# Can a large thermonuclear bomb be used for planetary defence?

How effective would a thermonuclear device of 47 megatons or higher like the zsar bomba be in space?

Could we use as planetary defence on somthing like an asteroid as long as it is detected in time and what effect if any would it have on our planet and our satellites in orbit?

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Nuclear physics is not the right tag for nuclear explosive devices. I was tempted by [nuclear-engineering] which is rather closer, but I doubt that practicing nuclear engineers would agree. Deflecting possible impactors is easy if you do it early enough and very hard (really very hard as in too much even for Bruce Willis) if you wait until the last minute. –  dmckee Jun 25 '12 at 21:57
Here are some real craters environmentalgraffiti.com/featured/… –  anna v Jun 26 '12 at 3:50

Have a look at http://impact.arc.nasa.gov/biblio.cfm for a comprehensive list of papers on the subject of asteroid deflection, or Google "deflecting asteroids site:nasa.gov" for a list of popular articles.

The summary of the Near Earth Orbit Program mentions the use of nuclear explosions, but makes it clear that a small device would be needed to prevent the asteroid from fracturing. A Tzar Bomba explosion would risk shattering the asteroid into bits, and even if all the bits missed the Earth this time round there is an excellent chance they would get us on the next orbit.

The favoured method seems to be laser ablation. If you shine a powerful laser at the asteroid you can boil off material from the surface, and the escaping gas will exert a force on the asteroid. This would be a very small force, but if you spot the asteroid far enough away it could divert the asteroid enough. See the NASA article for more info.

I seem to say this a lot, but the NASA site is a gold mine of useful information. If you want to research anything space related your first action should be to Google "whatever site:nasa.gov".

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An H-bomb can deflect asteroids in vacuum, because a sizable fraction of the explosive energy produces a wall of high-energy radiation and high temperature atoms at millions of degrees. This high temperature stuff heats the surface layers, and throws the material off the surface into space. The recoil from this ablation process makes a rocket, and this type of rocket pressure is effective enough to lift payloads of thousands of tons into space.

The way to estimate the thrust provided by atomic devices, arranged optimally, is to consider the orion spacecraft estimates. According to wikipedia, 800 bombs of size .15 kT (altogether only 100kT of bomb yield) will boost 4,000 tons into orbital velocity (a few km/s) in 10 minutes of explosions. A km scale asteroid is a million times more massive, so to deflect it's path by 10,000 km with the same explosive yeild would take many years.

With 50 mT of explosive yeild, you can reduce the time for deflection by a factor of 500, so you only need a few month's warning. But this is not going to work with a single big bomb, because a single big bomb will waste a lot of it's energy. But you should be able to deflect an 1km asteroid with less than a year's notice using one hundred small (like 50kT) nuclear devices planted one by one inside the surface a little and detonated, and then you would make an orion style deflector. This is a fine method, but considering that large asteroid impacts occur on a once in a million year time scale, we don't have much to worry about.

The distances involved are so large that there would be no effect on the Earth.

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I suppose smaller asteroids, that can still do a lot of damage to civilization as we know it, could be vaporized or turned to small fragments completely if necessary. It really seems a nuclear engineering problem. –  anna v Jun 26 '12 at 3:49

I can't help but think: "Why would you want to do that? Using a bomb to deflect an asteroid seems to me such a waste of energy.

We know already how to land on a comet (philae shown that it can work), so if you ignore the way to "get" to the comet (which is quite frankly the hardest problem, not the deflection itself). Why would you try to blow it away? Bombs are only good to blow things in pieces, but that's bad for obvious other reasons.

Why not land on the object, and then use a larger engine - with high specific impulse to deflect the asteroid? Here nuclear thermal engine (or maybe A fission fragment rocket. Can turn much more of the nuclear energy into useful thrust.

And then the question becomes simply a matter of the rocket equation

$$\Delta V = g_0 I_{sp} \ln\left( \frac{m_0}{m_1}\right)$$

Some simple calculation, notice this is a very naive way to deflect it.

Say you have an object of $10^{13}$ kg.
Typical $I_{sp}$ of thermal nuclear is in the order of 1000s. And it is rotating in a circular orbit at 1AU. ($150 \cdot 10^6 km$ To deflect it permanently we wish to move it to 1.05 AU. Using a hohman transfer this corresponds to two $\Delta V$ changes: ($V_0$ is speed in original orbit, $V_1$ in transfer, and $V_2$ final). $$a_0 = 1 Au$$ $$a_1 = 1.025 AU$$ $$a_2 = 1.05 AU$$ $$V_0 = \sqrt{\frac{\mu_s}{a_0}} \approx 29.78 \ km \ s^{-1}$$ $$V_{1_p} = \sqrt{\mu_s \left(\frac{2}{r_1} - \frac{1}{a_1} \right)} \approx 30.15 \ km \ s^{-1}$$ $$V_{1_a} = \sqrt{\mu_s \left(\frac{2}{r_2} - \frac{1}{a_1} \right)} \approx 28.71 \ km \ s^{-1}$$ $$V_2 = \sqrt{\frac{\mu_s}{a_2}} \approx 29.07 \ km \ s^{-1}$$

$$\Delta V_{tot} = 30.15 - 29.78 + 29.07-28.71 = 0.73 \ km \ s^{-1}$$

So using a thermo nuclear rocket with $I_{sp} = 1000 s$ the following minimal propellant mass can be found (ignoring structural mass of the rocket for now):

$$m_1 e^{\frac{\Delta V_{tot}}{g_0 I_{sp}}} - m_1 = 10^{13} e^{730/9810} -10^{13} \approx 7.73 \cdot 10^{11} kg$$. In coparison, the total amount of propellant the Saturn 5 used was about $2.5 \cdot 10^6 kg$

Theoretical fission fragement rockets have an $I_{sp}$ of about 1 million seconds. so that would lower the bar a lot on the fuel consumption, up to $7.44 \cdot 10^8 kg$ of fuel.

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