Why wasn't Earth destroyed in ID4?

[Wanderer]

After reading how the destruction of the Death Star killed the Ewoks by Dr. Saxton, I wondered why the Scientists weren't warning that to destroy the Mothership would wipe us out. After all it was 1/4 the Size of our moon, while the Death Star 2 was the size of Texas...

[Vehrec]

Hrumm, an excelent question. I do recall some of the novelization saying that pollution was leaking from pieces of some of the wrecks and doing massive global damage. Maybe some of that was parts of the mothership falling into the atmosphere. How one nuke was able to blow up a ship that big however. . . Yeah, I think this is one where they didn't do the numbers.

[Solauren]

Consider this:

Global environmental problems and possible extinctions vs

DEFINATE Rape, Strip-mining and extinction of the Planet.

They literally had no choice, so they didn't give a damn.

[Starglider]

The Death Star was at low altitude and supported by repulsors. The explosion appeared to be extremely violent. The ID4 mothership appeared to be in high orbit, and while it's been a long time since I've seen the film I don't think the explosion showed such high debris velocities. Assuming a relatively soft explosion, most of the debris would fly off into space, most of the rest would go into orbits of varying stability and only a small fraction would hit the earth.

That's the short answer. Maybe someone who has a copy of the film can do a proper analysis.

[CaptHawkeye]

Well, the mothership was mostly hollow.

[Drooling Iguana]

It's been a while since I've seen ID4 but wasn't the mothership out near the Moon's orbit? The Moon's gravitational pull could have sucked up a lot of the debris.

And if the mothership was in orbit when it exploded the debris would most likely just continue in that same orbit for quite a while.

[CaptHawkeye]

Well, now that I think about it, despite being hollow it still had A LOT of mass to it.

It's been a while since I've seen ID4 but wasn't the mothership out near the Moon's orbit? The Moon's gravitational pull could have sucked up a lot of the debris.

I don't think so. The mothership looked very close to Earth when it blew up. Also bear in mind that Goldblum and Smith weren't flying in space for very long before they got picked up.

[CaptainChewbacca]

The movie stated several times that the craft was near or around earth's moon, which means that even if the debris went towards earth it would have had a lot more time to spread out and disperse, meaning the bombardment would be less intense and the energy impact would be over a longer period of time.

[SAMAS]

On top of the Mothership, though, I remember a source stating that the mass of the City Destroyers alone should have fucked up the planet when they crashed.

[Patrick Degan]

On top of the Mothership, though, I remember a source stating that the mass of the City Destroyers alone should have fucked up the planet when they crashed.

Lawrence Krauss once pointed out in an article for SciFi Universe that the alien ships should have fucked the planet simply by orbiting it (braking the Earth's rotation) and passing through the atmosphere (with so much mass displacing that much atmosphere at even the "sedate" velocities they were moving at, the firestorms from massive wake friction as well as bow-shock effects should have swept the surface of everything in their paths).

[The Kernel]

On top of the Mothership, though, I remember a source stating that the mass of the City Destroyers alone should have fucked up the planet when they crashed.

Lawrence Krauss once pointed out in an article for SciFi Universe that the alien ships should have fucked the planet simply by orbiting it (braking the Earth's rotation) and passing through the atmosphere (with so much mass displacing that much atmosphere at even the "sedate" velocities they were moving at, the firestorms from massive wake friction as well as bow-shock effects should have swept the surface of everything in their paths).

He also addressed the ID4 doom scenarios in the beginning of his book Beyond Star Trek (they were quite numerous if memory serves).

[Starglider]

the alien ships should have fucked the planet simply by orbiting it (braking the Earth's rotation) and passing through the atmosphere (with so much mass displacing that much atmosphere at even the "sedate" velocities they were moving at, the firestorms from massive wake friction as well as bow-shock effects should have swept the surface of everything in their paths).

This is why Culture ships constantly displace thin layers of air from just in front of the craft to just behind it - well that and because they just love to show off by flying at hypersonic speeds in complete silence.

[Shroom Man 777]

Aside from the shields, the alien materials weren't all too impenetrable. Maybe the nuke blew up some fuel source and the Mothership got blown up to bite-sized pieces that easily disintegrated in the Earth's atmosphere? Disintegrating and forming some kind of black cloud that would block a portion of the sun's heat from affecting the planet, thus defeating Global Warming!

[tim31]

Yes... ala nuclear winter.

[NecronLord]

It was. We see the stuff entering the atmosphere, and the hero saying 'didn't I promise you fireworks'

It judiciously cuts to credits right before they all get immolated.

