What are the real chances of an asteroid hitting Earth?

[Catastrophe]

Helio, "Impacts use KE."

I understand that if we are talking asteroid hitting Earth. What we are looking at here is arranging an impact onto a moving asteroid to change its direction. Any ideas?

Cat

 

[Catastrophe]

Asteroid impact avoidance - Wikipedia

en.wikipedia.org

.

This is what I want . . . . . . . . .

Quote
The principle of the kinetic impactor mitigation method is that the NEO or Asteroid is deflected following an impact from an impactor spacecraft. The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of 10 km/s (36,000 km/h; 22,000 mph) or more.
Quote

Cat

 

[mmasse]

The odds of asteroid collision have been worked out by serious astronomers long ago.

The odds of the Earth being struck by an asteroid larger than the one that did in the dinosaurs is somewhat distressing. The odds are 1.000 plus or minus o.oo1. That a 100% chance.

But that isn't for any one year, it's over the lifetime of the planet.

It's happened at least five times over the past Billion years, or maybe just the past 3/4 of a billion years. On average it's once every hundred million years. It's been 65 Million years since the last one, so we are about due. Sometime in the next 35 million years that is.

For larger objects the odds go down dramatically. For smaller objects they go up quite dramatically as well.

Down at the bottom of the scale, the Earth gets hit several thousand times a day up to several million times a day by dust sized bits of space debris.

Rocks large enough to destroy a city strike somewhere between once a decade and three times a decade. The last one was over a city in Russia. Before that, several years before, there was one in Africa. There was also one reported over the open ocean.

Most strikes will be over the ocean because there is three times as much surface that is ocean as there is that is land.

So strikes by space rocks is a real thing and it happens several times a decade and always has.

The Tunguska Siberia strike in the early 1900's is one famous example. A Tunguska sized event we currently believe happens once every hundred years or so. Like the Chelyabinsk event more recently, it was an air burst, meaning that the actual body exploded high in the air and there was no crater to find on the ground. Still, it exploded like a hydrogen bomb blast.

So the real answer to 'Will it happen?' is Yes. It will happen. But when is very different question. But before 35 million years from now, there will be a planet wide disaster caused by a falling space rock. Before that, there will be hundreds of city sized disasters and a few continent sized disasters all caused by falling space debris.

Its all really just a consequence of us living in a somewhat littered solar system.

I never hear this mentioned so maybe it is just not possible but, is it possible to slow down a meteor so it does less or no damage coming in so slow that it just splashed down in a shallow ocean somewhere?

 

[mmasse]

The odds of asteroid collision have been worked out by serious astronomers long ago.

The odds of the Earth being struck by an asteroid larger than the one that did in the dinosaurs is somewhat distressing. The odds are 1.000 plus or minus o.oo1. That a 100% chance.

But that isn't for any one year, it's over the lifetime of the planet.

It's happened at least five times over the past Billion years, or maybe just the past 3/4 of a billion years. On average it's once every hundred million years. It's been 65 Million years since the last one, so we are about due. Sometime in the next 35 million years that is.

For larger objects the odds go down dramatically. For smaller objects they go up quite dramatically as well.

Down at the bottom of the scale, the Earth gets hit several thousand times a day up to several million times a day by dust sized bits of space debris.

Rocks large enough to destroy a city strike somewhere between once a decade and three times a decade. The last one was over a city in Russia. Before that, several years before, there was one in Africa. There was also one reported over the open ocean.

Most strikes will be over the ocean because there is three times as much surface that is ocean as there is that is land.

So strikes by space rocks is a real thing and it happens several times a decade and always has.

The Tunguska Siberia strike in the early 1900's is one famous example. A Tunguska sized event we currently believe happens once every hundred years or so. Like the Chelyabinsk event more recently, it was an air burst, meaning that the actual body exploded high in the air and there was no crater to find on the ground. Still, it exploded like a hydrogen bomb blast.

So the real answer to 'Will it happen?' is Yes. It will happen. But when is very different question. But before 35 million years from now, there will be a planet wide disaster caused by a falling space rock. Before that, there will be hundreds of city sized disasters and a few continent sized disasters all caused by falling space debris.

Its all really just a consequence of us living in a somewhat littered solar system.

