Science Questions

Should we blow up an Earth-bound object?

Sun, 15th Jul 2012

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Question

Andy John asked:

Hello Naked Scientists,

 

If a comet were heading toward the Earth, it is often stated that the worst thing that could happen would be to break it up into pieces because then you would compound the problem.

 

Why?

 

If I want an ice cube to melt faster in a glass of water, I break it into pieces which increases the surface area. It seems to me that 100 microcraters would be a big improvement over a Yucatan-sized, dinosaur-ending type rock.

 

Thanks,

 

Andy John from Asheville NC, USA

 

Answer

Dominic -   That’s a serious question that NASA have been doing research into and there are two approaches that you can take to avoid the comet from impacting the Earth.  You can either give it a very small nudge and try and change its orbit so it doesn’t actually end up on a collision course with the Earth, but it skims past us.  Or you can try and blow the comet into smithereens so that none of those small pieces can cause very much damage.  The problem is, you need a very large explosion to break one of these comets apart and if you for example, fire a nuclear missile at a comet, it’s then going to become radioactive and you may just end up with a lot of large fragments, each of which could wipe out a city of people and might be radioactive to boot.  It really actually all depends how far away the comet is and how much warning we have, and how much planning we can put into what the response would be.

 

“Hi, guys.  I think it’s not a good idea to blast it to pieces because the bits would break up and then they pound the earth with lots of craters and rubbish.”

Marc Hampson via Facebook

 

“It depends on how big the resulting rubble is.  If the individual pieces were small enough then they would all burn up in the atmosphere.  I prefer the idea of painting one part of it and using the extra absorption of light to steer it away from a collision course.  If we could convince some clever engineers to paint a logo on it, we can get some multinational company to sponsorship for the whole action.”

Andrew Reitemeyer

 

Dominic -   You would need quite a lot of warning because that would be quite a slow way of deflecting the comet.  One of the principal problemsis that we don’t have a very good idea of what comets are made of and how structurally sound they are.  So, in terms of firing something out and try and break them apart, we don’t really know how they would fragment.

Chris -   The point about sunlight though, just in case anyone doesn’t believe that that’s feasible, there is something called the YORP effect which is Yarkovsky–O'Keefe–Radzievskii–Paddack effect which is photon pressure.  So when photons, like packets, hit a surface, they impart a nudge or some momentum to it and it is capable of moving big objects given enough time.  There was some modelling done for the asteroid belt and a lot of the impactors that we think did away with the dinosaurs, you can retrace their history; they probably came out of the asteroid belt somewhere near Mars; and we think that some object was dislodged by the fact that you have this nudge from the pressure of light.

Dominic -   Yeah, and it’s a very small effect, but the first detection of that was actually made only about 2 years ago. There is an asteroid, but it has been detected that the sun is putting pressure on it and pushing it out.

Chris -   Hugh, you passed me a note here about Barnes Wallis.  Tell me more.  Is this in relation to the asteroid question?

Hugh -   Well, I'm not a great expert on asteroids, but one thing that Barnes Wallis found out during World War II was that dropping 40 250-pound bombs is not going to blow up the dams of Germany, but dropping one bomb which is 10,000 pounds would.  So, it strikes me then by similar argument that it’s probably better to have 40 small asteroids than one big one.

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I think part of why it does not make common sense is  due to the cube root, V=(4/3)pi*r^3. So, if the Yucatan impactor was 6 miles in diameter, breaking it into 100 pieces would yield 100 impactors each about 1.3 miles in diameter. Breaking it into 1000 (=10*10*10) pieces would reduce the diameter by 1/10, so 1000 objects 0.6 miles across, still pretty big impactors each with a many megaton explosion. Either way it is a bad day at the beach.

If you could vaporize the comet to dust size particles it may be a better situation but I can't say I've seen any models of the Earth running into a giant relatively dense dust cloud. distimpson, Sat, 14th Jul 2012

I agree; i think it comes down to the fact that one threat we have some hope of curtailing or mitigating, because at least the trajectory will be predictable.

But if we fragment the parent body, predicting the trajectories of the derivative pieces will be very difficult; plus, more impactors more more chances that one of them will hit somewhere important.

Chris chris, Sun, 15th Jul 2012

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