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You're an alien that really likes pyrotechnics. Nothing that goes boom on your home planet (in a safe and responsible manner) satisfies you any more. Why not blow up a planet and see what that's like? However, you're not allowed to do that in your home solar system. The next closest one is ours. Oh, but there are humans on Earth; they can't stop you, but you don't want to hurt them either. You're a bit crazy, but you're not a sociopath. You just wanna see the biggest planetary fireworks show possible.

In short, which is the most massive celestial body in our solar system that you could destroy without endangering human life on Earth, subject to the following constraints?

  • You don't want to harm humans, so you definitely can't destroy the Earth, Moon, or Sun.
  • It's okay if there is collateral damage, as long as that damage does not include human life (directly or indirectly).
    • This implies that human artifacts (like the Curiosity rover on Mars) on other bodies can be destroyed, even if they're in active use.
  • This is in the present day, so the only people off the planet are in orbit (like on the ISS). You don't want to hurt them either.
  • It's okay if you're detected before, during, or after the show. You're not interested in stealth.
    • This implies that there may be mass panic on Earth. That's not your problem, though.
  • You don't want to stick around or communicate with humanity. Get in, explode, and get out.
    • They might try to contact you, but you just ignore them.
    • You won't be around long enough for them to be able to attack you.
  • You don't want humans to have to accommodate your little stunt (e.g. humanity shouldn't have to move underground for a few months).
  • You do not have any way to prevent collateral damage except by careful planning. Better pick your target carefully.

The method that you use to actually destroy the body isn't relevant as long as it meets the following conditions:

  • You can pick with surgical precision which planetary body to destroy (i.e. you can pick Saturn but spare its moons).
  • You have fine control over how much energy is used. So you can avoid introducing excess energy (thus releasing massive heat or radiation).
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    $\begingroup$ You accept collateral damage, but you don't want direct or indirect loss of life: are you able to calculate the trajectories of all the fragments of the explosion and rule out debris induced asteroids bombardment? $\endgroup$ – L.Dutch May 29 '17 at 3:11
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    $\begingroup$ I'm trying to think of a reason why "Jupiter is off limits." And I can't. $\endgroup$ – Draco18s May 29 '17 at 3:18
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    $\begingroup$ @Draco18s -- the main hazard disposing of Jupiter would introduce is from the loss of its interplanetary trash collection function, I reckon :P (which might actually be unacceptable in this circumstance) $\endgroup$ – Shalvenay May 29 '17 at 3:42
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    $\begingroup$ No human collateral damage in what time span? Shooting one of the big masses out of the solar system by explosion might change the earth's orbit over a time span that might very well exceed a human life span but kill everyone afterwards. $\endgroup$ – Helmar May 29 '17 at 14:15
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    $\begingroup$ ...I just realized, I have a copy of Universe Sandbox 2. Let's try it out! $\endgroup$ – JesseTG Jun 1 '17 at 23:53
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Ganymede

No matter which one you go for, blowing up a planet is going to make a lot of shrapnel. An asteroid of several miles long is an extinction-level event, and you've just thrown millions of such asteroids, along with billions of smaller meteors, all over the Solar System, and mass tends to fall towards gravity wells. Chances are, enough of those are going to hit the Earth to give humanity a bad day.

If you want to blow something up, your best bet is to aim for an object that is already mostly constrained to a gravity well, minimizing the shrapnel that you'll be dealing with. Fortunately, the biggest moon in the Solar System is also close to the second-biggest gravity well. If you blow up Ganymede, most of the shrapnel will form a fancy ring around Jupiter instead of peppering the inner planets.

