What I learned from ARM

[zarakon]

The Orion drive is based on a proper deflector shield, and small, well timed nuclear explosions. Unless you install a deflector shield on the asteroid and launch multiple nuclear warheads, their efficiency is not really comparable.

A "proper deflector shield" wouldn't really be necessary. The major design elements of the Orion pusher plate involve allowing it to survive multiple blasts and dampening the impulse so as not to kill everyone onboard. Neither of those things really matter when it's an asteroid you're trying to push.

The efficiency comes from the design of the explosive charges themselves. They're designed to direct most of their energy toward the pusher plate as a "cigar-shaped" jet of plasma, rather than being just dumb bombs. They could probably be quite effective in pushing an asteroid

Of course, another application of the Orion drive would be to just send a high velocity impactor to the asteroid.

[MarvinKitFox]

Turning it to gravel is not bad option. It would burn in atmosphere instead of making large crater on surface.

Nope. At least, not for the ones we need to worry about, those 2-3km+ in size.

With a huge rock, it is *better* to have it bash into the ground, dissipate some energy in deforming/melting/crushing bedrock.

If you turn it to gravel, and the same mass enters, it will dump *all* of its energy into high atmosphere. Basically turning a significant patch of sky into whitehot plasma, thus ensuring extinction of life within line-of-sight. And line of sight for 60km altitude is very, very, very far.

Consider the case of a 1km stony meteor, impacting vertically at 35km/s , as observed from 500km away.

Crater = ~22km diameter wide, 800m deep.

Earthquake = 8.2 at impact. For observer, a good solid rattle, that may break a window but not topple a chimney.

A distinct light flash on the horizon, about as bright as car headlights at 400 meters.

25 minutes later, you hear a giant's burp over the horison. About as noisy as a diesel truck revving at 100m.

Over the next couple days, as much as 4mm of dust settle at your location.

Now take the same meteor, broken up into gravel. 1cm chunks, * 523598783333333 units. Yes, that is 523 598 783 333 333 = 523.5 trillion little shooting stars.

Each has about 1850Kj of energy, enough to heat one kilogram of air by 1.5 kelvin.

You get an airburst, at between 50km-75km, of some 9.6 quintillion joules. Thats about 230 gigatons of tnt, for the old school of thought.

This heats a 75km diameter spot of the atmosphere to more than 100000 degrees, initiating

1) a thermal pulse that heats everything within line of sight, including you at 500km, to several hundred degrees. You die, instantly.

2) a concussion wave that rips the top 10-20 meters of soil/rock/buildings/whatever off the surface of the earth, out to about 1000km., due to shockwave earthquake equavalent to richter 14 or so.

3) a BAD BAD day!

So, do you **really** think blowing a big rock to gravel is a good idea, hmm?

[MarvinKitFox]

What if you were to use a "bunker buster" bomb that penetrates the surface before detonating?

that's the idea in the armageddon, etc.. movies. At least when the idea was sensible, before the scriptwriters used it for toilet paper!

NOT to shatter the asteroid into tiny fragments, but simply to put to explosion deep enough under the ground so that the explosion converts virtually all its energy to (relatively) low-speed motion, thus huge momentum, and ejects all the moving debris in more-or-less one direction.

This acts like a very mass-inefficient but very energy-efficient rocket. And as a nuke has mountains of energy to spend, can result is a very hefty speed change for the larger asteroid as a whole.

By comparison, a surface or outside nuke blast will only boil off a very small amount of surface, and eject it at very high speeds. Efficient for a rocket, but inefficient use of the energy of the nuke, resulting is very little actual movement of the big rock.

Note, you seriously DONT want to fragment the whole rock, without causing it to miss the earth. Look at my scenario above. A gravelpile, scattered over a few km but still hitting atmosphere, is a WORST-case scenario.

[Kasuha]

Now take the same meteor, broken up into gravel. 1cm chunks, * 523598783333333 units. Yes, that is 523 598 783 333 333 = 523.5 trillion little shooting stars.

Each has about 1850Kj of energy, enough to heat one kilogram of air by 1.5 kelvin.

You get an airburst, at between 50km-75km, of some 9.6 quintillion joules. Thats about 230 gigatons of tnt, for the old school of thought.

This heats a 75km diameter spot of the atmosphere to more than 100000 degrees, initiating

1) a thermal pulse that heats everything within line of sight, including you at 500km, to several hundred degrees. You die, instantly.

2) a concussion wave that rips the top 10-20 meters of soil/rock/buildings/whatever off the surface of the earth, out to about 1000km., due to shockwave earthquake equavalent to richter 14 or so.

3) a BAD BAD day!

So, do you **really** think blowing a big rock to gravel is a good idea, hmm?

First of all I was not speaking about 2-3 km asteroids. But anyway. Let's take a look at it from a bit more realistic point of view.

Let's assume we really somehow broke a 2-3 km asteroid up to gravel.

