Any astrophysicists want to explain

I see what you're trying to say but I don't think that's really the point.

In certain circumstances, your analogue is valid. If the asteroid was 1,000 feet in diameter and could be broken up into chunks a foot in diameter - great. They'd burn up in the atmosphere and everything's fine.

(Don't quote me on exact sizes - I'm making them up. But you get the idea.)

But this doesn't help if you have a much bigger asteroid coming at you. We simply can't blow it up into small enough pieces. A choice between getting hit by a single 100-mile diameter asteroid versus 125,000 2-mile diameter asteroids is moot - both would wipe us out.

Even if 99% of the smaller (2-mile diameter) chunks were diverted and didn't hit earth, over 1,000 of them would. That's still more than enough to wipe us out.

In terms of a book using the scenario of a huge asteroid coming toward the earth, it's probably easier to figure out a way to nudge the (big) asteroid to one side than to figure out how to deal with thousands of smaller, but still Armageddon-sized asteroids.

(The movie 'Armageddon' got around this problem by making the asteroid have a 'fault-line' running right through it. An explosion in the right place would 'split' the asteroid in two and both pieces would be pushed sideways just enough to avoid the earth. Of course this is ridiculous but - heh, it worked in the movie.)

Not Bird shot

GeorgeK said:

why blowing up an asteroid on a collision course with the planet is such a terrible idea? Assume for the moment that it was otherwise guaranteed to impact. Getting hit by birdshot is preferrable to a deerslug. Why would asteroids be different?

Click to expand...

The problem is, the composition and density of most asteroids is unknown and each may be very different. Blowing one up may create two or three chunks, rather than hundreds.

Think SSG or buckshot, rather than bird shot. It would still be disastrous.

Check out this video with Neil DeGrasse Tyson. He explains it in pretty good detail (its after the black hole stuff). You might want to get his book too.

Pretty much what everyone else said. Especially this:

johnnycannuk said:

The problem is, the composition and density of most asteroids is unknown and each may be very different. Blowing one up may create two or three chunks, rather than hundreds.

Click to expand...

Of course, even if it does burn up, there is the problem that you've created a lot of dust that's going to be in the atmosphere (it has to go somewhere!). So you have all this dust that's obscuring the sun, which is basically the effect you were trying to avoid in the first place....

FR

GeorgeK said:

why blowing up an asteroid on a collision course with the planet is such a terrible idea? Assume for the moment that it was otherwise guaranteed to impact.

Click to expand...

The thing is, if you detect it far enough out, it probably wouldn't be. I mean, an asteroid isn't a bullet. You could conceivably detect one months in advance.

Izunya

Blowing up an asteroid isn't practical(at least if you're trying to ensure that the chunks don't hit earth), as both the energy requirements are huge AND the break up of the asteroid isn't really predictable. However deflection would be really easy with either a kinetic impactor or an adjacent nuke blast. Both of those circumstances would only be needed if a 1km+ asteroid was coming in, otherwise a simple gravity tug could probably manuever a smaller asteroid out of our way.

Izunya said:

The thing is, if you detect it far enough out, it probably wouldn't be. I mean, an asteroid isn't a bullet. You could conceivably detect one months in advance.

Izunya

Click to expand...

The problem is, the sky is BIG and an asteroid is comparatively really, really small.

If we're lucky enough to detect it that soon, I remember seeing a possible theory of sending probes to land on the asteroid and fire off enough thrust to push it out of the way.

GeorgeK, as many here have posted, it would be a bad idea to blow an asteroid anywhere near our planet for a number of reasons. It has been a pet theory of mine in many of my sci fi stories and I had the same problems with dealing with it. I have watched numerous programs on this and have gleaned some info for you from Discovery.com. and found another web posting that might be helpful.


“These are asteroids that are bigger than 460 feet in diameter — slightly smaller than the Superdome in New Orleans. They are a threat even if they don't hit Earth because if they explode while close enough — an event caused by heating in both the rock and the atmosphere — the devastation from the shockwaves is still immense.
The explosion alone could have with the power of 100 million tons of dynamite, enough to devastate an entire state, such as Maryland, they said.
The agency is already tracking bigger objects, at least 3,300 feet in diameter, that could wipe out most life on Earth, much like what is theorized to have happened to dinosaurs 65 million years ago. But even that search, which has spotted 769 asteroids and comets — none of which is on course to hit Earth — is behind schedule. It's supposed to be complete by the end of next year.”
“Earth got a scare in 2004, when initial readings suggested an 885-foot asteroid called 99942 Apophis seemed to have a chance of hitting Earth in 2029. But more observations showed that wouldn't happen. Scientists say there is a 1-in-45,000 chance that it could hit in 2036.
They think it would mostly likely strike the Pacific Ocean, which would cause a tsunami on the U.S. West Coast the size of the devastating 2004 Indian Ocean wave.”
(taken from www site http://dsc.discovery.com/news/2007/03/06/killerasteroid_spa.html?category=space&guid=20070306121530) (This may be protected info, check with site authors if used.)

