Russia may send spacecraft to knock away asteroid

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Tom Ligon
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Post by Tom Ligon »

If we could demolish an asteroid would depend on what qualifies as "big".

Apophis is rather dinky, maybe half a kilometer in length. A large nuke could probably pulverize it if carefully planted.

On the other hand, was it the Bruce Willis movie that supposed we were going to blow up one the size of Texas with four nukes? Whoever dreamed that up has never been to Texas. If I recall, the actual plot was to plant the nukes along a convenient fissure on said asteroid, which they knew all about because, I suppose, NASA scientists just know these things.

In actuality, we know pathetically little about the properties of asteroids, and so really don't know how they would respond to either direct application of nuclear hammers, or gentle pushing with nuclear feathers. Would the energy from even a very large weapon actually transmit efficiently thru an asteroid, or would it overkill around the weapon but leave most of it intact? Would a nearby blast push an asteroid, or break it up? How efficient would the push be, given the uncertainty in makeup (particularly the response to neutrons)? I've long been a proponent of getting out there and prospecting a fairly large number. The excuse of understanding them for Earth defense is as good an excuse as any, but my real hope is we find something on them worth exploiting.

Notice, in the article below, the sparse data on this rock. While Apophis is not a civilization-ender, it could be a city-killer, and it is the only big threat detected so far, so you can imagine it has been scrutinized more than most.

http://en.wikipedia.org/wiki/99942_Apophis

MSimon
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Post by MSimon »

CaptainBeowulf wrote:I seem to remember reading in a few places years ago that our nukes (<50 megatonnes) are not powerful enough to destroy a large, "earth-killer" asteroid composed of stuff like dense rock and iron. Even if you took up several of the largest available nukes on multiple launches, assembled them together, and sent them at the asteroid, you still probably wouldn't have enough power. The Orion/Daedalus approach is probably the most feasible: attach a series of nukes to the asteroid and use the explosions to deflect it.

Maybe if you had a >1000 megatonne antimatter warhead you could annihilate one of those things... if the particle accelerator won't work for finding a Higgs boson, maybe we could use it to make antihydrogen? :lol:
My understanding is that there is no practical limit to a boosted fission nuke. The only reason that they stopped making them bigger was that they had no value except as city busters. With more accurate rockets the need for city busters went away.
Engineering is the art of making what you want from what you can get at a profit.

Tom Ligon
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Post by Tom Ligon »

I used to hang around with a guy who might or might not have said more than he should have about devices with yields well beyond 50 MT, and things they could have been used for.

There is a limit to the utility blasting asteroids with really big nukes, though. Nukes tend to overkill the immediate vicinity of the explosion, but with an inverse square law falloff. All you may do by using a larger one as a pulverizer is overheat the vapor you make of the center of the rock, and fling the rest of the chunks a little harder. If the idea is to push one aside, a series of explosions would give a more certain result. If you put everything into one blast, what if it does not work?

Rick Kwan
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Post by Rick Kwan »

A while back, I tried to do some comparisons to grasp the magnitude of an Apophis impact. It turned out to be a really hard thing to grasp just because of the magnitude. Here are estimates I see from Wikipedia and other sources, given in terms of megatons of TNT (MT).

* 0.012-0.022 MT -- atomic bombs dropped on Hiroshima and Nagasaki
* 3-5 MT -- Tunguska event (Siberia, June 30, 1908)
* 150 MT -- 2009 VK184 impact (June 3, 2048, 1 chance in 3030)
* 510 MT -- Apophis impact (April 13, 2035, 1 chance in 233,000)
* 1100 MT -- 2009 WM1 impact (Nov 23, 2079, 1 chance in 29.4 million)

Basically, this puts Apophis at 23,000 to 42,000 times the impact of the atomic bombs dropped on Japan. Apophis is estimated to be 270 meters in diameter. It's orbit is roughly co-planar with Earth, with perihelion just beyond Venus and aphelion just beyond Earth; that is, as asteroids go, the impact delta-V should be pretty small. Things from the asteroid belt between Mars and Jupiter, or far out of the Earth's plane (like 2009 WM1, similar size to Apophis) will have a lot more energy.

But to me, any potentially impacting asteroid is also a potential candidate for mining. You already have an Earth fly-by. You need a delta-V to effect a rendezvous, and then enough energy to take it apart so that the pieces can be refined... somewhere...

