Planetary Defense Experiment

[Shadowy Man]

If they are coming in at escape velocity how long do you think the time difference is between impacts only 200km apart?

5 km/hour, it would take 40 hours, 2 days.

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

LOL, you meant they go one behind the other. No that's not how blowing up an asteroid works. They are blown up in all directions. And it does not take much time for remnants to separate enough. In fact, they would have enough time to separate so far that most of the rocks will miss the earth.

Ok. I thought based on the thread what was being suggested was that it was exploded so it comes in as small pieces and not all at once. Like in the movies. Sorry if I misread it.

I agree that if you can blow it apart far enough away so that pieces miss that would be better.

 

[barbos]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

 

[Shadowy Man]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

 

[barbos]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

 

[Shadowy Man]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

Possibly. But they will impact the Earth at over 11 km/sec so they’d better have enough time to spread a lot!

You had said 200 km apart, so that would only be about 20 seconds apart in impacting time at best.

We may be talking past each other as we may have different mental pictures of the scenario being considered.

 

[barbos]

You had said 200 km apart, so that would only be about 20 seconds apart in impacting time at best.

one more time, they will hit Earths atmosphere in different places spread over 200 km.
And the point of splitting asteroid is not really to have few smaller armagedons instead of one big. The point is to have parts small enough that they don't even reach upper atmosphere - render them harmless.

 

[Shadowy Man]

You had said 200 km apart, so that would only be about 20 seconds apart in impacting time at best.

one more time, they will hit Earths atmosphere in different places spread over 200 km.
And the point of splitting asteroid is not really to have few smaller armagedons instead of one big. The point is to have parts small enough that they don't even reach upper atmosphere - render them harmless.

My point was that if you break it apart into smaller pieces but those smaller pieces all still hit then you’re still depositing a lot of energy into atmosphere and that’d still be bad. Need to break it up enough so that most of it missed.

 

[bilby]

We already know how to break large masses of rock into small pieces. Quarries do it all the time.

You need a certain amount of explosives, though in the order of tonnes, rather than megatonnes, of TNT equivalent; But the critical thing is to spread these explosives in small packets throughout a large number of boreholes, and to time the detonation carefully - if you don't, the results are lots of dust that's far smaller than your target size, plus several unbroken bits far larger than your target maximum size.

In short, a big bang - even a really big, nuclear sized, bang - likely gets you lots of rock vapour and fine dust, plus a handful of still dangerously large fragments.

The way to reduce a big rock to small rubble is with a large number of small explosions carefully placed and timed.

To say that this would be extremely challenging to set up in deep space on a rigid timetable would be a huge understatement.

Any quarryman can tell you that a small number of large explosions, or worse still a single massive explosion, is a shithouse way to try to break up a large mass of rock.

What you need is a large number of explosions, each of which can be really very small.

 

[barbos]

You had said 200 km apart, so that would only be about 20 seconds apart in impacting time at best.

one more time, they will hit Earths atmosphere in different places spread over 200 km.
And the point of splitting asteroid is not really to have few smaller armagedons instead of one big. The point is to have parts small enough that they don't even reach upper atmosphere - render them harmless.

My point was that if you break it apart into smaller pieces but those smaller pieces all still hit then you’re still depositing a lot of energy into atmosphere and that’d still be bad. Need to break it up enough so that most of it missed.

No, atmosphere is perfectly capable of absorbing energy of 10km asteroid without much of consequences.

 

[Shadowy Man]

You had said 200 km apart, so that would only be about 20 seconds apart in impacting time at best.

one more time, they will hit Earths atmosphere in different places spread over 200 km.
And the point of splitting asteroid is not really to have few smaller armagedons instead of one big. The point is to have parts small enough that they don't even reach upper atmosphere - render them harmless.

My point was that if you break it apart into smaller pieces but those smaller pieces all still hit then you’re still depositing a lot of energy into atmosphere and that’d still be bad. Need to break it up enough so that most of it missed.

No, atmosphere is perfectly capable of absorbing energy of 10km asteroid without much of consequences.

How much energy do you think an asteroid 10km in diameter moving at Earth escape velocity has?

 

[barbos]

How much energy do you think an asteroid 10km in diameter moving at Earth escape velocity has?

for asteroid with R=5km and Density=2000kg/m^2 at Velosity=10km/sec
(4./3*M_PI*pow(5000,3)*2000*pow(10e3,2)/2) = 5.23598775598299e+22 Joules.

That's a lot, but don't forget that most of that energy will miss the Earth completely.
And most of the rest will be safely emitted into outer space. Without miss we are screwed though. You have to blow it so that most of it misses us.

Assuming it all is absorbed by the athmosphere that would be 10MJ per ton of the athmosphere, which is about 10kJ per kg, which is 2 degrees of celsius assuming heat capacity of water. So I would guess 5 degrees for air. Which is is not that bad but it would cause extremely strong winds.

But that's assuming it all absorbed by the athmosphere, in reality only tiny amount will heat atmosphere directly. I think most of the damage will be due to direct heating of the surface by radiation. Atmosphere mass is actually pretty huge compared to the surface mass.

 

[Shadowy Man]

How much energy do you think an asteroid 10km in diameter moving at Earth escape velocity has?

for asteroid with R=5km and Density=2000kg/m^2 at Velosity=10km/sec
(4./3*M_PI*pow(5000,3)*2000*pow(10e3,2)/2) = 5.23598775598299e+22 Joules.

