Destructive power of starships

[yurell]

The city ships should also crush the cities they're hovering above with their weight.

As for the Death Star, it was deliberately built to be spectacular, not efficient. If they wanted to depopulate a planet they could just Base Delta Zero the place to the ground with a couple of Star Destroyers.

[Izawwlgood]

Out of curiosity, what are you basing this size argument on? Because A ) The ship in Independence Day was big, but didn't seem to be 'planet gravity disrupting' size. Afterall, we see grounded ships at the end of the film, and they appear to be, I dunno, ballpark, maybe a mile long? Ish? And B ) The deathstar, while enormous, and the size of a moon, might not be as heavy as a moon. It's for one, not solid through and through, and almost certainly not composed of a core of heavy materials. I will grant that the death star is probably closer to 'planet gravity disruption' size, however.

[yurell]

The movie Independence Day states that the mothership "has a diameter of over 550 kilometres and a mass roughly 'one fourth' the size of our moon."

A quarter the mass of the moon should lead to some really nice destruction. If the smaller ships are even approaching that density, they'll be insanely massive. If you even input reasonable values for their mass based on their size you'll probably end up with planet crushing (the maths I saw for them assumed they were only ten times as heavy as air).

Edit: and the DS is impressively huge. Assuming that it's 10% structural material, and that has the average density of iron, it is about E18 kg, which isn't really that impressive (moon is E22), but if it moved to a low orbit (2000km above the surface) makes it 46 times closer to the Earth's centre than the moon (and nearly 200 times to its surface), so it would have roughly four times the moon's gravitational impact on the surface. Not really planet destroying, but it could screw up your weather, and shift a lot of water around, maybe even cause earthquakes.

[Izawwlgood]

Oh, wow. That's way bigger than they appeared in the film, I think?

[yurell]

550km is the mothership, the little city ships are 24 kilometres in diameter, although I'm not sure where that information came from.

Edit: btw, I just ran the maths — if we assume the mothership is a sphere (it isn't), it would have an average density of 211 tonnes / m³. If the city ships were the same density, it puts them at about E17 kg each, or about the mass of Prometheus

[Xanthir]

That's about 18 times the density of lead. Considering the mothership is *not* a sphere, but rather a disc (and thus has a substantially higher density), it seems like someone was exaggerating.

[Izawwlgood]

I could have sworn near the end of the film, when we saw a handful of them downed, they appeared at MOST a mile long.

[yurell]

An addendum to my previous post: the movie states that they're "over 15 miles" wide, so that's where the information came from.
As for looking smaller than they were ... maybe. They seem to change scale, and I do agree that they look smaller than 20km. Then again, looking at the one downed over Sydney I'd definitely argue more than a mile long, but definitely not an order of magnitude more. I guess it's a case of Sci-fi writers have no sense of scale.

[Tomlidich]

Sci-fi writers have no sense of scale.[/url]

just google searching images from the movie, i would say you are correct:


here we have the same class of ship, compared to buildings of differing sizes.

it is pretty clear from comparison to adjacent buildings, they didn't use any kind of scaling or logical sense in rendering them for cgi, and just went for cinematic effect.

[mfb]

The starship in Independence Day would have completely disrupted the entire planet's ecosystem.
Why, you may ask? Well, given how immense it was, and how close to the planet (after all you could see the dang thing hanging there in orbit), its very gravity would have been a destructive tidal force. Just being there would have caused immense damage.

Do you have any number for the distance ship<->earth? The size and mass are inconsistent anyway, but let's use the larger "1/4 of the moon mass": As long as its distance is half of the moon, it just gives the same influence. So it can alter the tides, but apart from that nothing serious happens.

>> It is similar to why the Death Star didn't just orbit Alderaan and screw with their ecosystem.
A big object so close to a planet would be subject to extreme tidal forces as well, probably ripping the ship apart unless it is made out of some unobtainium.

The city ships should also crush the cities they're hovering above with their weight.

Well, they have to have some way to hover without momentum exchange.

[yurell]

A big object so close to a planet would be subject to extreme tidal forces as well, probably ripping the ship apart unless it is made out of some unobtainium.

The difference in gravity across the station wouldn't have been nearly as great as the stress it absorbs when accelerating with its sublight engines — in fact, we see its larger successor orbiting Endor (or its moon, whatever the forest is called) without any apparent difficulty

As for the mothership, the size and mass aren't inconsistent, it's the city ships that are inconsistent. I don't know how close it was, but it should have just used its mass to wipe us out via gravity.

Well, they have to have some way to hover without momentum exchange.

I suppose it's as good a theory as any (since we saw them not crush the cities).

[gmalivuk]

A big object so close to a planet would be subject to extreme tidal forces as well, probably ripping the ship apart unless it is made out of some unobtainium.

Not necessarily. I suspect the Death Star is dense enough that it was orbiting well outside the Roche limit of itself and Endor.

[mfb]

Not necessarily. I suspect the Death Star is dense enough that it was orbiting well outside the Roche limit of itself and Endor.

Because it was "far away" from Endor. Well, I don't have numbers here.

@yurell:

The movie Independence Day states that the mothership "has a diameter of over 550 kilometres and a mass roughly 'one fourth' the size of our moon."

I think I can use 600km as upper limit for the diameter (even if 10000km are technically "over 550km" as well) and assume that the mothership is not larger in other directions.
That gives 160g/cm^3, consistent with your number of 211g/cm^3. Even without taking into account that the interior seems to have very large open areas and I would expect a ship design with a lot of accessible areas (with density << 1g/cm^3 as they breath air (iirc)), it is unlikely that any material reaches this density without extreme pressure. Pressure from what? And why? Is the open space to small to store everything with normal pressure values?