MSimon
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Post by MSimon »

Damage goes as the cube root of energy. So it is not as bad as it seems. Even if it goes as the square root of energy (detonation at optimum height) it takes 100X as much energy to get 10X the damage.
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Rick Kwan
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Post by Rick Kwan »

MSimon wrote:Damage goes as the cube root of energy. So it is not as bad as it seems. Even if it goes as the square root of energy (detonation at optimum height) it takes 100X as much energy to get 10X the damage.
Thanks for the insight. I presume the cube root is related to volume (rather than my naive linear) expansion. Taking my original factors of 23,000 and 42,000, that brings Apophis down to 28.3-34.6 times the damage of the individual A-bombs on Japan.

pfrit
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Post by pfrit »

Rick Kwan wrote:
MSimon wrote:Damage goes as the cube root of energy. So it is not as bad as it seems. Even if it goes as the square root of energy (detonation at optimum height) it takes 100X as much energy to get 10X the damage.
Thanks for the insight. I presume the cube root is related to volume (rather than my naive linear) expansion. Taking my original factors of 23,000 and 42,000, that brings Apophis down to 28.3-34.6 times the damage of the individual A-bombs on Japan.
Also the energy is not going to be applied in an optimum manner. Much is going to be dissapated by the atmosphere. It would probably first explode very high up and then come down as smaller fragments which would in turn explode lower in the atmosphere and on impact. The faster it goes, the lower the major explosions and the worse it is.
What is the difference between ignorance and apathy? I don't know and I don't care.

Tom Ligon
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Post by Tom Ligon »

How high an incoming object breaks up is also highly dependent on its compressive strength. Dynamic pressure is a strong function of velocity (the subsonic case will serve as a good approximation here: pressure in Pascals = 1/2 ro v^2 for air density ro in kg/m^3 and v in m/sec).

When the dynamic pressure equals the crush strength of the object, it breaks up. Comets, rubble-piles or pumice-like objects will tend to break up high in the atmosphere, as soon as ro is high enough to hit that pressure. The faster they are, the less density it takes to break them up, so the higher they break up and the less damage they do. Nickel-iron asteroids tend to hit the surface.

I will note a speculation regarding very large explosions above bodies of water even at orbital heights ... the overpressure may displace water and create a massive tsunami.

Objects coming straight down may not have time to spread much, so may be a bit like a shotgun at close range. You still catch nearly the full energy in a small area.

IntLibber
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Post by IntLibber »

CaptainBeowulf wrote:I seem to remember reading in a few places years ago that our nukes (<50 megatonnes) are not powerful enough to destroy a large, "earth-killer" asteroid composed of stuff like dense rock and iron. Even if you took up several of the largest available nukes on multiple launches, assembled them together, and sent them at the asteroid, you still probably wouldn't have enough power. The Orion/Daedalus approach is probably the most feasible: attach a series of nukes to the asteroid and use the explosions to deflect it.

Maybe if you had a >1000 megatonne antimatter warhead you could annihilate one of those things... if the particle accelerator won't work for finding a Higgs boson, maybe we could use it to make antihydrogen? :lol:
Well theres a serious issue with surface detonations on bodies without atmospheres. It turns out our atmosphere tamps a detonation on the ground and ensures most of its energy goes into doing damage. A nuke going off on the moon or an asteroid surface winds up losing most of the force to space.

Obviously, drilling a hole, like in Armageddon, and sinking a nuke down it, would allow the material of the asteroid to tamp the explosion so most of the energy goes into the asteroid. The more mass between the nuke and space, the better.

glemieux
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Post by glemieux »

Rick Kwan wrote:But to me, any potentially impacting asteroid is also a potential candidate for mining. You already have an Earth fly-by. You need a delta-V to effect a rendezvous, and then enough energy to take it apart so that the pieces can be refined... somewhere...
I second that! Wonder if an terrestrial mining companies would be interested in funding such a mission...

Rick Kwan
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Post by Rick Kwan »

glemieux wrote:
Rick Kwan wrote:But to me, any potentially impacting asteroid is also a potential candidate for mining. You already have an Earth fly-by. You need a delta-V to effect a rendezvous, and then enough energy to take it apart so that the pieces can be refined... somewhere...
I second that! Wonder if an terrestrial mining companies would be interested in funding such a mission...
A friend of mine who sits on a space colonization technical committee told me a few months ago that the mining companies are in fact extremely interested. The economic rewards are (no pun intended) astronomical. Designing special equipment to mine in hostile environments, and even having people live on site, is not a problem for them; it is what they do. What they lack is a way to get there; no one has designed a transportation system they can use yet.

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