Another good informative article would be Killer Asteroids by Armando Caussade. It can be found at http://www.armandocaussade.com/astronomy/killer_asteroids.html. (Note this is protected info and the author requests to be contacted if used in commercial work)

The article by Armando Caussade is particularly interesting and had both historical information in it and the current thoughts of how to prevent a killer asteroid from hitting the Earth. It also listed his references that might give you more information. Hope this has been helpful.

Calixus

For the same reason nuclear weapons with MIRVs are considered more damaging than one big bomb; There's a sort of economy of destructiveness, based on my favorite scientific principle, the Inverse Square Law.

The Inverse Square Law states that energy radiated from any source (virtually by any method) is reduced by the square of its distance from the detector. When it comes to explosions, that means that if you are standing 100 feet away from a certain explosion, you will experience the same amount of energy as you would standing 200 feet away from an explosion that is 4 times as big!

So its obvious that there's a greater economy of destructiveness to release lots of smaller explosives over a wider area than to have just one big one. I expect the same principle holds true to meteorite impacts.

Naturally though, any impacts big enough to cause global cooling through dust would make that kind of irrelevant.

It has nothing to do with collisions. In point of fact, blowing up a large asteroid is impossible with current technology. Period.

What does 'blow up' actually mean? We use the word fast and loose and generally our gut feel for what it means is based on either fictional accounts, or observations of rather small things pulverized by the shock wave of a high explosive and then dispered by the expanding gas.

Blowing up an asteroid would require simultaneously breaking it into pieces and pushing those pieces apart with more force than the still remaining and quite undiminished gravitational forces that kept it together in the first place.

Nuclear weapons, while daunting, are fairly ineffective in the vacuum of space. Most of the energy and destructive force we associate with nuclear weapons is transmitted through the conversion of heat from the fission process to a shockwave that travels through air. No air in space, all the radiation is hard, and there's very little 'push.' And chemical weapons just aren't powerful enough.

And if you're using Armageddon as a go-by, sure it was a fun movie, but as soon as you 'split' the asteroid, the gravational attraction of the two halves is going to pull it right back together.

The only realistic way to stop an asteroid impact, based on current or expected near-future technology, is to divert it. And the only way to divert it is to start EARLY, because as a proportion of asteriod mass, your diversion force is going to be incredibly small. Which means you have to KNOW about it and start early. Years early. Like ten.

And as far as building anything to get the job done? NASA is pretty much useless. Armageddon aside, NASA doesn't build anything, anymore. And the contractors who do would fight about money and spend it all hoping to get more and take forever to settle on a design, much less build the thing. Our only hope would be to use Russian rockets and paint one side of the asteroid white. That's right, paint it white. It might change the radiation pressure of the sun enough to alter the orbit. Or it might not.

Actually a lot of energy is lost in the fact that an atmosphere is around to absorb a lot of the radiation.

What a nuke would do, isn't blow up the asteroid, but vaporize the surface facing the nuke, and create a large amount of thrust that could "push" the asteroid off its course. In fact this thrust would actually be pretty massive, because it is possible to make "shape charged" nukes that actually can focus more of the blast in in a specific direction.(Think enough thrust to easily move up to 8,000,000 tons, as projected in some Orion starship designs) The biggest bonus is that the nuke itself would not break up the asteroid.

The only time you'd need to bust one out is if you only had a few months to act on an incoming asteroid. Otherwise there's other methods that could just as easily do the job, but would take longer(e.g. gravity tug, altering the reflective properties of the asteroid etc.).

I disagree.

The only reason a nuclear weapon has the kind of destructive force we see is because of the atmosphere. Oxygen and nitrogen are fairly opaque to high energy radiation which is a good thing or cosmic radiation would eventually kill us. Since it is opaque it absorbs prompt neutrons, x-rays, and gamma radiation and almost instantaneously converts them to heat. This heat is manifest in temperatures at the shock from of millions of degrees. So high that the air itself becomes incandescent. But because it is so hot, it expands as a true shock wave, carrying the energy of fission through the atmosphere.

