What caused the massive explosions in Russia?

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dydxforsn said:
heh, the quote in that asteroid picture reminds me to check Neil Tyson's twitter..

REDDITORS said the image came from Neil's feed.
 
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Borg said:
NASA is stating that the tracks are completely different.

'A man' on the radio said that one track is North South and the other is East West so just a coincidence. I hope we have no coincidences in the UK.
 
I'd like to see more confirmation, but here they've upped the size estimate by near an order of magnitude and the energy estimate by close to two:
The object that caused the Russian fireball, which damaged hundreds of buildings and wounded perhaps 1,000 people in the Chelyabinsk region, was originally probably about 50 feet (15 meters) in diameter and weighed roughly 7,000 tons, said Peter Brown, director of the Center for Planetary Science and Exploration at the University of Western Ontario in Canada.

From multiple sensors using multiple technologies, a best initial estimate of the total energy of the event is about 300 kilotons of TNT-equivalent
http://www.space.com/19822-russian-fireball-biggest-explosion-century.html

[edit] and the speed estimate to 40,000 mph (18km/s).
 
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...
 
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?
 
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I read this over a week ago.

At the current rate that near-Earth asteroids are being detected, it will take astronomers 15 years to identify every one of significant size

Scientists have estimated that 20,000 asteroids lurk in the solar system, of which only 6,000 have been identified, Beeson said.

Beeson and her colleagues looked at the historical rate of asteroid discovery and found that, at the current pace, it will take about 15 years to identify all the asteroids in the solar system that are wider than 100 meters (328 feet).

Most of the missing asteroids are traveling by during the daytime or traveling through a patch of the sky not watched by existing surveys, she found.

To speed discovery, the team should expand the patch of sky observed by two programs, the Mount Lemmon and Catalina sky surveys, she said. To find asteroids that are crossing by Earth only during the day, scientists should prioritize the B612 Sentinel mission, which aims to send a telescope into a Venus-like orbit around the sun, Beeson added.

http://news.yahoo.com/search-near-earth-asteroids-needs-speed-boost-114855823.html?_sr=1
 
aquitaine said:
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...

You have to ask yourself just how much of their income people would be prepared to spend on this. The money wouldn't just turn up by magic. 10% Asteroid tax??
 
This is crazy... same day the asteroid passes by too. However I fail to understand how any sort of rock barely half the size of a football field could possibly do any major, apocalyptic-scale harm to earth. What if it just landed in the ocean or something?
 
Color_of_Cyan said:
This is crazy... same day the asteroid passes by too.


However I fail to understand how any sort of rock barely half the size of a football field could possibly do any major, apocalyptic-scale harm to earth. What if it just landed in the ocean or something?

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.
 
russ_watters said:
To expand: 11 km/s is the minimum possible speed of an approaching meteor because that's escape velocity.
Wouldn't that make it the minimum speed of an escaping meteor?
 
But if I merely toss an object up, it comes back down without ever going 11 km/s
 
Actually, 11 km/s is the maximum speed of a meteor that almost escaped, then fell back down.
 
russ_watters said:
To expand: 11 km/s is the minimum possible speed of an approaching meteor because that's escape velocity.

Jimmy Snyder said:
Wouldn't that make it the minimum speed of an escaping meteor?

True.

However, for the speed to be less than 11 km/sec for a close approaching object, something had to change the speed of the object very recently or it would already be orbiting the Earth instead of just having a trajectory that intersects the Earth's orbit.

Both the Earth and the asteroid/comet/etc have to have the same specific potential energy per unit of mass (relative to the Sun) since they're intersecting. For the relative velocity to be small, both the Earth and the asteroid have to have a total specific energy per unit of mass that's almost equal. That means the orbit of both has to be close to the same size, meaning the two should come near each other repeatedly over eons of time.

Since you're talking relative velocity (the Earth is traveling around the Sun at nearly 30 km/sec), I guess you could conceivably come up with some other trajectories that just happen to finally synch up with the Earth's motion for the first time ever. It's just that the probability of those trajectories are going to be incredibly low.

For practical purposes, russ's statement is correct.
 
There is no minimum speed for a meteor colliding with the Earth (ignoring air resistance) because the meteor could have been going around the sun in the same orbit as the Earth before the collision. Indeed, it is possible that the Earth was made of such meteors and if so, there was negligible air resistance when it happened.
 
Jimmy Snyder said:
There is no minimum speed for a meteor colliding with the Earth (ignoring air resistance) because the meteor could have been going around the sun in the same orbit as the Earth before the collision. Indeed, it is possible that the Earth was made of such meteors and if so, there was negligible air resistance when it happened.

But ... if the meteor is going roughly the same speed as Earth when far away, it will speed way up (relative to us) as it gets closer. Gravitational attraction to Earth will make it speed up. The same escape speed you calculate from conservation of energy principles does give the minimum speed of a meteor strike.
 
First of all, Jimmy I never know when you are being serious. I'll be more serious now in case you really didn't understand this about ballistic trajectories: they are symmetrical. Absent atmospheric resistance, if you fire something up in the air, it hits the ground at exactly the same speed you fired it at. So if you fire an object up at just a little bit less than escape velocity, it hits the ground later at just a little bit less than escape velocity.

