere, and reach a speed of only a few hundred kilometers an hour while they are still many kilometers in the air. They reach terminal velocity, and go no faster. Note that a meteorite hit a car in New York, and while the car was ruined, there was no explosion. It was like dropping a chunk of metal from a few hundred meters up. That's how slowly these things move. http://www.badastronomy.com/bad/tv/asteroid.html Now please note carefully what Phil is stating here. First, he notes that an asteroid with a high velocity will penetrate the atmosphere to impact in a very short period of time. This is case one of the rule of thumb, in which the impact velocity of the asteroid greatly exceeds Earth’s terminal velocity and as a result, the atmosphere is no impediment. It effectively impacts a <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

Saiph · Jul 2, 2006

yevaud, slight mistake here in bold: <blockquote>In reply to:<hr />The other Case of the rule of thumb is fulfilled by the second example. Small asteroids, even to a modestly high approach velocity, will be significantly slowed down by passage through our atmosphere by friction with the medium. As you probably understand, once its velocity is lowered sufficiently, it will fall at Escape Velocity. I rather doubt it could fall slower than. <hr /></blockquote> Small asteroids with a small mass to surface area ratio (i.e. smaller masses compared to surface area) will be slowed to terminal velocity (typicall between 100 to 200 mph), which is much slower than escape velocity (11 km/s) <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

yevaud · Jul 2, 2006

Yes, true, I did say "escape" and I meant "terminal." My mistake. Thank you. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

nexium · Jul 3, 2006

A little eleteism is ok: The Elete keep society moving efficiently, but they rarely help society progress in important ways as the elete have typically been brain washed by the mainstream. May this forum serve both the common and the elte. Good results may follow. Neil

nexium · Jul 3, 2006

Hi Yevarud. It is entirely possible and probable that your college Astrophysics text has some errors. I think you should appologizes for the inference that it might be infallable and/or the sarcasm. My guess is the escape velosity is the minimum speed at which an unpowered object can approach Earth's atmosphere, but an outgasing asteroid is at least slightly powered. Neil

yevaud · Jul 3, 2006

Ok, let's carefully analyze this, please. We are both, presumably, utilizing textbooks (College, I would assume) as our sources of material. It appears to be implied here that what I use must, defacto, be incorrect, yet Wayne's is fully accurate. I have not stated that there may or may not be errors in the material I remember from school; there may well be (as an example of the possibility of error, I'd been saying "escape velocity" when I meant "terminal velocity." This was pointed out to me, and I so noted this). However, I have neither heard any admission of this possibility from Wayne. Yet, I am told mine must be incorrect. Hmmm. This is hardly logical or fair. Further, while in both cases they have been written and then vetted and published by professionals in this field, I have the additional correction factor of it having been selected and frequently used by Professors in their courses. One would think that such a glaring error would be noticed by the Professors, the students, or anyone else who utilized this publication. However, Wayne has stated that he is not a professional (however much a skilled amateur he may be), and presumably has self-selected his material. Further, he does not appear to have anyone in such numbers or ability to note and point out errors in his material. So why am I being hit by several of you as to the possibility of error in my referential material, yet Wayne has not? Is he Infallible? One wonders here. The next is about the nature of what I posted. What is the issue here? I am in error on a public message board that caters to the masses from speaking to those masses in understandable terms? Which is, in fact, a part of my job here? This too seems very questionable. I do disagree with what Wayne said. I found his information to be incomplete and somewhat misleading. The worst thing I have done here is make an addition/correction to his statemen <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

MeteorWayne · Jul 5, 2006

Hi, I'm back. Sorry it took so long, but I've been reading "Moons and Planets" by Hartmann, and collecting my own references. I'm going to split this reply into a few parts, since some of this discussion has drifted from the original question. I'd appreciate any suggestions as to where second and/or third parts of the discussion belong. First and foremost, let's look at the original question and my original response. Temporal Mechanic asked: "I'm thinking of the Cretacious asteroid, but it doesn't matter. How can we know the velocity of a meteor or asteroid as it APPROACHES the earth? It seems like it always hovers around 45,000 mph." My response began with this statement: "The minimum speed of an impacting object (IGNORING ATMOSPHERIC DECELERATION) is 11.2 kps(km per sec)=7 mps(miles per sec)=27,500mph(miles per hour); i.e. the earth's escape velocity. This is for an object just falling into the earths gravity well." I then went into the rest. I have capitalized some words for emphasis. Clearly he asked about approach velocities, and I clearly responded with pre atmospheric speeds. I continued with derivation of the maximum and probable APPROACH speeds. I gave the formula that derived the information, which I looked up (not because I didn't know it, but because I wanted to ensure I wrote it accurately) in the International Meteor Organization's Monograph "Visual Meteor Handbook. I have checked and include the following references, which mostly describe it in the form Vgeo^2=Vinc^2+Vesc^2 Vgeo= geocentric Velocity VInc=Approach Velocity before earth's gravity Vesc= Acceleration from earth's gravity. Here are the references, all from my personal library collection. "Meteor Science and Engineering", D.R.W McKinley McGraw Hill Book Company 1961 Equation 2-6 "Meteor Astronomy" A.C.B. Lovell Oxford University Press 1954 Equation 22 <</safety_wrapper> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