I never hear this mentioned so maybe it is just not possible but, is it possible to slow down a meteor so it does less or no damage coming in so slow that it just splashed down in a shallow ocean somewhere? Or if there would still be too much damage what about working it into orbit? Maybe it would take too much energy or something? I normally hear blow it up or redirect it but imagine the resources if we could slow it down and use it on earth...

 

[Wolfshadw]

I never hear this mentioned so maybe it is just not possible but, is it possible to slow down a meteor so it does less or no damage coming in so slow that it just splashed down in a shallow ocean somewhere? Or if there would still be too much damage what about working it into orbit? Maybe it would take too much energy or something? I normally hear blow it up or redirect it but imagine the resources if we could slow it down and use it on earth...

To my knowledge, it is relatively easier to nudge an incoming impactor off course than it would be to slow it down. If an object is so close that an impact is inevitable, then it's too late to do anything about it anyway.

As for a relatively soft landing, the amount of energy required to significantly slow an object far outweighs any potential resources gained; not to mention the risks involved...

"Hey! Let's slow down a meteor and put it on a slow course towards Earth!" Any kind of engine or directional failure means catastrophe.

-Wolf sends

 

[Catastrophe]

Wolfshadw, you posted "To my knowledge, it is relatively easier to nudge an incoming impactor off course than it would be to slow it down. If an object is so close that an impact is inevitable, then it's too late to do anything about it anyway."

I agree with your comment on nudging versus slowing down. However, I do have problems with the idea that it is easy and wide ranging. You are absolutely correct that if an object is too close, it will hit anyway. What I have not seen are any calculations based on triangular diagrams resolving momentum. Let me be clear, I have seen mention of energy, force, etcetera, but I think momentum is key.

Wiki has two important points of relevance:

Point 1
In Newtonian mechanics, linear momentum, translational momentum, or simply momentum (pl. momenta) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If m is an object's mass and v is its velocity (also a vector quantity), then the object's momentum is:
{\displaystyle \mathbf {p} =m\mathbf {v} .}
In SI units, momentum is measured in kilogram meters per second (kg⋅m/s)

BUT Point 2
Application to collisions
By itself, the law of conservation of momentum is not enough to determine the motion of particles after a collision. Another property of the motion, kinetic energy, must be known. This is not necessarily conserved. If it is conserved, the collision is called an elastic collision; if not, it is an inelastic collision.

Point 2 makes it clear that changing the direction of incoming asteroids is not as straightforward as may be suggested. It is even more complicated because we are not dealing with inelastic spheres, but with stony or iron bodies, some of which may shatter, producing a plurality of smaller, but still very dangerous, pieces.

I rest my case. 'Changing direction' is not a foolproof answer to the threat of impact.

Cat

 

[Wolfshadw]

However, I do have problems with the idea that it is easy and wide ranging.

Never said it was "easy". I said "relatively easier" as in less difficult.

-Wolf sends

 

[Catastrophe]

Never said it was "easy". I said "relatively easier" as in less difficult.

-Wolf sends

My apologies, I meant the comment generally. It was certainly not directed at you.

Cat

 

[Helio]

If my math is correct, and I'm "absolutely sure this could possibly work" (Cayde- 6, Destiny; stealth landing on the Dreadnaught), then the following is interesting:

Object: Dia. = 50 meters (2 gm/cu meter) [Size of Meteor for AZ. crater]
Tractor equal in mass to the ISS (420 tonne).
Object travel speed... 20 kps (Mars is 24kps)
Tractor distance from surface... 25 meters
Time of tractor effort: 5 months
Distance from Earth for initiation of tractor beam... 6 AU
If tangential, deflection (% of Earth diameter) ... 140%
If longitudinal, deflection (% of Earth diameter) ... 270% [Advantage as the Earth moves briskly in its orbit. Assumes perpendicular trajectory.]

[The mass of the object is not a factor, perhaps surprisingly. The extra gravity force with extra mass for a primary is canceled by the acceleration term. Is this right? It looks that way to me.]


[Added: oops, the 25 meter distance must include the radius of the object. I will correct tomorrow. Green color text are the corrections.]

 

[Catastrophe]

What are the real chances of an asteroid hitting Earth?

Just to keep on thread: We have established (I believe) that over a very long time frame, asteroid impact is a virtual certainty. We are now discussing avoiding that impact. I am not familiar with the term 'tractor' and am off to check it out.

Cat

 

[Helio]

What are the real chances of an asteroid hitting Earth?

