Planetary Defense Experiment

[Shadowy Man]

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.

Sure but once broken up the fragments moved on their own orbits and spread out following the principles of orbital mechanics.

 

[barbos]

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

Well, yes, it will never get back into one giant asteroid (assuming there was enough explosive). Every separate rock will gets its own orbit and will stay on it for pretty much forever.

 

[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.

Sure but once broken up the fragments moved on their own orbits and spread out following the principles of orbital mechanics.

Fragments have never been on the same orbit, that's the point of tidal forces.
It does not apply here. Blown up asteroid is an expanding ball of fragments, and if there is enough time most of them will miss the Earth.

 

[Shadowy Man]

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.

Sure but once broken up the fragments moved on their own orbits and spread out following the principles of orbital mechanics.

Fragments have never been on the same orbit, that's the point of tidal forces.
It does not apply here. Blown up asteroid is an expanding ball of fragments, and if there is enough time most of them will miss the Earth.

The Sun has tidal forces too. The expanding ball won’t stay a ball over time is my point.

I guess ultimately we can agree to disagree here. Orbital mechanics is not my specialty in astrophysics so I guess I can just defer to your expertise here, neh?

 

[barbos]

The Sun has tidal forces too. The expanding ball won’t stay a ball over time is my point.

Forget about tidal forces, I only mentioned them to expain that your example is not applicable here. Tidal breakup is different from explosion.

Yes, expanding ball will not stay perfect forever. But on the scale of one orbit it will stay pretty close to a ball. After a while it will spread over into a ring of debris.

 

[Shadowy Man]

The Sun has tidal forces too. The expanding ball won’t stay a ball over time is my point.

Forget about tidal forces, I only mentioned them to expain that your example is not applicable here. Tidal breakup is different from explosion.

Yes, expanding ball will not stay perfect forever. But on the scale of one orbit it will stay pretty close to a ball. After a while it will spread over into a ring of debris.

You seem to be both agreeing with me and disagreeing simultaneously.

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

If you break it up but all the debris still hits the Earth I am contending that that is just as bad as it hitting without breaking apart. Given the total energy involved, burning up in the atmosphere would be catastrophic. Look at a small event like the Tunguska impact and realize that is multiple orders of magnitude smaller than the hypothetical 10km asteroid you have invoked. Even if you broke it up into 10,000,000 pieces each of those would be a Tunguska event. Smaller pieces would indeed burn up but you’re still depositing that energy into the atmosphere.

Deflection is the only viable option whether that be done by nudging or exploding. Either way it has to be done far enough in advance to ensure the asteroid or its debris miss the Earth.

The movies would have you believe that blowing it up just prior to impact nullifies the threat. I believe that is incorrect.

 

[barbos]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

 

[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?

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

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg.

 

[barbos]

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg

No. it will be a perfect ball. Unless it was moving with relativistic speed in which case it will be a disk.

 

[Shadowy Man]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

So what is 1% of the energy you calculated earlier?

 

[Shadowy Man]

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg

No. it will be a perfect ball. Unless it was moving with relativistic speed in which case it will be a disk.

It will not remain a perfect ball. Each piece will have a new orbital energy and that will spread things out no longer spherically symmetric.

 

[barbos]

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg

No. it will be a perfect ball. Unless it was moving with relativistic speed in which case it will be a disk.

It will not remain a perfect ball. Each piece will have a new orbital energy and that will spread things out no longer spherically symmetric.

We (I and JH) were talking about free space.

 

[barbos]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

So what is 1% of the energy you calculated earlier?

it's 1/100 of that.

 

[Shadowy Man]

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg

No. it will be a perfect ball. Unless it was moving with relativistic speed in which case it will be a disk.

It will not remain a perfect ball. Each piece will have a new orbital energy and that will spread things out no longer spherically symmetric.

We (I and JH) were talking about free space.

Which isn’t relevant to the scenario of a near earth asteroid impacting earth, yes?

 

[Shadowy Man]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

So what is 1% of the energy you calculated earlier?

it's 1/100 of that.

I’m dazzled. So, instead of 10,000,000 Tunguska events, we only have 100,000 of them. Seems like a reasonable improvement I guess.

 

[barbos]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

So what is 1% of the energy you calculated earlier?

it's 1/100 of that.

I’m dazzled. So, instead of 10,000,000 Tunguska events, we only have 100,000 of them. Seems like a reasonable improvement I guess.

You do realize that 20 meters fragments don't really cause any damage and since they all will be spread over time frame of few hours, cumulative effect of heating of the surface from burning rocks will be absent?

 

[barbos]

But with it in motion, the explosion would accelerate particles in front, deceleration particles behind... though I think about it, that'd invert my egg

No. it will be a perfect ball. Unless it was moving with relativistic speed in which case it will be a disk.

It will not remain a perfect ball. Each piece will have a new orbital energy and that will spread things out no longer spherically symmetric.

We (I and JH) were talking about free space.

Which isn’t relevant to the scenario of a near earth asteroid impacting earth, yes?

Actually it is relevant for my back of the envelope calculation.
What is not relevant is your theory that anything other than perfect expanding ball of debris will be catastrophic. it won't be.

 

[Shadowy Man]

The question is if on a single orbit will the debris spread far enough apart so that most of it misses the Earth.

Blowing up and deflecting strategies are in principle the same thing.
Exploding is equivalent to taking asteroid apart and deflecting each part individually by giving it random dV.
in case of D=10km, dV is in the order 5 m/sec. You can do more, but less is problematic because it will fall back together.
5 m/sec is 400 km/day and 13,000km per month, 155,000 km per year.
So if you blow it up 1 year before impact ~99% of the debris will miss the Earth.

So what is 1% of the energy you calculated earlier?

it's 1/100 of that.

I’m dazzled. So, instead of 10,000,000 Tunguska events, we only have 100,000 of them. Seems like a reasonable improvement I guess.

You do realize that 20 meters fragments don't really cause any damage and since they all will be spread over time frame of few hours, cumulative effect of heating of the surface from burning rocks will be absent?

So, you think the Tunguska event didn’t cause any damage? The energy involved was about 1000 times greater than the Hiroshima bomb. Even your 1% of 5e22 Joules you calculated earlier is almost a million Hiroshimas. Even if you could hand wave away 99.9% of that energy as going away somewhere somehow you’d still have almost 10,000 Hiroshimas on the Earth in your few hours.

It’s not clear you are grasping the magnitude of the problem here.

 

[barbos]

So, you think the Tunguska event didn’t cause any damage? The energy involved was about 1000

tunguska was 60 meters and it had rather high speed (comet)
Chelabinsk was 20 meters and damage was minimal

 

[barbos]

you’d still have almost 10,000 Hiroshimas on the Earth

Not really a problem if it is high in the atmosphere.
Chelabinsk was 400 KT