MeteorWayne · Jul 5, 2006

Now , lets discuss the last post. This addresses the issues of scientific knowledge and elitism. Your post is in quotes, with my discussion preceded by Wayne />> "Ok, let's carefully analyze this, please." Wayne />> Works for me! "We are both, presumably, utilizing textbooks (College, I would assume) as our sources of material. It appears to be implied here that what I use must, defacto, be incorrect, yet Wayne's is fully accurate." Wayne />> I have not stated that, however mine is a law of physics, and yours in a recollection, and the reference you have provided is not correct. I have provided several; as a law of physics I could provide dozens more. I have an extensive library of books on this specific subject that I have read and absorbed. I have read HUNDREDS of articles in the journals Science, Nature, Astronomy and Astrophysics, Lunar and Planetary Science, et al. I am passionate about the very subject that the question was asked about. I am in fact, the most qualified person in this forum (so far) to answer the question. If not, PROVE me wrong. Y />"I have not stated that there may or may not be errors in the material I remember from school; there may well be (as an example of the possibility of error, I'd been saying "escape velocity" when I meant "terminal velocity." This was pointed out to me, and I so noted this)." Wayne />> Yeah, but that was likely just an error of misspeaking, so I didn't even bust your chops about that. Or maybe it is a subject that you are not familiar enough with, so as to catch such an error? After all it is what you remembered. I have all the facts right here in front of me right now. I also didn't speak up when you misused the term retrograde when you meant prograde (or direct) revolution of asteroids about the sun. "However, I have neither heard any admission of this possibility from Wayne. Yet, I am told mine must be incorrect. Hmmm. Thi <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

yevaud · Jul 5, 2006

Responses shortly. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

MeteorWayne · Jul 6, 2006

Back to more fruitful pusuits, the original question..... Yesterday Yevaud posted this list of recent close approaches to earth (He has since pulled it back, but I found the list). This is the last 10 close approaches, from June 21 to July 3. It's therefore kind of a semi-random sampling of the population. Vinc is velocity relative to earth before gravity in km/sec Vapp is approach speed BEFORE ATMOSPHERIC DRAG in km/sec kmph is the same in thousands of miles per hour, which is the original question TemporalMechanic asked. Object......Vinc......Vapp......kMPH 2006 MV1......5.04......12.3......28 2006 KC40....13.52.....17.5.....39 2006 KK103...6.41......12.9......29 2001 TX1.......18.39....21.5......48 2006 MB14.....10.56....15.4.....34 2006 MH10......6.90.....13.1.....29 2003 OS13......31.11....33.1.....70 2003 BX33.......11.22....15.8....36 2004 XP14......17.41.....20.7....47 2006 MA14......6.89......13.1.....30 So for these 10 close approaches, The Max PRE-ATMOSPHERIC speed was 70,000 mph, the lowest, 28,000, the Median 35,000 and the Mean 39,000 This compares pretty favorably the minimum possible of 25,200. 4 are less than 30,000 (catching up slowly from behind), 3 more 40,000 or less. Remebmber, most of the stuff in the solar system revolves the same direction as the earth, nearly in the plane of the earth. The object 2003 OS 13 is a real outlier. It's in an orbit with a 2.26 year period at an inclination of 41 degrees to the ecliptic. These account for the higher approach speed. It also happens to be the largest, between 1 and 2 km in size. That would be an ouchy! Fortunately the closest approach was 35 times the distance to the moon. Next, just for fun, and this will take a few days, I will estimate the potential impact speed at the surface taking atmospheric drag into account. These will be VERY rough estimates, but it's just for fun

Finally, I will a <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

yevaud · Jul 6, 2006

He has since pulled it, because responding to those 10,000 word posts is a royal pain. I'm re-writing it for clarity, and will repost it again, later. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

derekmcd · Jul 6, 2006

The angle of impact? <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

tony873004 · Jul 6, 2006

This is the same formula that MeteorWayne gives but written a slightly different way. It's identical to the formula used to launch a rocket from Earth on a hyperbolic trajectory with excess hyperbolic velocity (velocity at infinity).

yevaud · Jul 6, 2006

Gas density. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

MeteorWayne · Jul 6, 2006

Hmmm, well I guess angle of impact is correct, but it's not what I'm thinking of. I should award at least the palms on that