As you suggest, were a nuclear weapon to be detonated inside an asteroid it would indeed vaporize some amount of rock. We see this in the subsidence craters formed all over Yucca flats in Nevada. But the amount of rock vaporized is really quite small relative to the scale. Yucca flats isn't that large an area and there are hundreds of craters there. And Orion, as you correctly cite, was designed to use nuclear detonations as propulsion. Note though, that the mass of an Orion starship is miniscule compared to even a fairly small asteroid, say a mile across.

Something the size of Apophis, at 450 meters in the long dimension, would verly likely be broken into smaller pieces by a large nuclear weapon placed near the core. The gravity of Apophis is very low so may not pull them back together and the new trajectories might be altered enough from the initial to result in a miss. A nuclear weapon detonated on the surface though would probably do little to divert it. However Apophis is not a large asteroid, and while it would cause significant damage is no planet killer. Either way, anything you do is going to result in a changed orbit that still crosses Earth's path. The only way to fix that is to change it's trajectory twice in precisely the right direction at precisely the right time.

So the question really comes down to, how big is the object you're trying to 'blow up'? The Texas-sized object on Armageddon - not a chance. There would be absolutely nothing we could do about that. Fortunately, as far as we know, there are no NEO's that large. But there are hundreds of NEOs larger than a kilometer, and the largest is 32 kilometers across - far too large to break up with a nuclear weapon.

Good discussion. If you have any practical information on shaped-nukes I'd like to see it. It's sort of a hobby, collecting tidbits of public-domain nuclear technology. Back in the nineties after President Clinton created the Library Without Walls, which allowed you do download declassified technical reports from the Manhattan Project, I snagged a ton of that stuff. All about critical masses, crossections, neutron spectrums, and refining purified uranium from UF6. After they'd realized their gaffe they shut the whole thing down but we can all rest easier knowing Al Qaeda, and I, and hundreds of others, have pretty much all the information we need to design a 20 KT bomb. All we need is the fissionable material.

In fiction, if you blow up a comet or asteroid, every single piece of it hits a major city, whereas, if you leave it in one piece, only one city is destroyed. And as that is either New York or Los Angeles, no big deal.

Both them Russians and the Europeans have experimented in orbit with solar sails. The driving force is the sun's radiation primarily light (photons). They don;t have much thrust but the energy is free and for a decades long journey, they could build up a high velocity enough to deflect an asteroid from hitting the earth or, for that matter, sending one to hit Mars. The sails can be tacked just like on a sail boat so despite the fact that the photons are coming from the sunm you could tack towards it at an angle

thats always puzzled me. Tacking works in sailing because the water pressure against the keel counters the sideways component of the wind force. What would do that in space?

Sarpedon said:

thats always puzzled me. Tacking works in sailing because the water pressure against the keel counters the sideways component of the wind force. What would do that in space?

Click to expand...

I've been sailing on a small boat with a friend, it was interesting sailing into the wind. He was saying he could keep lowering that angle, but it could get unstable and he was being conservative. If I recall right, he said it's possible to sail into the wind FASTER than the wind is blowing...

You of course don't have a keel in space so you can't change the vector to push 'into the Sun.'* But you can still change it by changing the angle of the sail (it's a mirrored surface so it reflects the photons at the complementary angle to the normal, or however you say that in geometry), thus (if I'm thinking of this right) always giving force along the normal to the sail's surface. If you set the sail at 45 degrees to the Sun, photons will be reflected off at 90 degrees, transferring their force to the sail at that 45 degree angle. Of course the force is lower than with the sail full-on reflecting the sunlight straight back towards the Sun, and the force you get goes toward zero as you turn the sail toward 90 degrees to the Sun. Consult trig tables for the exact factors of how this force varies with angle.

* but you ARE presumably "in orbit" around the Sun while doing this, so using this to speed up or slow down your orbital speed might well have such an effect indirectly. Offhand, I don't have a good enough feel for orbital mechanics to say for sure.

Sarpedon said:

thats always puzzled me. Tacking works in sailing because the water pressure against the keel counters the sideways component of the wind force. What would do that in space?

Click to expand...

To answer a question with a question, what do the engines of the space shuttle or the engines of the various space probes push against when they are in space? They are in a vacuum. There is nothing to push against except, I suppose, their own inertia which is a fundamental property of matter.

As for the keel on a sail boat, I'm no sailor but isn't its purpose to prevent the vessel from capsizing due to the torsional effect of the wind on the sails? A sail boat without a keel would otherwise be top heavy.