Scenario 1: it for some reason still sticks together. Then it will behave more or less exactly as if the asteroid is still in one piece, air resistance would be only able to remove a few meters of its surface, increasing dust pollution by a small margin (most of air pollution would be Earth's own material ejected during impact). All of the consequences of direct asteroid impact will stay.

Scenario 2: pieces got ejected, creating an expanding cloud of gravel. There will probably still be a core (made of gravel) that would not be expanding and that would slowly start to attract closest of escaping particles. The density of particles would likely decrease exponentially from the center of this core and the size and mass of the core would be only part of the original asteroid mass. Then the result depends on how big we let this core stay because at one extreme we get to scenario 1 (no change) and at the other extreme we get just a cloud of meteors the Earth happens to be passing without any but pleasant consequences (lots of meteors in the sky). Consequences of the core impact would be similar to consequences of impact of asteroid of corresponding size.

Scenario 3: somehow we achieve a state when our evenly distributed cloud of gravel is exactly the size of Earth and hits Earth spot on. In that case, the atmosphere will not let a single piece to reach ground. The released energy distributed over the surface of Earth is negligible and will be released to space very fast. The major problem will be dust pollution which will obscure the atmosphere, decrease Earth temperature slightly for several years and may cause major food production problems. However, since some volcano eruptions already released bigger amounts of matter to atmosphere without any dire consequences, I expect things to return back to normal in a few years as well.

In any case, asteroid broken up to gravel appears as better option to unbroken asteroid hitting the earth.

[Zourin]

Nope. At least, not for the ones we need to worry about, those 2-3km+ in size.

With a huge rock, it is *better* to have it bash into the ground, dissipate some energy in deforming/melting/crushing bedrock.

If you turn it to gravel, and the same mass enters, it will dump *all* of its energy into high atmosphere. Basically turning a significant patch of sky into whitehot plasma, thus ensuring extinction of life within line-of-sight. And line of sight for 60km altitude is very, very, very far.

Consider the case of a 1km stony meteor, impacting vertically at 35km/s , as observed from 500km away.

Crater = ~22km diameter wide, 800m deep.

Earthquake = 8.2 at impact. For observer, a good solid rattle, that may break a window but not topple a chimney.

A distinct light flash on the horizon, about as bright as car headlights at 400 meters.

25 minutes later, you hear a giant's burp over the horison. About as noisy as a diesel truck revving at 100m.

Over the next couple days, as much as 4mm of dust settle at your location.

Now take the same meteor, broken up into gravel. 1cm chunks, * 523598783333333 units. Yes, that is 523 598 783 333 333 = 523.5 trillion little shooting stars.

Each has about 1850Kj of energy, enough to heat one kilogram of air by 1.5 kelvin.

You get an airburst, at between 50km-75km, of some 9.6 quintillion joules. Thats about 230 gigatons of tnt, for the old school of thought.

This heats a 75km diameter spot of the atmosphere to more than 100000 degrees, initiating

1) a thermal pulse that heats everything within line of sight, including you at 500km, to several hundred degrees. You die, instantly.

2) a concussion wave that rips the top 10-20 meters of soil/rock/buildings/whatever off the surface of the earth, out to about 1000km., due to shockwave earthquake equavalent to richter 14 or so.

3) a BAD BAD day!

So, do you **really** think blowing a big rock to gravel is a good idea, hmm?

So, in short, a thermic detonation instead of a tectonic shockwave. Such fun choices! Although, this doesn't particularly take into consideration the rate at which particles enter the atmosphere, as a larger 'spread' in combination with planetary rotation and wind would help diffuse a lot of that energy. There's probably a sizeable difference between a shotgun cluster of gravel (which isn't much different than a solid impact) and a 'cloud'.

I'm pretty sure the thermodynamics would play out a little better than a straight kinetics analysis would lead you to believe, as you're contending with early vaporization of portions of inbound gravel particles as the air heats up, windage, and atmospheric layers. I wouldn't say you could stand under it with SPF30 and some shades, however.

Edited April 14, 2014 by Zourin

[Xavven]

I'm no astrophysicist, so I turn to a certain famous astrophysicist to help with this discussion. Neil deGrasse Tyson is not a huge fan of nuking a certain asteroid named Apophis that could kill us in the year 2036.

To hear why, skip to the 46 minute mark on this video:

http://www.hulu.com/watch/97871

There's also this, but he doesn't explain why (he does in the above video)

http://www.wired.com/2012/04/opinion-tyson-killer-asteroids/

[Kasuha]

To hear why, skip to the 46 minute mark on this video:

http://www.hulu.com/watch/97871

Sorry, our videos are only available in United States.

Well, duh.

[Xavven]

Sorry, our videos are only available in United States.

Well, duh.

Sorry, I didn't know Hulu restricted its content like that. He explains that you now have multiple pieces of that rock still on a collision course that would "wipe out half the planet." He goes on to state that bombs are "unpredictable" and "dirty."