What caused the massive explosions in Russia?

[sophiecentaur]

Well if you scare people enough about something you'll get them to shovel as much as you could possibly need. In reality it wouldn't take *that* much, the costs of launching things into space is going down pretty significantly, and will continue to do so. In addition to that there has been some interest in surveying asteroids from private companies, particularly planetary resources which also adds our detection capability at no cost to the taxpayer.

Is there ever anything that will cost me nothing? Somehow, the cost will always get back to the individual (me) and, with such a massive project, how could this not affect my disposable income? If it's that important then whoever does it will be charging for it, surely. I am a big believer in considering the 'total costing' when someone says "it'll be free". I question your statement about the costs of launching - I mean the real costs.

PS People don't scare that easily. So many of them are still smoking themselves to death and the risks are clear.

 

[BobG]

It is of course possible. But that doesn't change the issue. An object that was just barely overtaking Earth (or vice versa) would still accelerate to (just barely above) escape velocity at impact.

So let's say the Earth is approaching perigee. It's speed with no orbital perturbations would be:

v=\sqrt{(\mu_h (\frac{2}{147.1x10^6 km} - \frac{1}{149.6x10^6 km})}=30.285 km/sec

heliocentric gravitational constant is 1.327x10^11 km^3/sec^2
semi-major axis of Earth's orbit is 149.6 million kilometers
perigee is 147.1 million kilometers

You have an asteroid with a semi-major axis of 145 million kilometers and its apogee is 147.2 million kilometers, meaning its speed is:
v=\sqrt{(\mu_h (\frac{2}{147.2x10^6 km} - \frac{1}{145x10^6 km})}=29.8 km/sec

The asteroid goes slightly further out from the Sun than the Earth and the Earth is overtaking it. The relative velocity between the two is 485 meters/sec (0.485 km/sec)

The asteroid's gravity adds a bit of energy to the Earth's orbit, while the Earth subtracts quite a bit of energy from the asteroid's orbit.

You're going to get into issues as to the directions of the velocity vectors, making this a tough calculation, however, it can be made easier by the calculating the specific energy due to gravitational attraction.

\epsilon=\frac{v^2}{2}-\frac{\mu_g}{d}

with the geocentric gravitational constant being 3.986x10^6 km^3/sec^2

When the specific energy is 0, the object has achieved escape velocity for that current position (11 km/sec is escape velocity at the surface of the Earth, which is actually a really bizarre place to calculate it given the problems of traveling 11 km/sec through the Earth's atmosphere).

If the Earth and the asteroid are within 3.4 million km when the asteroid is at apogee and the Earth is at perigee, then the asteroid will not achieve escape trajectory relative to the Earth. If the Earth and the asteroid are at least 3.4 million km apart, then the asteroid will achieve escape velocity relative to the Earth, but could still impact the Earth just because of the direction of its trajectory. (3.4 million sounds big, but that's actually about 1.3 degrees of the Earth's orbit.)

 

[russ_watters]

11 km/sec is escape velocity at the surface of the Earth, which is actually a really bizarre place to calculate it given the problems of traveling 11 km/sec through the Earth's atmosphere.

11.2 actually, but 10.9 at the altitude of low Earth orbit, where the atmosphere is negligible: http://en.wikipedia.org/wiki/Escape_velocity

I don't get the point of any of the rest of what you did in that post. Yes, orbital dynamics is very complicated and yes a lot of it is over my head, but none of that has any impact on the issue we are discussing. The dynamics of how they come to be approaching each other does not change the fact that the approach speed (by definition) starts at a value greater than zero.

 

[russ_watters]

I don't understand what you are saying here. Why would there be any 'minimum speed'? There are some 'very unlikely speeds', true, and we would have already hit any object which had been in a stable orbit 'near' us, but an object could find itself approaching Earth at a very low speed if it had been affected, 'recently' by another planet...

Or did I not get your drift? (All too common these days )

An object "approaching Earth at a very low" speed doesn't stay "at a very low speed". It gets accelerated by the Earth's gravity.

 

[sophiecentaur]

An object "approaching Earth at a very low" speed doesn't stay "at a very low speed". It gets accelerated by the Earth's gravity.

Right - I get what you mean. The actual impact speed will have to be at least something like escape velocity, even if you just released the object from a 'stationary position' in the Earth's frame but a few tens of Mm away.

 

[BobG]

It means that, physics wise, you can have objects impact the Earth at less than escape velocities (in fact, decayed satellites do that all the time).

