What caused the massive explosions in Russia?

[Jimmy Snyder]

Here is a contrived, but possible situation. An asteroid hits the Moon a glancing blow and is sent from there to a point just past the Earth-Moon L1 Lagrange point with a velocity nearly zero w.r.t. the Earth. At what speed will this asteroid hit the Earth? Neglect air resistance.

 

[D H]

Now we'll have to quibble over the definitions of "moon" and "asteroid" as it pertains to object size.

And longevity. Those chaotic orbits aren't very stable. Moons are things that are more or less permanently resident. (Some of the moons of Jupiter are dubious as "moons".) Those objects drift into the vicinity of the Earth inside what's called a weak stability boundary. They come for a visit, but they don't stick around for long.

A couple of examples of such temporary moons: J002E3, which turned out to be the upper stage of the Apollo 12 spacecraft , and 2006 RH₁₂₀, which is a natural satellite.

NASA recently used weak stability boundary transfer concepts to get the GRAIL satellites to the Moon. Instead of going directly to the Moon, the satellites went out almost to the Sun-Earth L1 point and then fell back toward the Moon on a ballistic capture trajectory. The 3 to 3.5 day journey to the Moon used by the Apollo program became a two month long slow drift, but saved a whole lot of fuel. (The so-called Interplanetary Highway System would be more aptly named as The Interplanetary Conestoga Wagon Trail. It's never fast to use those weak stability boundary transfers.)

 

[aquitaine]

Being "scared" by a brief flurry in the media is very different from pouring your hard earned cash into a tenuous form of insurance, which is what the anti-asteroid projects would be. A tremendous run on plastic sheeting and duck tape was the only example of substance that you quoted - how much did that cost the few people who bought it? What were the real figures? My point is that this would require a significant lifestyle change for the wealth nations, whatever you may say about it being 'free' because private companies are paying for it.
You forget that these so called good will projects are significantly funded through the tax breaks that the companies are allowed on the strength of them. Successful capitalists do nothing (altruistic) that doesn't make financial sense to them. Has Warren Buffett put money into it? I know he's given away a vast amount for charity so perhaps he's not a good example. Perhaps I should ask if he would have done something like that forty years ago.

Ok, what tax breaks do Planetary Resources get? They do have significant private investment from such people as Larry Page, Erich Schmidt, James Cameron and some other highly successful people. Show me where there is significant government intervention specifically for them.

The global warming thing is more in favour of my argument, I would say. It's only been a few governments that have actually encouraged action in that direction and the 'deniers' are the majority in big business, except where there's a profit to he had.

And not one of those governments that is "encouraging" action actually has a realistic plan to do it. Denmark has been spending huge amounts of tax money on wind farms. And 20 years later the percentage of electricity generated from coal is still 80%, higher than China's. There's a very basic reason for this, fear of nuclear power. This actually goes towards my point because it's a long running irrational hysteria. Germany in the past ten years has spent 100 billion Euros of tax money on solar alone plus a rather substantial amount of tax money on wind farms. So when it shutdown all of it's nuclear power plants (because of that fear factor I mentioned) despite the truly stunning amounts of money spent on the green saviors they ended up with major shortfalls in electricity production, an unstable grid, and skyrocketing electricity costs. So what are they going to do about? Why go back to COAL and NATURAL GAS of course. In other words, more fossil fuels. All because of fear. You can make people afraid of almost anything for a surprisingly long period of time as long as you control the narrative in a way that preys on their ignorance.

The reason the other governments aren't so into it is because most of them are poor. They want to develop their economies and that's what their constituents also want. The difference here is the narrative and in developing countries they don't have the same levels of fear mongering about this, so that's not high on their list of priorities. Interestingly enough one of the effects of this is to render all of our CO2 control efforts, ineffectual as they often are, a complete waste of time. Even if our carbon emissions went to zero it wouldn't matter because the growth of the third world.

There is no identifiable profit in perhaps pushing and, as yet unidentified, asteroid out of the way in twenty year's time - which is on a timescale that few company accountants work.