[Admiral Valdemar]

The craft obviousloy had anti-gravity engines anyway given you got 15 mile diameter city destroyers floating ominously. Patrick Moore in Independend Day UK pointed out that such a massive mother ship would have started affecting Earth in a Bad Way™ had it not been raping the laws of physics as we know it with some mass negating field etc.

There was something like near 200 City Destroyers, so given their size and estimating average density, we could ascertain their mass without any funky technology altering it somehow.

[RogueIce]

I have a related question: we saw the US try to nuke them, but they gave up after the first strike didn't work. Do we know if the other countries tried it? Did they also give up or keep trying? How much effect would that have (perhaps not on a global scale, but considering they were all over major population centers, rebuilding them probably won't be as attractive).

[Teleros]

Well the nukes used didn't seem to be that big to be honest, and if it struck the shields much of the city wouldn't be too adversely affected. I doubt it'd be much worse than Hiroshima or Nagasaki. It's when you throw a multi-megaton ICBM at it that you get a real mess.

[Sriad]

Easy math post, gotta keep the kinematics sharpened:

If the city ships are 24 km across and an average of 2 km thick (eyeballed, http://www.merzo.net/100mpp.htm has them at ~3.5 at thickest) their volume will be 9.05e11 cubic meters. If they're 5% metal as dense as steel (7.8g/cm^3, or 7800kg/m^3) and 95% empty (ID4 aliens like headroom, going for low-end figures anyway) we get 3.53e14 kg per ship.

Dropping that much mass let's say... 3000m yields 1.038e18 joules.

At 4.18e15 joules to the megaton, we get 248 megatons per falling ship. If you think 5% solid is way to low, just multiply linearly. In any case, that's enough for a quite brutal nuclear winter, but not in itself enough to assure mankind's final extinction. Though if whatever fuel stores and munitions they have aboard go up...

I don't know about the mothership fragments... Does anyone remember how long it took the Plucky Hackers to get back to Earth? IIRC the lightshow started about the same time, so we could get the velocity of the explosion's leading edge from that.

[Winston Blake]

At 4.18e15 joules to the megaton, we get 248 megatons per falling ship. If you think 5% solid is way to low, just multiply linearly. In any case, that's enough for a quite brutal nuclear winter, but not in itself enough to assure mankind's final extinction.

I don't know, I can't help but think that all that energy isn't going to be converted into heat radiation like in a nuclear explosion. When a ship hits the ground, both are going to deform a hell of a lot. So a fraction of that heat that would go into the air to make a blastwave/etc should get taken by work heating.

As a rough estimate, I'll use your mass number above and assume the ships are made of iron (specific heat capacity 0.45 J/g/K). Then all that 248 megatons could 'disappear' into increasing the temperature of the whole ship by just 6.5ºC (or equivalently, melting/softening a small fraction of it).

(248*4.18e15J) / ((0.45 J*g^-1*K^-1)* 3.53e14kg) = 6.53 kelvin

Unless I'm doing it wrong, it'd take about 57 gigatons just to melt a single ship, even if it only took heat as well as iron.

Though if whatever fuel stores and munitions they have aboard go up...

Given how easily the mothership blew up, that would seem likely. However, IIRC we see some of the defeated City Destroyers and they're just crashed on the ground, smoking, while people cheer. I can't really remember it that well, but it seems like there was no megaton-level explosion.

[Oni Koneko Damien]

The movie stated several times that the craft was near or around earth's moon, which means that even if the debris went towards earth it would have had a lot more time to spread out and disperse, meaning the bombardment would be less intense and the energy impact would be over a longer period of time.

IIRC, it's stated both in the movie and in the novelization that attempts to nuke the mothership in orbit would fail because it was hiding behind the moon...which means at the very least it was beyond the moon's orbit.

[Lost Soal]

It couldn't have been hiding behind the moon, at least not completely, otherwise the Virus couldn't have been downloaded onto the rest of the ships.

[Shroom Man 777]

Why? Can't the aliens have some kind of future radio that can bypass the moon?

[Teleros]

However, IIRC we see some of the defeated City Destroyers and they're just crashed on the ground, smoking, while people cheer. I can't really remember it that well, but it seems like there was no megaton-level explosion.

Yep, just lots of crashed starships all over the world.

It couldn't have been hiding behind the moon, at least not completely, otherwise the Virus couldn't have been downloaded onto the rest of the ships.

Not just that, but if it had the moon completely between itself and the Earth then I doubt we'd see much of it burning up in the atmosphere at all...

[Stark]

Why? Can't the aliens have some kind of future radio that can bypass the moon?

Then they wouldn't have to use our satellites to broadcast the countdown, and Our Heroes would all have been killed.