The reason the probability of an asteroid impacting us at less than escape velocity is virtually nil is that all of those objects had orbits so similar to ours that they've already impacted us.

And as for a stationary object starting far away from the Earth in a two-body scenario, it would be impossible for it to achieve escape velocity before impacting the Earth unless its initial distance was literally infinite. You'd need at least a little bit of tangential velocity to push it's energy level over the edge.

 

[sophiecentaur]

It means that, physics wise, you can have objects impact the Earth at less than escape velocities (in fact, decayed satellites do that all the time).

The reason the probability of an asteroid impacting us at less than escape velocity is virtually nil is that all of those objects had orbits so similar to ours that they've already impacted us.

And as for a stationary object starting far away from the Earth in a two-body scenario, it would be impossible for it to achieve escape velocity before impacting the Earth unless its initial distance was literally infinite. You'd need at least a little bit of tangential velocity to push it's energy level over the edge.

Not necessarily, surely. Could they not find themselves in that orbit for the first time ever, due to interaction with another large body (Slingshot)? Unlikely, of course.

 

[russ_watters]

It means that, physics wise, you can have objects impact the Earth at less than escape velocities (in fact, decayed satellites do that all the time).

Decayed satellites are "dropped" from low Earth orbit. Their GPE is relatively low and the trajectories fairly flat.

What it looked to me like you calculated is that for your hypothetical object, it would start its approach at 0.485 km/sec, which would result in an impact velocity of 11.685 km/sec.

And as for a stationary object starting far away from the Earth in a two-body scenario, it would be impossible for it to achieve escape velocity before impacting the Earth unless its initial distance was literally infinite.

Understood, but our number of significant digits here is pretty small, so "infinite" is not a very large number. Throwing an escape velocity equation into Excel, I find that at 750,000 km, escape velocity is 1 km/sec so an object "dropped" from that distance would still hit the Earth at more than 10 km/sec. That's close enough for our purposes.

And you're talking about probabilities here -- did you look at the scholarly paper I linked? Probability = 0.

 

[russ_watters]

Additional sources:

ESCAPE VELOCITY - the velocity required to escape entirely from the gravitational field of an orbit; also the minimum impact velocity for anybody arriving from a very great distance.

http://www.namnmeteors.org/appendixE.html

. The minimum impact
velocity for collisions with Earth is 11.2 km/s; this is, by
definition, equal to the escape velocity for an object launched
into space from Earth’s surface

http://www.lpi.usra.edu/publications/books/CB-954/chapter2.pdf

The minimum velocity of an asteroid impact is 11 km/sec. This is speed acquired simply from the Earth's gravitation.

http://www.ips.gov.au/Educational/4/3/1

The minimum impact speed is the planet’s escape velocity...

https://www.google.com/url?sa=t&rct...=r3o7W010wa7kbFtypTigOQ&bvm=bv.42553238,d.dmQ

 

[Astronuc]

This meteor was unusual because its material was so hard — it may have been made of iron, the statement said — which allowed some small fragments, or meteorites, perhaps 5 percent of the meteor’s mass, to reach the Earth’s surface. Nothing similar has been recorded in Russian territory since 2002, the statement said.

. . . .

So there was a similar event in 2002.

http://www.nytimes.com/2013/02/16/w...agments-are-said-to-rain-down-on-siberia.html

Size estimates cover a broad range - from "10-ton meteor around 10 feet in diameter" to "it was closer to 50 feet in diameter and probably weighed around 7,000 tons". But a 50 ft diameter sphere has 125 x volume of 10 ft diameter, and so assuming 10 ton for 10 ft diameter, it could be more like 1250 ton, which is still less than 7 kt. So the estimates are assuming different densities, with one assuming something closer to iron and another more like a lighter mineral.

 

[Jimmy Snyder]

So, I've gathered from this that we never saw this coming. I wonder if this is going to add impetus to develop a planetary defense system against this sort of thing...

Yes.
The Aquitaine bill.

The dramatic meteor explosion over Russia Friday (Feb. 15) highlights the need for more attention to be paid to the threat of near-Earth asteroids, an influential American congressman says

 

[BobG]

Not necessarily, surely. Could they not find themselves in that orbit for the first time ever, due to interaction with another large body (Slingshot)? Unlikely, of course.