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    $\begingroup$ This is the only good answer on this page. Orbiting bodies act differently than do objects most people answering are used to. Blowing something up means accelerating most of it's mass beyond it's own escape velocity in all directions. Any sun-orbiting mass will thus send debris raining down on the inner solar system. And even though space is big, the planets all orbit close to the ecliptic plane and that debris will now be in elliptical orbits. Over time that debris will rain down on the inner planets and the outer planets will get a spaying as well. Best to leave it in Jovian orbit. $\endgroup$ – dotancohen May 29 '17 at 17:49
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    $\begingroup$ @dotancohen Why in all directions? The question doesn't say that. Any alien capable of blowing up an entire planetary body should be able to make this a directional explosion. Then the alien can place itself between the Sun and Uranus and blow its pieces Oort-cloud-high. And the alien should be happy to do that, because he won't have to care about limiting energy to prevent human deaths. The more energy he puts in, the more quickly the Uranus cone of debris will exit the solar system and, more importantly, the better the fireworks show. $\endgroup$ – Iwillnotexist Idonotexist May 30 '17 at 5:02
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    $\begingroup$ @IwillnotexistIdonotexist: Orbits don't work that way. $\endgroup$ – dotancohen May 30 '17 at 6:36
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    $\begingroup$ @dotancohen Care to elaborate a bit? I haven't said anything about orbits, only about blowing a planet up such that all the ejecta flies outwards from the Sun. $\endgroup$ – Iwillnotexist Idonotexist May 30 '17 at 9:24
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    $\begingroup$ Conservation of linear momentum doesn't help either, in other words recoil (@IwillnotexistIdonotexist) $\endgroup$ – Chris H May 30 '17 at 13:05
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I'd strongly suggest your Alien to blow up Neptune or Uranus (latter would make for a nice joke, too)

Saturn and Jupiter have too many moons. With that many moons, you're creating a fleet of rogue-objects that could potentially start a chain-reaction and murderdeathkill everything on Earth. Also too much risk for your Alien.

So Neptune and Uranus. Neptune: Fewer moons than Uranus, most are smaller, too. Triton might be making trouble, but that should be okay.

Uranus: Funny joke, iirc slightly bigger than Neptune and looks prettier. Blowing pretty things up is funnier, isn't it?

Conclusion: don't blow up Jupiter or Saturn. The others are fine, but as size matters (im going strong with the jokes today), Neptune or Uranus should be your Alien's target.

And most importantly:Do NOT hurt Pluto.

Edited out because scientifically unaccurate (could still be added via hand-waving):

As one can read up on this question, Jupiter does not orbit the sun. Both J and S orbit around their combined center of mass wich rests just outside of the sun. By destroying jupiter, you might disturb the suns orbit and screw everything up. So that's probably too risky for your alien.

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    $\begingroup$ @OlafKlausson Plus it's not even a planet. Where's the fun in that? $\endgroup$ – JMac May 29 '17 at 11:46
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    $\begingroup$ @JMac you take that back, he will forever be a planet in my heart :( $\endgroup$ – Olaf Klausson May 29 '17 at 11:55
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    $\begingroup$ Why would Jupiter's moons go anywhere? Wouldn't they simply orbit the debris field that's left of the great Jupiter fireworks show? And even if you removed Jupiter's mass somehow, the odds of hitting Earth with one of the moons are astronomical, aren't they? $\endgroup$ – nikie May 29 '17 at 17:49
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    $\begingroup$ "A Look Behind: The Day They Blew Up Uranus. Join us as we plunge the depths of the back-story of the infamous crime that is the destruction of Uranus. We have looked at the other end of the solar system and our probes came up empty, but this book builds from the bottom to finally fill the void and asks: where is Uranus? Will we ever again look at the solar system to see it's whole?" $\endgroup$ – Jeroen Mostert May 29 '17 at 20:44
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    $\begingroup$ @nikie - to be fair, no matter how high or low the chance of hitting Earth, the odds will always be astronomical. $\endgroup$ – sirjonsnow May 30 '17 at 15:33
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The answer is go for the biggest and the best. Blow up Jupiter!