That's a lot, but don't forget that most of that energy will miss the Earth completely.
And most of the rest will be safely emitted into outer space. Without miss we are screwed though. You have to blow it so that most of it misses us.

I agree. Especially when you consider that 1 megaton of TNT is 4e15 Joules. Most of it needs to miss. But that wasn’t what you appeared to be saying.

You said: “atmosphere is perfectly capable of absorbing energy of 10km asteroid without much of consequences.” I disagree with that.

 

[barbos]

You said: “atmosphere is perfectly capable of absorbing energy of 10km asteroid without much of consequences.” I disagree with that.

Well, I am not wrong. it is capable, it just won't happen because radiation will pass through atmosphere without much absorption

So, yeah, we need most of the 10km asteroid actually miss the Earth.
Smaller ones like let say 1km don't need to miss, you just need to spread it over.

 

[Jimmy Higgins]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

If a large asteroid is moving with a substantial amount of momentum, wouldn't the blast fragments (assuming within the object) be spread out in more of an off-centered egg shaped distribution, with more pieces in front of what was the asteroid?

 

[Jimmy Higgins]

We already know how to break large masses of rock into small pieces. Quarries do it all the time.

You need a certain amount of explosives, though in the order of tonnes, rather than megatonnes, of TNT equivalent; But the critical thing is to spread these explosives in small packets throughout a large number of boreholes, and to time the detonation carefully - if you don't, the results are lots of dust that's far smaller than your target size, plus several unbroken bits far larger than your target maximum size.

In short, a big bang - even a really big, nuclear sized, bang - likely gets you lots of rock vapour and fine dust, plus a handful of still dangerously large fragments.

The way to reduce a big rock to small rubble is with a large number of small explosions carefully placed and timed.

To say that this would be extremely challenging to set up in deep space on a rigid timetable would be a huge understatement.

Any quarryman can tell you that a small number of large explosions, or worse still a single massive explosion, is a shithouse way to try to break up a large mass of rock.

What you need is a large number of explosions, each of which can be really very small.

Of course, that rock is relatively static and not moving, and also, not 1 to 10 km in diameter size and all needs to be dismantled... quickly. Quarries and rock cuts usually involve small chunks at the edges.

Which is why the best option to fight inertia is with more inertia. The idea of landing something on an asteroid, drilling, and getting a large bomb to blow it up is much more fantasy at this point than science fiction.

 

[Shadowy Man]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

If a large asteroid is moving with a substantial amount of momentum, wouldn't the blast fragments (assuming within the object) be spread out in more of an off-centered egg shaped distribution, with more pieces in front of what was the asteroid?

It is most likely that fragments would get spread out along the original orbit, similarly to Comet Shoemaker-Levy 9, which impacted Jupiter in the summer of 1994.

I think you wouldn’t see just a moving cloud of debris expanding symmetrically because the change of energy for the different parts would impact their orbital energy.

 

[barbos]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

If a large asteroid is moving with a substantial amount of momentum, wouldn't the blast fragments (assuming within the object) be spread out in more of an off-centered egg shaped distribution, with more pieces in front of what was the asteroid?

No, in free space it will be a perfect expanding ball moving with original speed.

 

[steve_bank]

All the kinetic energy in the asteroid has to go somewhere. Mostly heat no different than disk brakes getting hot when stopping a car.

Tunguska event - Wikipedia

en.wikipedia.org

The Tunguska event (occasionally also called the Tunguska incident) was a tremendous ~12 megaton[2] explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of June 30, 1908.[1][3] The explosion over the sparsely populated Eastern Siberian Taiga flattened an estimated 80 million trees over an area of 2,150 km2 (830 sq mi) of forest, and eyewitness reports suggest that at least three people may have died in the event.[4][5][6][7][2] The explosion is generally attributed to a meteor air burst: the atmospheric explosion of a stony meteoroid about 50–60 metres (160–200 feet) in size.[2][8]: p. 178  The meteoroid approached from the east-southeast, and likely with a relatively high speed of about 27 km/s (60,000 mph) (~Ma 80).[2] It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 5 to 10 kilometres (3 to 6 miles) rather than to have hit the surface of the Earth.[9]

The Tunguska event is the largest impact event on Earth in recorded history, though much larger impacts have occurred in prehistoric times. An explosion of this magnitude would be capable of destroying a large metropolitan area.[10] It has been mentioned numerous times in popular culture, and has also inspired real-world discussion of asteroid impact avoidance.[citation needed]

So called kinetic energy weapons have been developed. Launch a mass in a suborbital arc and when it hits it has the effect of an explosive.

 

[barbos]

It is most likely that fragments would get spread out along the original orbit, similarly to Comet Shoemaker-Levy 9, which impacted Jupiter in the summer of 1994.

No, does not apply here. That comet was broken up by tidal forces which are directional.

 

[Shadowy Man]

I agree that if an asteroid impacts at 5km/hour it would not be an issue.

Escape velocity from 10km asteroid is 5 km/hour

But the relevant escape velocity for an impacting asteroid is Earth’s, not the asteroid’s.

not relevant because asteroid parts will be blown in all directions uniformly.

If a large asteroid is moving with a substantial amount of momentum, wouldn't the blast fragments (assuming within the object) be spread out in more of an off-centered egg shaped distribution, with more pieces in front of what was the asteroid?

No, in free space it will be a perfect expanding ball moving with original speed.

It’s not in “free space” if it is in orbit around the Sun.

Over short enough timescales what you say is likely a good approximation but over time it won’t stay that way.