Yes, just as in post #52. It's just that "unlikely" is a gross understatement. You'd be more likely to be struck by lightning while carrying a winning Powerball ticket at just the precise instant this falling asteroid struck you. (Okay, that's technically a mathematical impossibility for the existence of this asteroid to be less than the chances of it striking you, but whatever.)

I kind of feel like it's splitting hairs since the result is the same regardless of the reason, but the references stating that the minimum impact velocity equals the escape velocity don't explain why that's the case.

We're not talking about asteroids/comets that are orbiting the Earth. All of those asteroids/comets are either in stable orbits around the Earth, were in orbits where perigee happened to be below the Earth's surface, or in an orbit that already decayed due to orbital perturbations. Who knows how many of those type objects once existed, but they're gone regardless of how many there may have once been.

We're only talking about asteroids/comets that are not orbiting the Earth and the relative velocity of all of those objects is at least escape velocity.

 

[russ_watters]

I kind of feel like it's splitting hairs since the result is the same regardless of the reason, but the references stating that the minimum impact velocity equals the escape velocity don't explain why that's the case.

Most of them do explain it, it's just that the explanation is short and simple: when an object approaches the earth, it is already moving toward the Earth and then the Earth provides additional acceleration. The minimum therefore has to be when "already moving toward earth" is a very small number, equivalent to releasing the object from a stationary position above earth, very far away.

I also think the difference between "very unlikely" and "impossible" is significant enough to matter. It is very unlikely that I'll win the Powerball, but people still do win.

We're not talking about asteroids/comets that are orbiting the Earth.

We're only talking about asteroids/comets that are not orbiting the Earth and the relative velocity of all of those objects is at least escape velocity.

You meant to say orbiting the sun, not earth, right? There is only one celestial body orbiting Earth and there has never been more than a couple (a few years back we captured an asteroid for a few orbits, then lost it again).

The upper-left graph in the scholarly paper I linked was for near-earth objects, which are the type of object you are hypothesizing about. They are on low-eccentricity orbits of similar distance from the sun as Earth (that's the definition of a NEO). There aren't many left, but there are enough to care. http://en.wikipedia.org/wiki/Near-Earth_object

2012DA14, which passed us yesterday, is such an object and has such an orbit (0.8-1.0AU): http://en.wikipedia.org/wiki/2012_DA14

Since it is only moving a little bit faster than Earth in its orbit (since the orbit is usually a bit closer to the sun), if it impacted (wiki says "atmospheric entry velocity"), it would only be going a little bit faster than escape velocity: 12.7 km/sec.

 

[BobG]

You meant to say orbiting the sun, not earth, right? There is only one celestial body orbiting Earth and there has never been more than a couple (a few years back we captured an asteroid for a few orbits, then lost it again).

The upper-left graph in the scholarly paper I linked was for near-earth objects, which are the type of object you are hypothesizing about. They are on low-eccentricity orbits of similar distance from the sun as Earth (that's the definition of a NEO). There aren't many left, but there are enough to care. http://en.wikipedia.org/wiki/Near-Earth_object

2012DA14, which passed us yesterday, is such an object and has such an orbit (0.8-1.0AU): http://en.wikipedia.org/wiki/2012_DA14

Since it is only moving a little bit faster than Earth in its orbit (since the orbit is usually a bit closer to the sun), if it impacted (wiki says "atmospheric entry velocity"), it would only be going a little bit faster than escape velocity: 12.7 km/sec.

No, I meant that none of the asteroids/comets we're talking about orbit the Earth, so they must be traveling faster than escape trajectory (or else their speed was radically changed very recently to make the relative velocity less than escape velocity).

And while 2012DA14 has a similar size orbit, it's also inclined 11.6 degrees relative to the Earth's orbit. While it's speed was only around 1.5 km/sec less than the Earth's speed, the angle between the two orbits gave a relative velocity of over 6 km/sec (Law of Cosines). At the distance of closest approach (34,050 km from the center of the Earth), escape velocity is 4.84 km/sec.

A different angle and you could have a drastically different result. Even so, the close approach reduced the orbital period of the asteroid from 368 days to 317 days, which is a pretty drastic change in itself.

With the angle between the orbits, it's hard to get the relative velocity below escape velocity, but it's conceivable that a different asteroid (such as Apophis with an inclination of only 3.3 degrees) could be changed enough to reduce the relative velocity below escape velocity. If 2012DA14 had only a 3.3 degree inclination, the relative velocity would have only been 2.26 km/sec, which would have been below escape velocity.