That wasn't my point. My point was that the means are private now which has reduced the costs of launching a probably government paid for asteroid pusher thing into space pretty significantly, which does matter a lot.

 

[BobG]

Bob, I don't know if you've changed your position or if I misunderstood earlier, but your examples of the Earth sweeping out its orbit and Saturn's rings lead me to think that you were under the impression that all near-earth asteroids would necessarily hit earth/earth's atmosphere significantly below escape velocity. Edit: note, you continue to use wording that implies you think all NEOs would impact at a speed below escape velocity: Do you really mean 'could result in an impact at less than escape velocity if properly perturbed just prior to impact'?

If all you meant was that if some event like a close encounter with the moon perturbs its trajectory just before it hits Earth (or bouncing off the atmosphere or getting hit by a nuclear bomb), an asteroid could hit the Earth a little below escape velocity, then we're all good. The paper I linked mentions the effect of the moon. Heck, the moon has its own mass and escape velocity and will often act to increase the impact velocity on earth!

Perhaps you have misunderstood my position, since this started with a post where I agreed you were essentially correct, but only because the probability of an impact at less than escape velocity was so incredibly low - not because they were impossible.

Which eventually prompted your reply...

I also think the difference between "very unlikely" and "impossible" is significant enough to matter.

But it isn't impossible and the perturbation doesn't have to occur just before impact (at least not if you mean the type of duration described by being perturbed by the Moon just before impact). It's original orbit has to be altered so that it's new orbit is almost the same size as the Earth's orbit, it's orbital plane has to be very close to the Earth's orbital plane, the trailing object has to be slightly lower than the leading object's position as they approach towards the collision - and the collision practically has to happen on the first close approach of the two objects, since the orbit of the asteroid is going to be drastically changed by the close approach. (Or perhaps an asteroid starts out not meeting those criteria, but a close approach changes the orbit to meet those criteria.)

It has to be an asteroid that has moved into the Earth region recently, since the Earth tends to clean out debris too close to its own orbit. While that happens continuously (or else we wouldn't have as many near Earth asteroids as we do and also means "clearing the neighbourhood" is an unending task, as DH pointed out), the probability of altering it to meet the criteria for a low velocity collision makes "very unlikely" an understatement.

Actually, I think I did say, or at least implied, that there was a time in the past (early in the solar system) that low velocity impacts were common (even if a very low percentage). They aren't common now.

 

[OmCheeto]

I can see several reasons why a planetary defense system would not be feasible against meteors.

I came across the website of the American Meteor Society and noticed that the number of reports of 'fireballs' increased from zero in 2004 - 2006 to 2219 in 2012..

Here is the full result

https://dl.dropbox.com/u/22026080/meteor-rep.jpg

Now which of the following is true?:

A. it's unreliable/fake;
B. armageddon 12-21-12-hype;
C. it merely reflects the profileration of the site;
D. Earth is actually entering a dirty piece of the cosmos
E. a bit of all of the above?

"D" sounds plausible, based on the numbers.
With an orbital period of 240,000,000 years for the sun to go around the galactic center, we should look for clues around that time period in the past.

It looks like we transitioned from the Paleozoic to the Mesozoic eras around 250,000,000 years ago. Did anything special happen back then? I'm not much of a historian. Lot's-o-space rocks maybe? This might be fun. I've never seen a big meteorite before.

hmmm...

The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct.

Oh my. I think I'll stock up on canned beans and toilet paper.

 

[russ_watters]

But it isn't impossible and the perturbation doesn't have to occur just before impact...

I'm still unclear. It still sounds like you are saying that an object can approach from a long distance (say, 10 million km since its last major interaction), not interact with the moon on its way in, and hit Earth significantly below escape velocity and that a significant class of NEOs fit that description, regardless of if we've cleared most of them out (note: according to the paper I linked, these are still the most likely objects to hit us).

Or put another way, it sounds like you think an unperturbed trajectory (again, of at least the last 10 million km) exists that can cause a below escape velocity impact and that most or all NEOs that hit us have such such a trajectory by definition of being NEOs. It doesn't make sense to me that you used the Earth sweeping out its NEOs as an example unless you think a significant fraction of them (all of them?) Impacted at below escape velocity.