[yurell]

Because it was "far away" from Endor. Well, I don't have numbers here.

The DS can accelerate at c. 100g's with its sublight engines, so it should be able to survive Endor's gravitational field.

Pressure from what? And why? Is the open space to small to store everything with normal pressure values?

No idea, it seems obscenely and unnecessarily dense to me. Unfortunately, it's also an observation that we have to accept (just like we saw the DS destroy Alderaan, and so we have to accept the obscene power generation / energy storage / efficiency of the DS). Maybe they use some sort of exotic material that's responsible for that density? Or they could by hauling around a small black hole for power generation.

[Tomlidich]

Because it was "far away" from Endor. Well, I don't have numbers here.

The DS can accelerate at c. 100g's with its sublight engines, so it should be able to survive Endor's gravitational field.

Pressure from what? And why? Is the open space to small to store everything with normal pressure values?

No idea, it seems obscenely and unnecessarily dense to me. Unfortunately, it's also an observation that we have to accept (just like we saw the DS destroy Alderaan, and so we have to accept the obscene power generation / energy storage / efficiency of the DS). Maybe they use some sort of exotic material that's responsible for that density? Or they could by hauling around a small black hole for power generation.

i am not sure that destroying a planet would require as much energy to be unfeasible.

suppose it just had very powerful scanning devices and computer simulations.

it could then decide the most sensitive areas of the planet to strike, such as high stress fault lines, or areas where the crust is thinner, causing the planet to collapse in on itself. that eliminates alot of the work of breaking up the planet into smaller peices, which can then themselves be blown up, distributed, destroyed, etc.

[letterX]

Because it was "far away" from Endor. Well, I don't have numbers here.

The DS can accelerate at c. 100g's with its sublight engines, so it should be able to survive Endor's gravitational field.

Pressure from what? And why? Is the open space to small to store everything with normal pressure values?

No idea, it seems obscenely and unnecessarily dense to me. Unfortunately, it's also an observation that we have to accept (just like we saw the DS destroy Alderaan, and so we have to accept the obscene power generation / energy storage / efficiency of the DS). Maybe they use some sort of exotic material that's responsible for that density? Or they could by hauling around a small black hole for power generation.

i am not sure that destroying a planet would require as much energy to be unfeasible.

suppose it just had very powerful scanning devices and computer simulations.

it could then decide the most sensitive areas of the planet to strike, such as high stress fault lines, or areas where the crust is thinner, causing the planet to collapse in on itself. that eliminates alot of the work of breaking up the planet into smaller peices, which can then themselves be blown up, distributed, destroyed, etc.

Read this. It's possible you don't quite grasp the sheer mindboggling scale of actually destroying a planet.

[gmalivuk]

It wasn't just destroyed, though. It was completely blown apart. The gravitational binding energy of a body doesn't really depend at all on structural things like fault lines, and counts for the vast majority of the energy needed to completely destroy a planet.

[yurell]

i am not sure that destroying a planet would require as much energy to be unfeasible.

suppose it just had very powerful scanning devices and computer simulations.

it could then decide the most sensitive areas of the planet to strike, such as high stress fault lines, or areas where the crust is thinner, causing the planet to collapse in on itself. that eliminates alot of the work of breaking up the planet into smaller peices, which can then themselves be blown up, distributed, destroyed, etc.

Except we saw it being completely destroyed. Did you see how vast the chunks of crust that were blown outwards were, and how fast they were travelling? I don't think the gravitational binding energy would come close to the energy required to do that ... if you watch the clip, you can see the planet quickly expand to more than double its size, which means that it's being blasted apart far exceeding escape velocity.
I think the energy is somewhere between E32 and E38 joules, the former number being the GBE, the latter based on Mike Wong's analysis, which I'm inclined to trust to within order of magnitude (it agrees with my back-of-the-envelope calculations, anyway). Personally, I'm impressed that the DS managed it without melting.

Besides, with your 'break-up' idea, you still need at least the gravitational binding energy to send the smaller pieces flying away, and in the couple of seconds it took to go from planet to empty space certainly doesn't support the planet gently being torn apart.

[rath358]

According to the project RHO boom table, it would take 2.9e31 J to blow up the Earth and reduce it to gravel in the same orbit. Reducing it to gravel escaping the Sun's pull would take ten times that.
This roughly matches yurell's numbers.

[yurell]

Just FTR, my E32 figure comes from assuming the Earth is a perfect sphere of uniform density and using the formula (3/5)*(GM²/R) for binding energy. More detailed analysis can be found here where he takes into account the Earth is not uniform density, and it drops the value by about 30% to 1.7E32 J.

At any rate, the E31-E32 joules is the minimum required to destroy the planet, and the entire thing will drift apart at escape velocity, which is definitely not what we saw in the movie. So the Death Star has energy output at least comparable to the sun each day, and manages to use it without reducing itself to plasma. It is particularly impressive overkill, though, since the energy needed to melt the crust is orders of magnitude less, and that still leaves you with a planet at the end.

[Diadem]

Read this. It's possible you don't quite grasp the sheer mindboggling scale of actually destroying a planet.

I love that site. It's brilliantly written.

This is perhaps my favourite part, and nicely demonstrates the difficulty in blowing up a planet. Italics from the original:

There is evidence that in the past, asteroids have hit the Earth with the explosive yield of five billion Hiroshima bombs - and such evidence is difficult to find.