So, I concede the possibility of an impact below escape velocity is probably a lot higher than just looking at current orbits. But, given all the possible changes that could occur to the orbit as a result of a close approach, the chances the change results in a collision below escape velocity is still very small. Plus, the chance of collision between coplanar objects (lower velocity collisions) is going to be a lot higher than for objects in a significantly different orbit plane, and I think most of those are self-cleaning, if you will.

We only discovered Aphophis in 2004. It would be interesting to know the history of its orbit, since surely it couldn't have been in its current orbit for very long, astronomically speaking. It's orbit is due for another large change at the next close approach.

 

[Astronuc]

Russian Meteor Blast Bigger Than Thought, NASA Says
http://news.yahoo.com/russian-meteor-blast-bigger-thought-nasa-says-234920189.html

. . . late Friday, NASA revised its estimates on the size and power of the devastating meteor explosion. The meteor's size is now thought to be slightly larger — about 55 feet (17 m) wide — with the power of the blast estimate of about 500 kilotons, 30 kilotons higher than before, NASA officials said in a statement.

. . . Scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., now say the meteor weighed about 10,000 tons and was traveling 40,000 mph (64,373 km/h) when it exploded.

. . .

Meteor That Hit Russia Presents Rare Opportunity For Scientists
http://news.yahoo.com/video/meteor-hit-russia-presents-rare-234000073.html

Now that's the proper perspective.

 

[aquitaine]

Is there ever anything that will cost me nothing? Somehow, the cost will always get back to the individual (me) and, with such a massive project, how could this not affect my disposable income?

The magic of privately funded enterprises. It won't affect your disposable income if you don't buy their products. Their efforts with surveying asteroids are privately funded.

If it's that important then whoever does it will be charging for it, surely. I am a big believer in considering the 'total costing' when someone says "it'll be free". I question your statement about the costs of launching - I mean the real costs.

I never said it would be free. Their survey satellites will be launched per a contract they signed with SpaceX, another privately funded venture. The money they are spending is from their investors. Launch costs with SpaceX are quite a lot lower than with other rockets.

PS People don't scare that easily. So many of them are still smoking themselves to death and the risks are clear.

Here's some proof to the contrary:

1.) Lots of people were scared that Y2K would cause the end of the world as we know it.

2.) Lots more people were terrified by the color coded terror alerts of about a decade ago. In one particular case there was a vague terror alert about an imminent terrorist attack with chemical weapons. That one caused a tremendous run on plastic sheeting and duct tape, across the country for days you couldn't buy that stuff anywhere.

3.) More than a few people bought into the whole 2012 Maya doomsday thing.

4.) There are lots of people who think climate change will cause billions of people around the world to die and cause the end of the world as we know it. A few years ago an ABC "documentary" called Earth 2100 was all about this.

So yes, people do scare easily.

 

[sophiecentaur]

The magic of privately funded enterprises. It won't affect your disposable income if you don't buy their products. Their efforts with surveying asteroids are privately funded.

I never said it would be free. Their survey satellites will be launched per a contract they signed with SpaceX, another privately funded venture. The money they are spending is from their investors. Launch costs with SpaceX are quite a lot lower than with other rockets.
Here's some proof to the contrary:

1.) Lots of people were scared that Y2K would cause the end of the world as we know it.

2.) Lots more people were terrified by the color coded terror alerts of about a decade ago. In one particular case there was a vague terror alert about an imminent terrorist attack with chemical weapons. That one caused a tremendous run on plastic sheeting and duct tape, across the country for days you couldn't buy that stuff anywhere.

3.) More than a few people bought into the whole 2012 Maya doomsday thing.

4.) There are lots of people who think climate change will cause billions of people around the world to die and cause the end of the world as we know it. A few years ago an ABC "documentary" called Earth 2100 was all about this.

So yes, people do scare easily.

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.

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. 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.

Don't misunderstand me. I think it would be a good idea - but no better than feeding the World and avoiding the Ice Caps melting. Perhaps I'm just old and cynical.

 

[Color_of_Cyan]

Kinetic energy is 1/2 mv^2

The velocity of objects in space tend to be very high - with the relative velocity even higher if the two objects are traveling towards each other.

With a speed of 18km/sec, the kinetic energy of a train locomotive would be equivalent to a kiloton of TNT.

