Skin of Evil
Explosion of torpedo sends air blast/shockwave of around 150 kilometers in radius. Wong's nuclear effects calculator gives us 500 megatons for thermal radiation and 9.83 gigatons for air blast radius (lowest value).
However, fireball seems to be 50 kilometers in diameter which gives us 87 to 144 gigatons. So torpedo is anywhere between 500 megatons and 144 gigatons.
Pegasus
In beginning, we have this dialogue:
Pressman: "The Pegasus was prototype. Experimental engine, new weapon systems, in fact some of our designs were used in constructing the Enterprise. There are lot of things that Romulans would like to have their hands on"
Picard: "What are our orders?"
Pressman: "To find Pegasus. Salvage if possible, destroy if necessary."
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RIKER: I recommend we destroy the asteroid. It would take most of our photon torpedoes, but it would preclude any possibility of the Pegasus falling into Romulan hands.
PRESSMAN: What the hell is the matter with you? Destroy the Pegasus before we've even taken a look at it?
RIKER: I thought it was more important that the Romulans
PRESSMAN: Well, you were wrong! We have a chance here to change the balance of power in this quadrant, but we can't very well do that if we destroy the Pegasus, now can we?
RIKER: No, sir.
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Later on, Enterprise is taken inside asteroid.
From these screenshots, we can conclude that asteroid is 8974 meters long, and 4487 meters wide on average. That would give volume of around 1.419 x 10e11 cubic meters. Given that quote number 3 was about destroying both Pegasus and asteroid, I will take it to mean that asteroid was about to be "vaporized". I will use energy required to destroy asteroid, and not any additional energy for destroying Pegasus, which should be vastly more difficult to destroy than asteroid.
Sphere of equivalent volume should be 3235.6869 meters in diameter. According to Mike Wong's asteroid destruction calculator, energy requirements for destruction of such asteroid are between 130 and 254 gigatons.
Now, we don't know exact number of torpedoes, but Enterprise-D carries 250 torpedoes, so I will take "most of" to mean anywhere between 140 and 240 torpedoes.
Conclusion: Federation torpedoes as of this episode are 542 megatons and 1.8 gigatons.
Die is Cast
We see orbital bombardment and learn that 30% of planetary crust is destroyed in single volley. So individual torpedo would have yield of several to several thousand gigatons at least, depending on interpretation. For calculations based on it, see
here.
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Photon torpedos seem to have produced omni-directional explosions in early TNG, but after that it seems they've upgraded them to the point where they can produce directional explosions. Evidence of this is that after Q Who and The Nth Degree we never hear or see effects of photon torpedo backlash again, and in TNG's Quality of Life we find out they can shape the explosion of a photon torpedo:
RIKER: Then we need to shut down the particle fountain. What if we detonated a low yield photon torpedo within the particle stream? Wouldn't that shut it down?
FARALLON: We'd have to configure the torpedo very carefully. The shape of the shock wave would be critical. But it could work.
So photon torpedoes ARE shaped charges and direct almost entire blast towards their targets, despite
some warsie arguments stating otherwise.
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Variable yield has not only been confirmed by pure visual evidence, but also by dialogue from Star Trek : Enterprise, episode "Expanse":
Reed : "Photonic torpedoes. Their range is over fifty times greater than our conventional torpedoes. And they have a variable yield. They can knock the comm array off a shuttle pod without scratching the hull, or they can put a three kilometre crater into an asteroid."
as well as TNG episode "Code of Honor":
Picard : "Set them for a display blast a thousand metres short of the planets surface."
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Some on SDN have claimed that episode "Genesis" proves low yield for photon torpedoes.
Link here:
http://bbs.stardestroyer.net/viewtopic.php?f=2&t=142124
That, however, is just intentional misinterpretation of sources. We are never said that torpedoes WERE fired with 11% greater yield; to contrary, torpedoes also got new guidance system, so it is logical that you will
lower yield of torpedo before firing it. That, however, is one of last TNG episodes, so if that 11% rise in yield was kept, it could mean standard yield for new torpedoes is 602 megatons.
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Some have claimed that visual effects disprove above yields. That, however, is wrong.
http://www.wwheaton.com/waw/mad/mad12.html
First, it is M/AM explosion, as opposed to classical nuclear explosion. Main product of that explosion are various types of radiation, mostly invisible but very damaging. Second, reactants in photon torpedo get annihilated relatively fast. So my guess is that, regardless of yield, we will see just very small, very bright flash which will disappear quickly, as soon as reaction finishes. But photon torpedoes are portrayed like strong chemical weapons by visual effects team (especially in newer instances, "First Contact" comes to mind). So, if we take stance that visual effects override everything, it would mean that torpedoes are not M/AM weapons, but rather just large, glowy artillery shells.
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C is constant, and is always 299 792 458 meters per second.
Kiloton is 4.184 x 10e12 joules, and more precise converter is
here.
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Ultra-dense deuterium:
http://www.sciencedaily.com/releases/2009/05/090511181356.htm
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Conclusion: Anti-ship torpedoes are
500 megatons as of TNG and 600 megatons as of late TNG. Yields for orbital bombardment end up from high megaton to high gigaton range.
On my site.