Just to keep on thread: We have established (I believe) that over a very long time frame, asteroid impact is a virtual certainty. We are now discussing avoiding that impact. I am not familiar with the term 'tractor' and am off to check it out.

The idea seems to have originated by two astronauts, Lu and Love. Here

 

[Catastrophe]

OK. That did not take long. This is from NASA:

Tractor | NASA

"If an approaching asteroid were detected early enough, it could be possible to divert its path using the gravity of a spacecraft. Instead of sending an impactor to ram into an approaching object, a gravity tractor device would fly alongside the asteroid for a long period of time (years to decades) and slowly pull it out of Earth’s path. Gravity tractors would be most likely to work on any shape or composition of approaching asteroid, even if it were just a pile of rubble. However, gravity tractors might not be effective for the largest asteroids of over 500 meters in diameter which might be the greatest threat to Earth. Gravity tractors offer the greatest control and could perhaps even divert an approaching asteroid to other locations in space where people could theoretically use them for research or commercial purposes. However, these techniques have never been tried and would require decades for building, launching, and carrying out a mitigation mission."

My emphasis.

Cat

 

[Catastrophe]

The idea seems to have originated by two astronauts, Lu and Love. Here

From the Lu and Love reference:

"The tractor would then gradually pull the asteroid off course, using nothing more than the gravitational pull between the two bodies." My emphasis.

Am I not correct in thinking that:
The equation for universal gravitation thus takes the form:

{\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}},} F = Gm1m2/r^2
where F is the gravitational force acting between two objects, m1 and m2 are the masses of the objects, r is the distance between the centers of their masses, and G is the gravitational constant. [Wiki]

So if one object is much more massive than the other. like me standing on Earth, then Earth is going to move me more likely than I am going to move Earth. So does this not mean that to move a massive asteroid, one is going to need a traction object of comparable mass to achieve an effect. Then we have to look at the fuel to get a massive object, especially if high acceleration is required, up to meet that object in time to have any effect. To borrow from Tennis, I would say that is "game to love" (small l) against the idea. It seems to me that the idea is only useful against objects which you see a very long way off, and which are too small to do any serious damage anyway. Objects which could, in any case, probably be shot down using more conventional means.

Cat

 

[Helio]

From the Lu and Love reference:

"The tractor would then gradually pull the asteroid off course, using nothing more than the gravitational pull between the two bodies." My emphasis.

Am I not correct in thinking that:
The equation for universal gravitation thus takes the form:

{\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}},} F = Gm1m2/r^2
where F is the gravitational force acting between two objects, m1 and m2 are the masses of the objects, r is the distance between the centers of their masses, and G is the gravitational constant. [Wiki]

Right. So we can equate this force with F = m1*a, which cancels m1. So a = G* m2 / r^2.

So if one object is much more massive than the other. like me standing on Earth, then Earth is going to move me more likely than I am going to move Earth. So does this not mean that to move a massive asteroid, one is going to need a traction object of comparable mass to achieve an effect.

I think, perhaps I’m missing something, that if we increase the mass of the object we increase the gravitational force to them, but the acceleration of this greater mass is unchanged; the masses cancel. But increasing the mass of the tractor does increase acceleration.

 

[Catastrophe]

Momentum is a vector quantity: it has both magnitude and direction. Since momentum has a direction, it can be used to predict the resulting direction and speed of motion of objects after they collide.
We have seen that there are further considerations.

But how else do we calculate the change in direction of a guided impact onto an asteroid approaching Earth? We are not talking perfect inelastic spheres and mathematical abstractions. Any ideas as to how we actually calculate the mass and velocity of the impactor to do the job?

Cat

 

[Catastrophe]

I just found this in Asteroid Impact Avoidance [Wiki]:

Quote
The principle of momentum transfer is used, as the impactor crashes into the NEO at a very high velocity of 10 km/s (36,000 km/h; 22,000 mph) or more. The momentum of the impactor is transferred to the NEO, causing a change in velocity and therefore making it deviate from its course slightly.[103]
Quote

My emphasis.

