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Dialogue from the TNG episode Pegasus has long been used as evidence of low yield for Federation torpedoes. The argument, briefly, comes from Riker's statement that the asteroid could be destroyed but:

"It would take most of our photon torpedoes, but it would preclude any possibility of the Pegasus falling into Romulan hands."

Taking the size of the asteroid and then determining the energy required to fragment the asteroid, then dividing that by the number of torpedoes the E-D carries, you come out with a yield of somewhere in the hundreds of kilotons range.

However.

Upon reviewing the episode, it becomes clear that the destruction Riker is talking about does not simply equate to fragmentation of the asteroid. The whole premise of Pegasus is that there is a Starfleet ship carrying valuable technologies that the Romulans are trying to get their hands on. Riker and Admiral Pressman know that it's an experimental cloaking device, while Picard believes that the ship is carrying an "experimental engine" and "new weapons systems".

If the Enterprise were to destroy the asteroid as Riker suggested, therefore, they wouldn't simply be trying to fragment or vaporize the rock, they would be bombarding it to the extent that any useful technology contained within the hull of the Pegasus would be destroyed. Moreover, this presumably requires the destruction of sensitive data on the ship's computers. So, in context, the quote doesn't tell us that it would take most of the Enterprise's torpedoes to just fragment the asteroid, but to bombard it with such intensity that the ship and any sizable pieces of debris would be annihilated too. Without knowing how tough the hull of the ship is or how easily the Enterprise will be able to detect if the Pegasus has been sufficiently destroyed, this example doesn't tell us anything concrete about the yield of a photon torpedo.

there is how ever the matter that the Pegasus is not an inert target, IIRC the only reason ENT-D was on the mission was that it was found that accident was not in fact a warpcore breach and the Pegasus was largely intact so there was tech that could be found.

That's true, but it doesn't make it any more quantifiable, at least that I can see.

Sure it does, at least for an order-of-magnitude estimate. Scenarios:
1. Shattering the asteroid is enough to destroy Pegasus to everybody's satisfaction.
2. It's not, but guess what-the damned asteroid is out the way and you can now phaser her out of existence.
3. They for some arcane reason need to actually vapourize the asteroid.
4. For some even more arcane reason, Pegasus survives that.
5. The damned asteroid is out of the way so they can phaser the ship out of existence.
So yes, I'm very much afraid that all they have to do is destroy the asteroid, which would take...most of their photon torpedoes. And yes, nobody said they were fully stocked, but nobody mentioned they were particularly low, either. I absolutely do expect the closest thing Starfleet has to a warship to top off her torpedo loadout any chance she can, and I can't help notice the complete lack of qualifiers. Like 'it'd take most of our photon torpedoes since we're down to 50 percent ', 'it'd take most of our photon torpedoes since we're down to 17 and a maybe', 'it'd take most of the photon torpedoes we have left'...
Do we have any reason to assume the E-D isn't carrying a full load or something close to it?

What was the status of the Pegasus' warp core? Was it still intact? Did the Pegasus' hull still contain antimatter? If part of the asteroid's destruction could be contributed to the Pegasus suffering a warp core / antimatter containment breach on the process, wouldn't that mean that actual yield of the photon torpedoes would be even lower?

There's a couple of factors you haven't accounted for. The first is that the Pegasus is not what they're trying to destroy. They're trying to destroy the technology on the Pegasus. In particular a pretty small cloaking device and quite probably the ship's computer cores as well.

Secondly, this asteroid is obviously not an easy place to scan. Even once the Enterprise knows which asteroid the Pegasus is in, Data still says it will take six hours to locate the ship beneath the surface. And as Geordi tells us earlier in the episode:

"There's an awful lot of ionising radiation in this system, Captain. That and the sheer amount of rock is going to slow down our search a little."

So there's another scenario: Enterprise fragments the asteroid and Pegasus is blown to pieces, but there are still bits of wreckage that survive. Bits of wreckage that could include the cloaking device or computer cores from which the Romulans can derive useful technology. Bits of wreckage that, because of the conditions in the system, the Enterprise won't necessarily be able to pin down before the Romulans.