If Jupiter can be destroyed with surgical precision, so we can safely assume its moons will be left intact, then the collateral damage will be minimal. More so, if your pyrotechnic explodomaniac alien waits until Jupiter on the other side of the Sun when the explosion occurs.

Jupiter is approximately half a light hour distant from Earth. Its moons will be mainly in Jupiter orbit. While the Jovian barycentre is dispersing across the solar system, the moons will retain their velocity around the Sun and any additional velocity they effectively acquire due to their orbital velocities around Jupiter. Generally this will mean the moons will either move into higher or lower heliocentric orbits. Essentially they will be half a light hour away from the earth and the centre of the solar system, so the Sun and the Earth are safe.

The OP hasn't specified the precise method for making planets going KA-BOOM!! So it is possible that whatever remains of Jupiter might constitute hazardous materials and fragments for the inner solar system. But this should be considered as quite unlikely because the orbital velocities of Jupiter and its moons indicates that Jovian matter and the moons will remain far from earth.

However, if the aliens are zealous in their regard for human life they enjoy taking pot shots and blowing up any fragments that might be heading towards the Earth. But if their method for precisely demolishing Jupiter is sufficiently precise, then they could be capable of blasting Jupiter away from the Sun. Shoot so it gets blasted out and away. This means there shouldn't be problems for the sky falling in on Earthlings.

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  • $\begingroup$ I don't have the mathematical expertise nor enough knowledge in astrophysics, but I guess this is about right. Any explosion of Jupiter is going to increase its moon's velocity, so even if they change their orbits, they'll end orbiting far, far away from the sun, thus never crossing the Earth's orbit except in case of extremely elliptical ones. A precise timing wiht the explosion would render this outcome impossible. Since the barycenter of the solar system is inside the Sun, the vanishing of Jupiter is not going to affect the orbits of the inner planets. $\endgroup$ – Rekesoft May 29 '17 at 9:19
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    $\begingroup$ Blowing up Jupiter might compromise the dynamic stability of the solar system in the long run (timescales of hundreds of millions of years), about which a lot remains to be understood. In particular, it might lead to instability in the asteroid belt resulting in an increased frequency of impacts on Earth. I think Saturn would be safer. Or perhaps Planet Nine, which is at a safe distance. $\endgroup$ – Mike Scott May 29 '17 at 9:29
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    $\begingroup$ I have to disagree with your idea that Jupiter's moons will stay in a far orbit. Io, for example, has a velocity relative to Jupiter of about 17km/s, while Jupiter has a velocity around the Sun of 13km/s. This means that, if your aliens aren't caution, the resulting velocity of Io could range from 30km/s (periapsis at Jupiter orbit, we're fine) to -4km/s (change of direction, and a low periapsis). A non-negligeable range of velocities in between could very well result in a eccentric orbit crossing ours. So... not safe. $\endgroup$ – Keelhaul May 29 '17 at 9:40
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    $\begingroup$ @Keelhaul Very grateful for your useful information. At the high end of its velocity range Io's velocity should exceed solar escape velocity. Certainly at - 4 km/s Io would drop towards the Sun. That's why I suggested the aliens could blow stray hazardous objects. If they can blow up Jupiter, Io should be dead set easy. I also suggested Jupiter's blow up should happen on the other side of the Sun. Space is biggish. The cross-section of intersection between Earth & Io will be negligible. Not so dangerous. $\endgroup$ – a4android May 29 '17 at 12:29
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    $\begingroup$ If you'd destroy Jupiter it would fuck up the whole system. Jupiter absorbs arount 80% of meteroids and stuff that would otherwise fall on earth and other celestial objects, it has a huge influence on all orbits due to its mass, the gases would cloud the sun, you would probably destabilize orbits of other objects, the moons would search their own orbit and/or fall on a celestial object themselves. Don't blow up a gas giant $\endgroup$ – Fl.pf. May 30 '17 at 4:45
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There are a number of ways to annihilate a planet to consider, all of which have different effects. In addition, it depends on the interpretation of "not harming humans" both in terms of timescales (within the next year, next millenia, next million years, etc.) and in terms of harm (not killing/injuring immediately, not increasing likelihood of death or injury, etc.).