Cat

P.S. Several ways of avoiding an asteroid impact have been described.[6] Nonetheless, in March 2019, scientists reported that asteroids may be much more difficult to destroy than thought earlier.[7][8] In addition, an asteroid may reassemble itself due to gravity after being disrupted.[9] [Ibid]

Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has yet been developed. [Ibid]

An impact by a 10 kilometres (6.2 mi) asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets entering the inner Solar System. The impact speed of a long-period comet would likely be several times greater than that of a near-Earth asteroid, making its impact much more destructive; in addition, the warning time is unlikely to be more than a few months.[17] Impacts from objects as small as 50 metres (160 ft) in diameter, which are far more common, are historically extremely destructive regionally (see Barringer crater). [Ibid]

 

[Catastrophe]

"if we increase the mass of an object, we increase the force of attraction to it," Yes

"but the acceleration of this greater mass remains unchanged" Hmmmmm?
Momentum = mass x speed

Cat

 

[Helio]

See Post #59 for the corrections.

hello, it's obvious that if we increase the mass of an object, we increase the force of attraction to it, but the acceleration of this greater mass remains unchanged. Am I thinking right?

Yes, we need to look at both equations to see it.

As Cat noted:
F = G*m1*m2 / R^2 ; (m1 is the tractor, m2 the asteroid)
So there is a linear increase in the gravitational force with an increase in the mass of either object.

Also,

F = m2*a, so the net equations becomes....

F = G*m1*m2 / R^2 = m1*a...
G*m2 / R^2 = a
or
a = G*m2 / R^2

So increasing m2 (mass of the asteroid), say by 2x, will double the value of F (gravity), but with F = m*a with a now doubled value of F but a double value of m, makes a the same value as before.

IOW, a = 2F/2m, which is the same value as F/m.

I'm open to criticism on this, but I think it's correct.

 

[Helio]

What isn't in the calculations (post 59) , however, is the acceleration loss due to the thrust from the tractor to maintain position. This thrust can be symmetrically diverted around the asteroid, but at considerable fuel expense.

[Added: Also note how many months were added to get the deflection stated in Post 59]

The tractor could be equipped with heating probes that could be launched into the icy body to produce thrust (ice spewing steam) for the asteroid itself, though the techniques to make this work would need to be tested on a number of objects to get this to work feasibly, no doubt.

The tractor also could carry some serious nuking alternatives if all else fails, I suppose.

 

[Catastrophe]

Helio, I believe I have shown that the tractor idea has terminal disadvantages. Also your nuke of last resort might have the pieces reassembling due to gravity. #63.

Cat

 

[Helio]

Helio, I believe I have shown that the tractor idea has terminal disadvantages.

I doubt the disadvantages are terminal, if enough time exists to make a difference.

Also your nuke of last resort might have the pieces reassembling due to gravity. #63.

The key there is the escape velocity of the object based on the c.g. Huge fragmentation of a small, say 100 meter, asteroid will greatly exceed, IMO, its tiny escape velocity.

 

[Catastrophe]

Helio, am I not correct in saying that the tractor system requires a large mass (and hence great fuel consumption) whereas the impact system just needs sufficient fuel to hit the incoming asteroid at great speed?

Cat

 

[Catastrophe]

Helio, post #72. You suggest small object. Larger ones are obviously more dangerous. All the books quote reaggregation in similar circumstances, viz naturally occurring impacts in the asteroid belt (depending on relative sizes).

On another aspect: I would like to see a triangle of momentum figures giving a resultant for new direction. It depends on the momentum of the "guided missile".

Cat

 

[Helio]

Helio, post #72. You suggest small object. Larger ones are obviously more dangerous. All the books quote reaggregation in similar circumstances, viz naturally occurring impacts in the asteroid belt (depending on relative sizes).

I suspect that the ejecta from mutual impacts will "reaggregate" on any body they gently encounter, but not necessarily back onto the original body unless it is very massive (e.g. Ceres).

But, even in the case of Ceres, we may be talking many years, so a nuked asteroid wouldn't have time to reaggregate much if any of its ejecta, IMO.

Notice that the escape velocity for Ceres is only 0.5 kps. So I would assume any serious impact would have a large percentage exceeding this speed.

One of the big challenges for asteroid and planetary formation theories is that impacts for objects that grow to about 1 meter in diameter will break these objects apart, disallowing them to grow larger. Some models, in recent years, have solutions around this.

 

[Catastrophe]

I have more shortly, but my immediate reaction is that much smaller bodies than Ceres are involved. Have you not seen the possibilities of asteroid impacts depending on their relative sizes? From crater to breakup and reaggregation, depending on relative size?

Anyway, even if they did not reaggregate to a 'solid' body (which is not the suggestion) any association, however loose, is going to have multiple hits on Earth (if not diverted)?

Cat