Tribble wrote:What was the status of the Pegasus' warp core? Was it still intact? Did the Pegasus' hull still contain antimatter? If part of the asteroid's destruction could be contributed to the Pegasus suffering a warp core / antimatter containment breach on the process, wouldn't that mean that actual yield of the photon torpedoes would be even lower?

All they say is that most of the ship's warp core is intact, but that there's no power on the ship. No mention of antimatter in the script.

No. It would mean the asteroid destruction could've been done with lower yield torpedoes, yes. But they couldn't be sure Pegasus would make sure of own destruction if they did enough damage to the asteroid

It's been awhile since I saw the episode, but I thought the goal of using the torpedoes was simply for the E-D to escape, and not the total destruction of the Pegasus. I don't seem to recall them destroying the Pegasus (or trying to) after obtaining the phase cloak and leaving.

Also, why weren't they able to use the phasers to re-open the fissure or create a new one in order to escape? Was it because the Romulan warbird outside would just seal it up again the moment they tried? Or was there some technobabble which prevented phasers from working on the inside, but allowed disruptors to be used on the outside?

The 'destruction of the asteroid' talks happened before they ever went in there (and since the whole point of the exercise at the point was to destroy the phase cloak...) By the time they had the phase cloak there was no longer any point to destroying the asteroid because they had the phase cloak? Nobody gave a damn about the actual asteroid , its destruction was only relevant insofar as it enabled them to destroy Pegasus.
Not to mention that for the big E to try destroy the asteroid from the inside would probably not have been a particularly smart idea?

Tribble wrote:Also, why weren't they able to use the phasers to re-open the fissure or create a new one in order to escape? Was it because the Romulan warbird outside would just seal it up again the moment they tried? Or was there some technobabble which prevented phasers from working on the inside, but allowed disruptors to be used on the outside?

How would the fissure even be sealed in the first place ?

This isn't a planet. You can just knock loose some rocks and rely on gravity to keep them in place.

bilateralrope wrote:How would the fissure even be sealed in the first place ?

This isn't a planet. You can just knock loose some rocks and rely on gravity to keep them in place.

The Romulans collapsed the entrance and fused the rocks in place with disruptor fire.

Tribble wrote:What was the status of the Pegasus' warp core? Was it still intact? Did the Pegasus' hull still contain antimatter? If part of the asteroid's destruction could be contributed to the Pegasus suffering a warp core / antimatter containment breach on the process, wouldn't that mean that actual yield of the photon torpedoes would be even lower?

I don't think Riker would rely on the Pegasus to contribute to its own destruction.

They're trying to destroy the technology on the Pegasus. In particular a pretty small cloaking device and quite probably the ship's computer cores as well.

Two relatively tiny pieces of technology buried deep inside the hull of the ship. Best way to ensure destruction of them is to make sure the ship is destroyed. Thus, shatter the asteroid into pieces, locate the hull of the Pegasus, and hit it with several photorps or phaser strikes. Still don't need to vaporize the entire asteroid for that.

Captain Seafort wrote:

bilateralrope wrote:How would the fissure even be sealed in the first place ?

This isn't a planet. You can just knock loose some rocks and rely on gravity to keep them in place.

The Romulans collapsed the entrance and fused the rocks in place with disruptor fire.

So if disruptor fire collapsed the entrance and fused the rocks in place, what prevented the E-D from using phaser fire to reopen it, or create another entrance?

EDIT: Worf suggested using phasers, but Data stated that they would collapse the rest of the tunnel and destroy the Enterprise. How would that work given the low gravity? Between phaser fire and tractor beams, the E-D should of had no problems.

Ted C wrote:

Tribble wrote:What was the status of the Pegasus' warp core? Was it still intact? Did the Pegasus' hull still contain antimatter? If part of the asteroid's destruction could be contributed to the Pegasus suffering a warp core / antimatter containment breach on the process, wouldn't that mean that actual yield of the photon torpedoes would be even lower?