Blow it up

This leaves large fragments (=asteroids) floating around,some of which are likely to cross the Earth's orbit, with a likelihood of collision. A collision definitely falls under the category of harming humans, but in the short term, not something that is likely to happen.

It also removes a gravitational well from that planetary orbit, which had its own effects. In the case of the gas giants (especially Jupiter), they sweep up much of the asteroids and comets that could threaten the inner solar system (and thus us), providing a shield. They also have large numbers of moons whose orbits would change, possibly sending then careering off in random directions, with one or more likely to cross Earth's orbit.

So, if "not harming humans" means "not immediately harming humans", then anything, other than the moon, is possible. (The moon, tidally locked to the earth, performs several vital functions, one of which is keeping the earth steady on its axis. If it were to be removed, earth's rotation would become chaotic (like that of Mars) and our current, predictable seasons would eventually vary wildly.) If it means "not in the next hundred years" then I would add Venus, Mars and the gas giants to that list of no-goes (due to the likelihood of creating a large number of earth-crossing asteroids). If it means "not significantly changing the risk to humanity in the next million years" then probably only Pluto or any of the Kuiper Belt or Oort Cloud objects would be feasible to blow up.


Reduce it to insignificant pieces

The main effect of this would probably be the loss of gravitational well, but depending on how spread out the particles are, that might not be significant, and they would coalesce back into a (slightly smaller) planet over the period of a few million years.

If the particles are widely spread out, we would have a problem with loss of gravity well.


Vaporise it (convert to energy).

I'm not sure what the minimum safe distance for watching the mass-conversion of a planet might be, but it is likely to be quite far. Without doing the sums, I would hazard a guess that only the smaller asteroids and anything in the Kuiper Belt or Oort Cloud would be suitable for us to survive the blast of radiation.

It would, however, be very flashy!

The loss of the gravity well would probably not be a problem with those small distant objects.

If we assume that Jupiter was vaporised, and that it produced the same amount of energy as the fusion process in the sun (0.7% mass conversion into energy) it would produce 1.2x1042 J. For comparison, the Sun puts out an average of 3.846x1026 W, so if the explosion occurred over 10 seconds, it would be about 3x1014 times more energetic than the Sun, and outshine the entire galaxy (about 100 billion (=111 stars). If we assume that Jupiter was on the opposite side of the Sun from the Earth, it would be 968 million km away, and the energy flux would be about 1016 W/m2 at the Earth's distance, lasting 10 seconds.

A similar calculation for Neptune results in an energy flux at the Earth's distance of 4.6x1013 W/m2.

Now, the Sun is very opaque to radiation (it takes about 4 million years for photons to make their way out of the core), but I don't think it's going to be sufficiently opaque.

By comparison, a Type II supernova puts out about 1046 J over a period of about 10 seconds. A Type II supernova occurring 8 parsecs from the Earth might destroy more than half the ozone layer, producing an average flux of 3.05x1010 W/m2 at the Earth, so vaporising Jupiter would produce about 330,00 times more radiation on the Earth than the supernova. Neptune would produce about 1500 times more radiation. I don't think that any of the gas giants would be a good choice to vaporise, even on the opposite side of the sun.


Convert into a black hole

(EDIT: I've done some very rough calculations, using http://xaonon.dyndns.org/hawking/, and come to a different conclusion than before.)

This would be rather dull, as the planet disappears into a tiny black hole. For example, a Jupiter-mass black hole would be 2.8m in radius, and be barely detectable as its temperature would be barely be above absolute zero. It would evaporate too slowly to notice (well in excess of the lifetime of the universe so far at 1.82x1058 years. (I'd previously thought that it would evaporate soon after forming in a flash of gamma rays, but apparently that's only true for really tiny black holes which are also a lot hotter.)