Could you please try to answer more succinctly because all of the qualifiers you are putting on this don't add clarity and seem to contradict your examples.

 

[D H]

I'm still unclear. It still sounds like you are saying that an object can approach from a long distance (say, 10 million km since its last major interaction), not interact with the moon on its way in, and hit Earth significantly below escape velocity

Yes, that is possible Russ. Significantly below escape velocity? No, but below is possible. Likely? No. It's rather unlikely. It is not, however impossible. It is impossible in the context of the two body problem (the Earth, the asteroid, and effectively nothing else). Add a big third body, i.e. the Sun, and things get weird.

and that a significant class of NEOs fit that description, regardless of if we've cleared most of them out (note: according to the paper I linked, these are still the most likely objects to hit us).

It's a small fraction of NEOs that fit this description. The vast majority that impact the Earth will do so with a velocity greater than escape velocity. Saying that all of them will isn't quite true, but it's pretty close to being true. It's close enough to being true that a lay article can legitimately say that it is true.

Regarding the paper you linked, the authors didn't write clearly, which in turn led you to misread/misinterpret what they wrote. This is, I think, the bit from that article that you are interpreting incorrectly:

Most asteroids snagged by Earth's gravity don't zoom around our planet in neat circles, according to the simulation. Instead, they follow complicated, twisting paths, tugged this way and that by the gravitational pulls of Earth, the moon and the sun.​

That "snagged by Earth's gravity" was not alluding to the asteroids that impact the Earth. It was alluding to the very, very few asteroids that, at least temporarily, orbit the Earth. Most (almost all) asteroids that impact the Earth aren't "snagged by Earth's gravity" (i.e., orbiting). They came into the vicinity of the Earth on a hyperbolic trajectory and they would have immediately have left on a hyperbolic trajectory had the Earth not have been in the way.

What that part of the article was alluding to is that it is unlikely to capture an asteroid and have it end up in a circular orbit. Very, very unlikely. For example, the "captured asteroid" explanation for the moons of Mars has come under increased scrutiny because this explanation doesn't make sense from a dynamical point of view.

 

[g.lemaitre]

If the Russian meteorite released 20 Hiroshimas of energy, then how come only two people were injured? I'm guessing they're measuring energy from the time it entered the Earth's atmosphere until it hit the ground, whereas the Hiroshima Bomb released all of its energy right at one time, the meteorite released its energy over a long period of time. Still, that doesn't sound like a good explanation to me.

 

[Borek]

If the Russian meteorite released 20 Hiroshimas of energy, then how come only two people were injured? I'm guessing they're measuring energy from the time it entered the Earth's atmosphere until it hit the ground, whereas the Hiroshima Bomb released all of its energy right at one time, the meteorite released its energy over a long period of time. Still, that doesn't sound like a good explanation to me.

Not necessarily - it is a matter of distance from the explosion.

 

[Borg]

If the Russian meteorite released 20 Hiroshimas of energy, then how come only two people were injured? I'm guessing they're measuring energy from the time it entered the Earth's atmosphere until it hit the ground, whereas the Hiroshima Bomb released all of its energy right at one time, the meteorite released its energy over a long period of time. Still, that doesn't sound like a good explanation to me.

As Borek stated, it's the distance. And there were 1200 people injured - mostly from flying glass. The main shock wave was strong enough to blow out windows all over the city but only one building's roof partially collapsed. If it was a little bigger, this event could have been a lot worse. Also, it's been reported that there were three separate shock waves. The first one, along with the initial flash got everyones attention. Then, when people went to look out the window, the second shockwave hit strong enough to blow the windows in their faces. Nature can be unforgiving sometimes.

 

[OmCheeto]

Not necessarily - it is a matter of distance from the explosion.

