Exploring this new star system you have been pretty lucky to find an Earth like planet, with plenty of Oxygen, both in gaseous and in chemically bonded state (water, rocks, etc.).

Just before you were opening that bottle of Chateau Centaurix 68 to toast this discover, your supercomputer starts beeping like crazy: due to gravitational perturbation, in some decades your new found planet is going to impact that blue Neptune-like giant planet over there.

You don't need a triple Ph.D. to guess that a blue giant planet is very rich in methane and other hydrocarbon gases, and you don't even dare deviating the orbit of the planets, as you know from kindergarden it is impossible with your means.

You bid farewell to your colonization dreams, but you start thinking that this is going to be a show that can benefit science: a bubble of Oxygen impacting a bigger bubble of Methane; quite a big 4th July firework!

What kind of observation equipment are you going to prepare, and where are you going to stand to observe it without being broiled?

Assuming the Earth as reference, its atmosphere mass is 5x10^21 g (source), of which roughly 1/5 is Oxygen, which gives about 6x10^19 mol of O2.

These can burn 3x10^19 mol of CH4, releasing about 3x10^22 kJ, which converts to 6x10^9 Megatons

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    $\begingroup$ You may want to wait a day, before accepting an answer, there are some freaks around here, who may and will give a more detailed and better answer than me (although I'm happy about the accepting ;) ) $\endgroup$ Dec 8 '16 at 7:28
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    $\begingroup$ The energy of the impact alone is greater than 10^29 kJ. The chemical energy of burning is entirely swamped by the gravitational energy released by the impact. $\endgroup$
    – James K
    Dec 8 '16 at 17:16

There'd be much more interesting things than the chemical reaction between oxygen and methane, which you can reproduce in every lab, it's just a bit bigger.

But you can observe the gravitation forces between the planets and their influence on the tectonics and the influence on the tides of the earth-like planet.

You can watch the result of the magnetic fields crashing. You can place cameras on the planets to get the First Person view of the crash.

And you can observe the influence on the rest of the solar system too. Since the two planets become more or less one gravitational force, it'll influence other planets too.

But the oxygen/methane reaction is, to quote one of the biggest reference books in the entire galaxy, mostly harmless.

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    $\begingroup$ Depending on how fast they're moving towards each other, things might get fun when they enter each other's Roche limits, I'd imagine. $\endgroup$ Dec 8 '16 at 13:06

The oxygen and methane are irrelevant to this question. The chemical energy released by them combining will be insignificant compared to the kinetic energy of the collision. So you observe this collision in the same way as any other planetary collision.

  • $\begingroup$ I am not sure the amount of chemical energy is neglectable, see my calculation in the edit $\endgroup$
    – L.Dutch
    Dec 8 '16 at 7:08
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    $\begingroup$ 6x10^9 megatons for the chemical energy sounds reasonable. Now assuming a 30km/sec impact speed and a planet massing 6x10^24kg, we get 2.7X10^33 joules or 6.5x10^18 megatons of kinetic energy, a billion times as much as the chemical energy. $\endgroup$
    – Mike Scott
    Dec 8 '16 at 7:17
  • $\begingroup$ @Mike Scot: At least double that, no? Because you have two planets impacting, and the blue giant is probably more massive than the earthlike one. $\endgroup$
    – jamesqf
    Dec 8 '16 at 7:20
  • $\begingroup$ @jamesqf For the sake of simplicity, I'm treating the larger planet as an immovable object. $\endgroup$
    – Mike Scott
    Dec 8 '16 at 7:21
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    $\begingroup$ @L.Dutch: The minimum impact velocity for a large body and a small body is the same as the escape velocity of the larger body, since the velocity lost leaving a gravity well is identical to the velocity gained entering it. In Neptune's case that would be 23.8 km/s. I don't know what it is for two large bodies, but it would be larger still since they would be pulled towards each other. So 5 km/s impact is impossible, while 30 km/s would be a pretty good estimate. $\endgroup$ Dec 8 '16 at 13:34

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