As a result, just about any planetary object can be safely converted into a black hole; either they're too large to noticeably decay, or they're too small to put out the kind of energy that might harm us.

The loss of the gravity well would not be a problem with those objects; the larger ones with a noticeable gravity well would survive for a very long time, and the smaller ones wouldn't have a noticeable gravity well anyway.

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  • $\begingroup$ For the 100 year risk, it's any planetary body. Destroying a planet completely means accelerating all its mass to escape velocity in all directions. It's going to make one hell of a mess all over the solar system, no matter which planet it is. $\endgroup$ – Leliel May 29 '17 at 23:09
  • $\begingroup$ I would argue that fragments from the more distant bodies are likely to take more than 100 years to reach us; it takes an awful lot of energy for a body to quickly change direction from a regular orbit to diving into the solar system. Nonetheless, I take your point about the larger planets and will edit accordingly. $\endgroup$ – Pak May 30 '17 at 16:48
  • $\begingroup$ blowing it up hard enough to be exciting requires quite a bit of velocity over escape, otherwise the parts just kinds drift apart at planetary scale, so imo it's either boring, or changing debris orbits radically.. $\endgroup$ – Leliel May 30 '17 at 17:44
  • $\begingroup$ Hmm....what would you consider exciting enough? Double the escape velocity? An explosion that double the size of the planet in a second? $\endgroup$ – Pak May 30 '17 at 23:11
  • $\begingroup$ Using earth as an example, ~11Kps escape velocity, that means that 10 seconds after the boom, the surface has barely risen 110Km, or, through the atmosphere. it would take ~40 seconds to take out the ISS and about 9 hours to reach the moon.. that's not all that exciting to watch. If it's at 10 times escape velocity, that's a lot more interesting to watch. $\endgroup$ – Leliel May 30 '17 at 23:15
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While you could certainly blow up Jupiter, being the largest of the bodies in our solar system, I think the biggest factor is distance here, not size. In other words, the riskiest body to blow up is likely Venus due to its relative distance to Earth.

Though the pieces would go in every direction, the amount of space it would have to traverse before it reached Earth would be enormous. As a result, very little, as it turns out, would reach earth, and the pieces which do end up orbiting Earth would burn up in Earth's atmosphere. The truly dangerous pieces would have to have a straight trajectory and would have to be several kilometers in size, something that is not likely to happen statistically.

Most of the debris would simply follow its natural orbit around the sun, and although some would fall back together due to gravity, it would not reform any resemblance of a planet until much much later.

TL;DR - In other words, distance, not size is relevant, and the distances that you'd have to traverse to get to the planets in our solar system mean the really only truly dangerous planet to blow up for the inhabitants of Earth would be Earth itself.

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Did you ever hear of the Endor Holocaust? You can find sites on the internet that argue for and against the Endor Holocaust, the theory that the Endor moon would have been devastated by the explosion of the second Death Star.

http://www.businessinsider.com/endor-holocaust-star-wars-science-physics-2015-121

And what about the Yavin IV holcaust? What happened to the habitable and partly jungle covered fourth moon of Yavin when the expanding cloud of gas that resulted from the explosion of the first Death Star struck Yavin IV? Could any life survive on Yavin IV? Did the rebels have to rapidly evacuate Yavin IV before the shock wave from the Death Star struck the moon?

And what about the deadly radiation emitted by the explosions? Would everyone and all life forms on the near sides of Endor's Moon and Yavin IV receive fatal does of radiation or perhaps even be instantly vaporized even at distances of thousands or millions of miles?

The only way to know is to have someone do the calculations and see.

In the meantime I would guess that it might not be safe for Earth and Earth life to blow up even distant Pluto.