So that means that if a random object in space is dropped from stationary far above earth, it hits the ground at close to escape velocity. But there are no objects just sitting there stationary, held up by unseen forces. Everything is already in motion, so any object that approaches us would have to hit at about escape velocity + initial approach velocity.

BobG said:
Since you're talking relative velocity (the Earth is traveling around the Sun at nearly 30 km/sec), I guess you could conceivably come up with some other trajectories that just happen to finally synch up with the Earth's motion for the first time ever. It's just that the probability of those trajectories are going to be incredibly low.

For practical purposes, russ's statement is correct.
I don't think that's true. If an asteroid is on an orbit that is just barely overtaking ours, eventually our gravity takes hold and pulls it in and the speed ends up above escape velocity. It has to because we are starting with a non-zero relative velocity towards each other.

I'd like a confirmation from berke or DH or one of our other aero/astro guys though to make sure I'm not missing something...
 
Are we talking ever? Or are we talking now?

The gaps in the rings of Saturn are the perfect example. A moon has swept out virtually all of the objects in a similar orbit, creating a gap in the rings.

The same is true of each of the planets orbiting the Sun, even if the Sun's debris disk isn't as spectacular as Saturn's.

Virtually all of the objects that could intersect the Earth at less than escape velocity have already been captured (aside from a few exceptions, such as Cruithne's orbit).

Virtually all objects that might intersect the Earth now or in the future have probably escaped Earth's gravity in the past unless this is the very first time the object has approached the Earth. And the fact that the object has never approached the Earth in the past actually reduces the chances that the velocity of the object would be anywhere close to the Earth's velocity.
 
Jimmy Snyder said:
There is no minimum speed for a meteor colliding with the Earth (ignoring air resistance) because the meteor could have been going around the sun in the same orbit as the Earth before the collision.

Ignoring air resistance, the trajectory of the meteorite is reversible. So try to figure out a way to throw a rock into space, starting from a very slow speed on the surface.

[Ducks behind wall to avoid being hit by a failed experiment.]
 
There is no mechanism that prevents an asteroid from injecting into a solar orbit similar to but not exactly the same as the Earth's.

Are there no asteroids in the same orbit as the Earth such as the trojans in the orbit of Jupiter? This site indicates that there might be.

Another important group is called Trojans; they are in the orbit of Jupiter, on two Lagrangian points. Similar sets of asteriods appear to lie in the trojan points in almost all planetary orbits.
http://www.redorbit.com/education/reference_library/space_1/universe/2574624/asteroid/

If the Earth is included in 'almost all', then in order to prevent such Trojans from colliding with the earth, the center of the Trojan would have to be exactly the same distance from the sun as the Earth's center, and unlikely situation.
 
BobG said:
Are we talking ever? Or are we talking now?

The gaps in the rings of Saturn are the perfect example. A moon has swept out virtually all of the objects in a similar orbit, creating a gap in the rings.

The same is true of each of the planets orbiting the Sun, even if the Sun's debris disk isn't as spectacular as Saturn's.

Virtually all of the objects that could intersect the Earth at less than escape velocity have already been captured (aside from a few exceptions, such as Cruithne's orbit).
I'm talking ever. I think all of those examples involve collisions above their respective escape velocities. Indeed, the examples you gave would indicate to me that over time the chances of a minimum speed (escape velocity) impact go down and the average impact speed rises.
 
Jimmy Snyder said:
There is no mechanism that prevents an asteroid from injecting into a solar orbit similar to but not exactly the same as the Earth's.
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.
 
By the way, the link I provided earlier to the impact calculator doesn't explain, but says this:
The minimum impact velocity on Earth is 11 km/s. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity for objects orbiting the sun is 72 km/s. [emphasis added]

[edit] Ok, I found a paper: http://www.arm.ac.uk/~ambn/356.pdf

Page 2 shows (in graph form), impact velocities and probabilities for known near Earth objects. The top left graph is asteroids, which have the lowest impact speed due to their relatively low orbit eccentricity compared to comets. There is a sharp cutoff at the 11 km/sec minimum.
 
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sophiecentaur said:
You have to ask yourself just how much of their income people would be prepared to spend on this. The money wouldn't just turn up by magic. 10% Asteroid tax??


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.
 
russ_watters said:
To expand: 11 km/s is the minimum possible speed of an approaching meteor because that's escape velocity. Most objects in the solar system orbit in the same direction, but if one is on an opposite course or is a sun-diving comet, it could be as high as 72 km/sec. http://impact.ese.ic.ac.uk/ImpactEffects/

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. It could have been nudged into an orbit with almost the same speed, position and direction as Earth at a given time. There are many choices of elliptical orbit with a range of energies to achieve this 'first-time possible' event. Clearly, that situation has not occurred very frequently in Earth's orbital position but there will the other places in the Solar System where it could happen more often (where there a many more asteroid bodies and a nearby massive planet.

Or did I not get your drift? (All too common these days :biggrin:)
 
Sorry, I see it now. At any event, it will have at a minimum, the potential energy indistinguishable from an object at infinity and in addition, at least some component of velocity toward the earth.
 

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