Gas density is not relevant for a K-T class object, since the atmosphere has no effect Here are two interesting links for those that wish to pursue this a little. http://neo.jpl.nasa.gov/ca/ This is the NASA/Jet Propulsion Lab page which lists those closest approaches. A great site relevant to the question. http://www.lpl.arizona.edu/impacteffects/ Now this one is fun. It (approximately) calculates the impact effect for objects coming in. You fill in how big the object is, what it's made of, and how fast it is travelling at the top of the atmosphere. It tells you what the effects are. There's a lot to consider, so it's only an approximation, but they do explain, if you click on the link, what assumptions they've made. For example, if the largest (2.1 km) and fastest asteroid in the list above (2003 OS 13) were made of iron (like my Canyon Diablo meteorite that created Meteor Crater) came in at 90 degrees (straight down) at a PRE-ATMOSPHERIC speed of 33.1 kps it would make a crater 40 miles wide. If you were 320 km (200 miles) away: 1.7 seconds after impact (which would be at 33.1 kps/69,975 mph since the atmosphere would not slow it down a whit) the fireball would be 34X the size of the sun, and 112 X as bright. All wood would ignite. The ground shake would be eqiv to a 9.1 Mag earthquake about a minute later. Almost all buildings would collapse. The airblast wind speed would be 600 mph. Oh, and 2 feet of ejecta would cover your blown away, toasted body. Talk about an ouchy! Using the slowest and smallest object, having it made of light stone, and impact angle of 30 degrees helps a lot. No crater; it bursts apart at 24 miles and i <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

yevaud · Jul 6, 2006

Gas density is not relevant for a K-T class object, since the atmosphere has no effect You didn't mention a K-T impactor, or any masses at all (well, the table of NEO's did, that is, but your question itself was fairly generic). If you had, my answer would have been somewhat different. One thing: In the post for which we are discussing now, you'd said, "Remebmber, most of the stuff in the solar system revolves the same direction as the earth, nearly in the plane of the earth." Counting large and moderate sized bodies (Planets, Asteroids, random debris), this is largely true. However... A few days ago, you'd said “Gee, you don’t even know Prograde from Retrograde.” The better part of the material that impacts Earth (or ablates in the atmosphere en route) is Cometary in nature (not deriving from the Asteroid belt, the source of much of the Chondritic, Stony, or nickel/iron material that does enter our atmosphere). Of those – virtually all short-period Comets travel in retrograde orbits, and somewhat more than half of the long-term period Comets do. And of those long-period Comets, somewhat around half will not end up in elliptical orbits, but hyperbolic, and will thus be ejected from the solar system. In point of fact, there are only five well-known long-period Comets that move in a Prograde orbit. If you look at the Annual meteor showers - everything from the Quadrantids through Alpha Puppids - they are all Cometary in origin. Which, btw, is why I said what I did the other day. Seemed I owed you a response. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

MeteorWayne · Jul 7, 2006

Yevaud, OK I've taken my deep breaths. You are correct, I did not state what size object in my challenge. So you get the bronze oak leaf palms, too, but the brass figligee remains in play. Interestingly, I discovered another subtle effect last night while researching some errors in a popular astronomy magazine. No one's figured it out yet, so I'll hold that in abeyance for now as well. You said the better part of the material that impacts earth is cometary in nature. That may be true, at this point I can't figure out how to prove it one way or the other. However, you stated that virtually all periodic comets travel in retrograde orbits. Sorry but you've got that one backwards. It's very hard to find a current list of periodic comets that list inclinations, because so many are being discovered right now. I have used as a source http://cometography.com/periodic_comets.html A marvelous site by cometary historian Gary Kronk, current trough at least 2004 He shows 173 Periodic comets (1P through 173P) of which 169 have inclinations less than 90 degrees. These are in a prograde, direct orbit (revolving around the sun the same direction as we do.) That's 97.7% There are 16 periodic comets with good orbits that were never recovered. 15 are prograde. There are 77 periodic comets (with good orbits) awaiting their first return, of which 74 are prograde. Overall that's 258 of 266 being prograde, or 97.0% I use that number in my meteor talks all the time, I'm glad it's still accurate, since I haven't checked in a year or so. Of the 4 retrograde short period comets that have been recovered, comets, 3 cause major meteor showers. More on that later. They are: 1P Halley (eta-Aquarids and Orionids) inclination 162.2 degrees. Meteor shower entry velocity 66 km/sec. 55P Tempel-Tuttle (Leonids), incl 1 <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

spacester · Jul 7, 2006

There is one other thing to take into account. It does not affect the approach speed, but does affect the impact speed. It is a small effect, that varies across the earth's surface. Any takers?

Lumpy Gravity. The local gravitational field matters, and Earth's gravity field is not uniform. But surely this is a very tiny factor? During most of the time the object is falling down the gravity well, deviation from the mean must be infinitesimally small. <div class="Discussion_UserSignature"> </div>

MeteorWayne · Jul 7, 2006

Good one spacester. Yeah, a really tiny effect, but not one you could ignore if you wanted to account for everything. Bronze oak leaf palms for you too. I can see I need to stock up! Still, not what I was looking for. However I love all the answers, it shows how complicated science can be, giving good reason for rules of thumb if they're accurate enough. MW <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

spacester · Jul 7, 2006

Local surface velocity. The impact velocity is going to use vector arithmetic involving the incoming velocity relative to the center of the Earth, and the surface velocity, which depends on latitude. <div class="Discussion_UserSignature"> </div>

MeteorWayne · Jul 7, 2006

BINGO! That's the one I was originally looking for. Now where did I put that brass figligee???? Wayne <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

The speed of incomming asteroids?

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