I don't think Riker would rely on the Pegasus to contribute to its own destruction.

But that doesn't change the fact that the Pegasus had a (mostly) intact warp core. And potentially still had antimatter on board from the tanks which supply the warp core and/or it's own compliment of photon torpedoes. Why he wouldn't rely on the ship contributing to its own destruction, in all likelihood it probably would have.

As was noted here, we do not know how many photon torpedoes the Enterprise carried in this episode.

As Riker could assume that Picard knew how many photon torpedoes the Enterprise had, there was no need to mention that they were particularly low on them or to add any qualifiers like »it'd take most of our photon torpedoes since we're down to 50 percent«, »it'd take most of our photon torpedoes since we're down to 17 and a maybe«, »it'd take most of the photon torpedoes we have left«.

The fact allone that the calculation of the yield of the photon torpedoes - based on the assumption that 250 or 275 photon torpedoes could have been used - is contradicted by other observations in the canon is reason to assume the Enterprise wasn't carrying a full load of photon torpedoes or something close to it.

Furthermore the asteroid was not a normal asteroid.

It was difficult to scan:

• • • • • • RIKER:
        Sir, the Romulan warbird has altered course again... they are heading toward our position.
        
        • • RIKER:
        They probably want to see what we're so interested in over here.
        
        • • PICARD:
        Mister Data, how long will it take to determine the exact location of the Pegasus?
        
        • • DATA:
        At least another six hours, Captain.

It had unusual gravimetric or magnetic characteristics:

• • • • • • DATA:
        For the past several hours, I have been scanning the area where we detected the resonance signature of the Pegasus. From the strength and polarity of the signature, it would appear that most of the warp core is still intact.
        
        • • PRESSMAN:
        If we're lucky, the entire engineering section could be down there.
        
        • • RIKER:
        So how do we get to it? I wouldn't want to try to transport through that much solid rock.
        
        • • PICARD:
        Agreed. What about a shuttle? We could send it down through one of these fissures.
        
        • • DATA:
        I would recommend against that, sir. There may be gravimetric or magnetic fluctuations inside the asteroid which would overpower the engines on a shuttlecraft.

And it had a surprisingly strong gravitation:

• • • • • • PICARD:
        The Romulans fired on the entrance to the chasm with their disruptors... they've sealed us in.
        
        • • [...]
        • • PICARD:
        Options?
        
        • • WORF:
        We could use the phasers to cut our way out.
        
        • • DATA:
        The asteroid's internal structure is highly unstable. Any attempt to cut through the rock could cause the entire chasm to collapse.

As was noted before, the gravity of a normal asteroid should not be strong enough to seal the chasm. »This wasn't a planet. You can just knock loose some rocks and rely on gravity to keep them in place« [1]. And in low gravity the rest of the chasm wouldn't collapse if they had used the phaser to cut through the rock.

Conclusion: We know next to nothing about this event. We do not know how many photon torpedoes the Enterprise carried in this episode. And we do know next to nothing about the asteroid. We only know that it was a highly peculiar asteroid - what could explain that it was able to overload the cloacking device while the Pegasus flew through it.

what could explain that it was able to overload the cloacking device while the Pegasus flew through it.

As I recall, it didn't. The cloak overloaded due to it being a highly unstable prototype and it happened to fail inside the asteroid. There was no mention of the asteroid being the cause of the failure.

"Surprisingly strong gravitational field" is a hell of an understatement. Shuttlecraft can take off from planets that are at least Earth-sized. This asteroid had a strong enough gravitational/magnetic field to potentially overload a shuttlecraft's engines. That would imply that at the very least it has a greater gravitational / magnetic field than Earth does. What kind of material would the asteroid have to be made of? Something that was extremely dense, I would imagine.