Hah, thanks. Looks like that would still make it the equivalent of a small nuke then. Well, for sure it still would be devastating to the immediate impact area and probably the surrounding area (my apologies for insensitively looking over this issue). I was originally talking something more on the scale of horribly physically affecting the entire Earth / ecology in some way though (dinosaur extinction, etc.). It just seems that obvious to tell certain things like that too.

 

[mheslep]

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?

My guess(es):
i) the recent availability of cheap and always on the person cameras in personal phones and the like, the theory being that a sighting with imagery to back it up is much more likely to be reported;
ii) increasing confidence in the Chicxulub meteor dinosaur extinction theory, with media coverage following the increasing scientific confidence to a point recently of near certainty;
iii) increasingly accurate account of near Earth objects made possible by ever improving observations, again with corresponding media coverage which also prompts reports.

 

[Redbelly98]

.
.
.
And while 2012DA14 has a similar size orbit, it's also inclined 11.6 degrees relative to the Earth's orbit. While it's speed was only around 1.5 km/sec less than the Earth's speed, the angle between the two orbits gave a relative velocity of over 6 km/sec (Law of Cosines). At the distance of closest approach (34,050 km from the center of the Earth), escape velocity is 4.84 km/sec.

A different angle and you could have a drastically different result. Even so, the close approach reduced the orbital period of the asteroid from 368 days to 317 days, which is a pretty drastic change in itself.

With the angle between the orbits, it's hard to get the relative velocity below escape velocity, but it's conceivable that a different asteroid (such as Apophis with an inclination of only 3.3 degrees) could be changed enough to reduce the relative velocity below escape velocity. If 2012DA14 had only a 3.3 degree inclination, the relative velocity would have only been 2.26 km/sec, which would have been below escape velocity.
.
.
.

You seem to be talking about the relative velocity when the meteor is still many Earth radii away from Earth -- that is, the relative velocity as given by its normal orbit around the sun, neglecting the effect of Earth's gravity on the meteor. I think we all agree that that speed can be well below escape velocity.

However, Earth's gravity will increase the meteor's speed as it approaches Earth. It looks like you are ignoring this.

 

[BobG]

You seem to be talking about the relative velocity when the meteor is still many Earth radii away from Earth -- that is, the relative velocity as given by its normal orbit around the sun, neglecting the effect of Earth's gravity on the meteor. I think we all agree that that speed can be well below escape velocity.

However, Earth's gravity will increase the meteor's speed as it approaches Earth. It looks like you are ignoring this.

No, I'm not.

If you know how fast the Earth's gravity will accelerate an object (which we do), then no matter how far away or how close the object starts out, I know how fast it will be going when it hits the surface of the Earth (or at least the denser part of the atmosphere). The value of escape velocity may be low (if you're far away from Earth) or it may be high (if you're close to Earth).

But you're either always above escape velocity or you're always below escape velocity.

You can't start out below escape velocity and then exceed escape velocity unless something besides the Earth accelerates you because the Earth's gravitational acceleration is already taken into account. Likewise you can't start out above escape velocity, but drop below escape velocity as the Earth's gravity slows you down.

This phenomenon is more commonly known as Conservation of Energy.

Of course, both the asteroid and the Earth are actually orbiting the Sun, which makes it a three-body problem instead of just a two-body problem. To collide at less than escape velocity, both the asteroid and the Earth have to have very similar orbits. And the 'trailing' object has to start out just a little lower than the 'leading' object, since the 'leading' object accelerates the 'trailing' object, increasing the size of its orbit, resulting in a longer orbital period and the 'trailing' object slowing down relative to the 'leading' object.

The problem with the three-body scenario is that the two objects won't stay in a very similar orbit for very long, at least on astronomical time scales. The two objects will have close approaches repeatedly until the small object is diverted into a non-similar orbit, finds a semi-stable balance with the larger object (Cruithne, for example), the small object is captured and orbits the Earth, or the small object collides with the Earth. And the chances of finding a stable balance with the Earth that will last for eons is very, very, very small. Virtually all of those objects are gone, one way or the other.

 

[D H]

But you're either always above escape velocity or you're always below escape velocity.

In the two body problem, yes. In the N-body problem, not necessarily.

There are a few ways that a meteor could hit the upper atmosphere with less than escape velocity. They all involve weak stability boundaries or ballistic capture trajectories. Suppose an object enters the vicinity of the Earth near the Sun-Earth L1 or L2 point. If the velocity is right, this object will be on a ballistic capture trajectory (it will lie inside a weak stability boundary). Some of these weak stability boundaries come fairly close to the Earth, meaning that the object can impact the Earth with less than escape velocity.