And I would imagine, duration. The meteor would have been dissipating energy the entire time, from entry, to disintegration. Does anyone know how far it traveled through the atmosphere?

hmmm...

from my calculations, this meteor arrived with a speed of 17.8 km/sec

Earthsky.org lists these numbers for various showers:

Leonids: 71 kilometers per second
Perseids: 61 kilometers per second
Orionids: 67 kilometers per second
Lyrids: 48 kilometers per second
Geminids: 35 kilometers per second
Fall Taurids: 30 kilometers per second
Delta Leonids: 23 kilometers per second
Draconids: 23 kilometers per second

ref

Bah! Lame! Slower than a Draconid!

 

[BobG]

Or put another way, it sounds like you think an unperturbed trajectory (again, of at least the last 10 million km) exists that can cause a below escape velocity impact and that most or all NEOs that hit us have such such a trajectory by definition of being NEOs. It doesn't make sense to me that you used the Earth sweeping out its NEOs as an example unless you think a significant fraction of them (all of them?) Impacted at below escape velocity.

Could you please try to answer more succinctly because all of the qualifiers you are putting on this don't add clarity and seem to contradict your examples.

If there's a lot of debris sharing the Earth's orbit (as there was early in the Solar System), then there's a lot of collisions. Low velocity impacts are still the exception, but there's still a lot of them.

You can't have an unperturbed trajectory. Both the asteroid and the Earth are orbiting the Sun and the Earth is perturbing the asteroid's orbit (and, likewise, the asteroid is just slightly perturbing the Earth's orbit).

To glance at the orbits of near Earth asteroids, you'd say a lot of them look pretty similar, but having a similar size isn't enough. They have to match other parameters as well. And they have to collide before their orbit is changed again.

 

[Solon]

Now that the large chunk of meteorite has been removed from the Chelyabinsk lake, I'm puzzled as to how it remained intact and also ended up 6 feet or so below the lake bed. I'm getting mixed results from different experts on meteorites as to the terminal velocity on impact with the water, anywhere from 100 to 600 miles per hour. !00 seems slow to me, 600 probably too high. The streamlined kinetic impact weapons will reach up to Mach 2 from 50,000 ft, 900 mph from 30,000 ft, so an irregular object must be at the lower end of the estimates it seems.

Using this calculator, with a "brick" drag coefficient, 600 Kg, and 1 meter cross section gives impact speed of 70 M/s. (150 mph, 230 ft/sec)
http://www.calctool.org/CALC/eng/aerospace/terminal
From info on the chunk:

"The rock crumbled into several chunks as scientists began lifting it from the ground with the help of levers and ropes."

Crumbled would suggest a relatively soft object, and that should not have survived impact. Could a much harder object, even at 100 mph, remain whole on initial contact? Water is incompressible and quite dense. It will stop a 50 caliber machine gun bullet in 5 feet or less of water, so how far could the meteorite have traveled before being stopped? How much energy would be dissipated? I'm stuck trying to form a complete, mechanically sound model of this work in my head, so hoping someone can put some numbers together that do make sense!

 

[Bandersnatch]

Now that the large chunk of meteorite has been removed from the Chelyabinsk lake, I'm puzzled as to how it remained intact and also ended up 6 feet or so below the lake bed. I'm getting mixed results from different experts on meteorites as to the terminal velocity on impact with the water, anywhere from 100 to 600 miles per hour. !00 seems slow to me, 600 probably too high. The streamlined kinetic impact weapons will reach up to Mach 2 from 50,000 ft, 900 mph from 30,000 ft, so an irregular object must be at the lower end of the estimates it seems.

Using this calculator, with a "brick" drag coefficient, 600 Kg, and 1 meter cross section gives impact speed of 70 M/s. (150 mph, 230 ft/sec)
http://www.calctool.org/CALC/eng/aerospace/terminal

Why do you think it was falling at terminal velocity? (hint: see post #101)

 

[Solon]

"Meteorites hit the ground at terminal velocity, about 200-400 miles per hour."

http://meteorites.wustl.edu/realities.htm

I thought that some of the forward velocity the meteorite carried from the meteor entering the atmosphere would have been added to the gravitational acceleration, but that's not the case, so I am informed.