Tribble wrote:"Surprisingly strong gravitational field" is a hell of an understatement. Shuttlecraft can take off from planets that are at least Earth-sized. This asteroid had a strong enough gravitational/magnetic field to potentially overload a shuttlecraft's engines. That would imply that at the very least it has a greater gravitational / magnetic field than Earth does. What kind of material would the asteroid have to be made of? Something that was extremely dense, I would imagine.

I can't image any material dense enough to have such a strong gravitational field that would not also have already collapsed into a solid ball instead of remaining the Swiss cheese that this asteroid was. Yet another scientific absurdity from Trek.

There may be gravimetric or magnetic fluctuations inside the asteroid which would overpower the engines on a shuttlecraft.

I wish to emphasize "MAY BE" in that sentence. Data has proven to be less than reliable when it comes to numbers in the past...despite being a computer. In this case, he could simply be overly cautious, or incorrect as to the composition of the asteroid. Since it's never actually stated what the asteroid is made of, I'm guessing it's a bit of both.

Did they ever state in the Ep why they couldn't just fire torps into the opening and have them detonate inside? Those things are guided, right? Or, heck, if they were able to transport some people over to the Pegasus, they could have transported a few torps over to it as well.

Borgholio wrote:

They're trying to destroy the technology on the Pegasus. In particular a pretty small cloaking device and quite probably the ship's computer cores as well.

Two relatively tiny pieces of technology buried deep inside the hull of the ship. Best way to ensure destruction of them is to make sure the ship is destroyed. Thus, shatter the asteroid into pieces, locate the hull of the Pegasus, and hit it with several photorps or phaser strikes. Still don't need to vaporize the entire asteroid for that.

The problem is we don't know whether it's possible to fragment the asteroid without fragmenting the Pegasus, and also whether that will actually make it any easier to find the Pegasus or all the little bits of wreckage. Indeed, the fact that Riker argues for blowing up the asteroid instead of trying to find it would seem to suggest that there's no way of simply blowing up the asteroid while leaving the ship somewhat intact.

biostem wrote:Did they ever state in the Ep why they couldn't just fire torps into the opening and have them detonate inside? Those things are guided, right? Or, heck, if they were able to transport some people over to the Pegasus, they could have transported a few torps over to it as well.

It wasn't a straight path to where the Pegasus was half embedded in the rock, so a torpedo might be able to negotiate the turns to get there.

There weren't able to transport to the Pegasus until they flew the Enterprise into the asteroid and found it, so by the time they could transport torpedoes or other explosives over, the Romulans would be in a position to seal them inside.

Borgholio wrote:

what could explain that it was able to overload the cloacking device while the Pegasus flew through it.

As I recall, it didn't. The cloak overloaded due to it being a highly unstable prototype and it happened to fail inside the asteroid. There was no mention of the asteroid being the cause of the failure.

• • • • • • GEORDI:
        Commander, I've routed the impulse engines through the warp plasma conduits... but you'll have to watch the intercooler levels... if they go too high, we'll blow out the entire relay system.
        
        • • RIKER:
        Understood. I think that's what happened twelve years ago. The cloak blew out the plasma relays on the Pegasus after we left the ship... the plasma ignited in space, and it looked like the ship had been destroyed.
        
        • • PRESSMAN:
        So the ship drifted into this system... still in a phased state... and when it passed through this asteroid...
        
        • • RIKER:
        The cloak failed. And half the ship materialized in solid rock.

As I recall, it was a prototype but it was nothing said of it being unstable. The relay system of the ship wasn't able to handle too high intercooler levels. And even with blown out relay system, the claocking device still worked. The ship remained in its phased state, drifted into this system and only when it passed through the asteroid, the cloak failed. Implication: As it is not very probably that it was a coincidence that the cloak failed just in the moment the ship passed through the asteroid, we have to assume, that the passing through the asteroid - as it is a very peculiar asteroid - caused the failure.

Ted C wrote:I can't image any material dense enough to have such a strong gravitational field that would not also have already collapsed into a solid ball instead of remaining the Swiss cheese that this asteroid was. Yet another scientific absurdity from Trek.

While I agree that is what was depicted. We have to suspense our disbelief.