So maybe the explosion loving alien should take his super bomb into interstellar space and explode the planet buster in empty space. Thus he can see a very, very, very, very, very big explosion.

If the explosion loving alien needs to see a huge object be destroyed, maybe he can go the Oort Cloud in our solar system (his fellow aliens would prefer he does it in our solar system instead of theirs). The Oort cloud can be considered to both be in interstellar space and in the outer reaches of our solar system.

The Oort cloud is believed to consist of an inner torus shaped cloud of comets stretching from 2,000 to 20,000 Astronomical Units (AU) from the Sun and an outer spherical cloud that stretches from about 20,000 AU (0.32 light year) to about 50,000 AU (0.79 light year) or possibly several times that distance from the Sun.

It is believed that the billions and trillions of comets in the outer Oort cloud may have a total mass of about 5 earth masses. It is believed that the inner Oort cloud contains tens or hundreds of times as many comets as the outer Oort cloud.

So the bang loving alien can spend centuries, or millions of years, or however long it takes, to change the orbits of trillions of comets in the Oort could and assemble them into a planet large enough to satisfy his lust for explosions and then blast it with his planet busting bomb.

And maybe that will be far enough from Earth that the radiation does not kill any Earth life. And maybe that will be far enough from Earth that the expanding sphere of gases will not destroy all life on Earth. And maybe that will be far enough from Earth that the expanding sphere of debris will not be thick enough when it reaches Earth to cause any extinction level impacts.

Or maybe the alien will build not one planet but two planets that orbit each other. And when the planets are lined up with the inner solar system he will explode the planet that is farther away from the Sun. And the planet closer to the Sun might shield the solar system, absorbing radiation and shock waves and debris and creating a safe shadow that covers the planets of the solar system.

Or maybe the alien will plant his bomb in the Sun, at one of the poles pointing perpendicular to the plane the planets orbit in. And his giant explosion might blast away a chunk of the sun the size of a planet and send it on its way out into interstellar space. But the planet sized chunk of material ejected from the Sun will be only a tiny insignificant fraction of the Sun's mass and the Sun will continue basically unchanged - we hope.

It is certainly possible to create a very, very, very, big explosion on the Sun that would be very spectacular at close range (but far enough away to be safe, of course) but not be noticed at the distance of Earth.

In any case the only safe solar system planet to blow up might have to be an artificial planet as far out in the Oort could as possible.

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Well if u blow up any planet it will destroy the whole solar system as taking up Neptune. Neptune will be destroy than its explosion will effect Uranus due to this Uranus will also be come to an end then comes Saturn my favourite planet but if Saturn will be dead it will be disasterous explosion as it has so many moons it's rings then Jupiter it's destruction is very much dangerous as it has bigger mass much explosion material harmful gases and it will effect asteroid belt if that disturbs the earth will come back where it was started from...... 💥

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    $\begingroup$ Sorry, but this answer is wrong. Blowing up planets is hard, and we have discussed that previously, but with over 99.9% of the total solar system mass concentrated to the big glowy thingy near the center of the solar system (the mass of Jupiter is less than 1/1000 of that of the Sun, and excluding the Sun, Jupiter is by far the most massive object in the solar system), the solar system as a whole would barely notice if one of the planets disappeared. It might cause some mild orbital perturbations (or lack of same), but not much more than that. $\endgroup$ – a CVn May 29 '17 at 14:17
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    $\begingroup$ Blowing up a gas giant is also nothing even remotely similar to just dropping a match into a pool of hydrogen, because you need to bring along a much larger amount of oxizider for the show. Compare the answers to How can I destroy a gas giant planet? that discuss this in some detail. $\endgroup$ – a CVn May 29 '17 at 14:18
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    $\begingroup$ This answer is so spectacularly wrong I can't even wrap my head around it. Please don't give an answer to a question when you have no clue what you're talking about. $\endgroup$ – Arthur Dent May 30 '17 at 18:03

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