This in general doesn't work with something that enters near the Earth-Moon L2 point. The WSBs of the Earth-Moon system don't come close to the Earth -- unless the Earth-Moon L2 point is close to the Sun-Earth L1 or L2 point. Now we're in the domain of the restricted four body problem, something that makes the restricted three body problem look like child's play.

 

[Redbelly98]

BobG, I'm reading back more carefully on earlier posts, it looks like we were in agreement after all:

We're only talking about asteroids/comets that are not orbiting the Earth and the relative velocity of all of those objects is at least escape velocity.

Sorry about the misunderstanding.

 

[BobG]

In the two body problem, yes. In the N-body problem, not necessarily.

Yes, that's the point. It's physcially possible to have impacts at less than escape velocity. It's just very rare because an asteroid can't survive very long in an orbit that would result in an impact at less than escape velocity. It's going to be kicked out, collide, etc.

In fact, that refers directly to one of the IAU definitions of a planet:

(1) A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(bolding mine)

The only asteroids that can collide at less than escape velocity would be asteroids that have had their orbits changed very recently (astronomically speaking), since those asteroids probably won't survive in that orbit for more than a few million years. It's a low probability to be changed into a similar orbit and it's a low probability to survive very long if it is changed into a similar orbit.

 

[Astronuc]

The Rush is on!
http://news.yahoo.com/meteorite-rush-begins-russian-scientists-fragments-111415119.html

. . . .
Scientists at the Urals Federal University were the first to announce a significant find - 53 small, stony, black objects around Lake Chebarkul, near Chelyabinsk, which tests confirmed were small meteorites.
. . . .

More video - http://news.yahoo.com/video/local-expert-russia-meteor-strike-155657134.html


Watch out for 'infrasound':

The waves can bounce off buildings and be stronger in some places than others; they can also resonate with glass, explaining why bottles and dishes might have shattered inside undamaged kitchens, as if crushed by the airy hand of the meteor itself.
. . . .

http://www.nytimes.com/2013/02/18/w...-ruined-and-spared-by-meteor-share-space.html

 

[D H]

In fact, that refers directly to one of the IAU definitions of a planet:

(1) A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(bolding mine)

You are misinterpreting. That "has cleared the neighborhood around its orbit" clause doesn't mean that every last bit of stuff has either been swept up, ejected, or captured as a moon. It means almost everything. IIRC, a planet-like body is deemed a planet if the mass of that planet-like body is 100 times more than the mass of all the other stuff that orbits in the same orbital zone as the body in question.

There are plenty of small solar system bodies that orbit in the same orbital zone about the Sun as does the Earth. The total mass of all of those bodies doesn't come anywhere close to the mass of the Earth.

 

[Oldfart]

Almost free, relative to development of an effective early warning system that can detect incoming meteors, would be an advertising effort that simply explains that it's necessary to seek cover away from windows in the event of a brilliant flash of light.

I suspect that many of the injuries were caused by people going TO windows after the flash to see what was going on...

 

[Jimmy Snyder]

There are plenty of small solar system bodies that orbit in the same orbital zone about the Sun as does the Earth. The total mass of all of those bodies doesn't come anywhere close to the mass of the Earth.

Does this help?
2010 TK7

2010 TK7 has a diameter of about 300 meters (1,000 ft). Its path oscillates about the Sun–Earth L4 Lagrangian point (60 degrees ahead of Earth), shuttling between its closest approach to Earth and its closest approach to the L3 point (180 degrees from Earth) about every 400 years.

 

[russ_watters]

No, I meant that none of the asteroids/comets we're talking about orbit the Earth, so they must be traveling faster than escape trajectory (or else their speed was radically changed very recently to make the relative velocity less than escape velocity).

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:

It's just very rare because an asteroid can't survive very long in an orbit that would result in an impact at less than escape velocity. [emphasis added]

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!

 

[russ_watters]

Does this help?
2010 TK7

Very cool. Something else I learned:

Earth Usually Has More than One Moon, Study Suggests

The team concluded that at least one asteroid with a diameter of 3 feet (1 meter) or more is likely orbiting our planet at anyone time. There may be many smaller objects circling Earth, too, but the study didn't address them; it was tough enough to model the motions of the bigger space rocks.

http://www.space.com/15151-earth-multiple-moons-asteroids.html

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