If I were the secretary of the UN, and a corporation whose fundings exceeds that of several nations, and that employs people such as Lovecraft (Inventor of the Dark dreamer project, which became the avatar project) wanted to design a spaceship with weapon capabilities and equip it with nukes, and send that to an alien planet, of which the only intelligent live lies on stone age levels, I'd sure as hell withdraw that mining license* and dismantle the company if I could.
Though are you factoring in that if the ships like the one seen in the movie exist, and even smaller more mundane ones exist that these, even without and nuke can cause a catastrophe on earth already? It seems to me that if you were to remove their mining license on this charge you'd run up against the precedent already set by having start ships zip around space at fractions of the speed of light. If mining for this material through out space is already wide spread, nuclear reactors are going to be the main stay of these mining operations.
And besides, if nukes do hold that much power in that world, you're probably not going to anger any organization that has them. The lead up to acquiring nuclear weapons might be fraught with embargoes and sanctions, but once you have them and have demonstrated their existence, people tend to leave you alone unless you have a reputation as an asshole.
There' a difference between ships armed with active weaponry and shuttles that could be used to crash on Earth. Besides, as evidenced by the post above, the RDA was required to sign a contract that they would not use/own any WMD or participate in the militarization of space. This implies they had no WMD's at the moment of signing, and weren't allowed to acquire any later on. Mining for unobtanium is not widespread. There are 12 ISV's, and Pandora is the only confirmed source. All other space travel is implied to be seriously subluminal, considering the giant expenses required in maintaining faster then light speed. The other mines (On the moon and Mars), mine He-3 and some other resources.
The first mission to Pandora was done at great expense, using a 4 km ship. Considering the enormous energy costs involved, there would be no space for redundant features such as weaponry. After that they knew about the Na'vi, and using nukes and orbital weaponry against stone age people seems a bit redundant, doesn't it. Especially when you are having serious PR problems already, and your ship only has a 350 metric tonnes of cargo capacity. (Which isn't a lot on astronomical scales, believe me)
*The RDA was only permitted the right to mine the planet after PETA screwed up their campaign. Turns out using a human with implants as Na' Vi was a bad idea.
Last but not Least:
THERE IS NO FTL TRAVEL IN AVATAR (0.7 c is fast, but not FTL)
This is a bit worse so to speak. There might be some weird physics cheat in an FTL drive that might lessen the impact, but a straight on torch ship has problems that ships with reactionless drives have. Namely that it goes so fast that if it were to collide with earth it would probably crack the crust and kill everything on it. A nuke, and especially a nuke with the power of WWII bombs, aren't going to be on the list of things you need to worry about. Their ship, if it moves this fast, can effectively hold the earth hostage.
They're already beyond the dangers that nukes can pose after they built that ship, the ship effectively doubles as a planet cracker. Shoving a few Hiroshima sized bombs onto it doesn't seem to be that much of an escalation of danger this ship poses to all human life on earth.
As for weight, with their level of technology, a fusion or even a fission bomb can be made quite small I imagine, especially if it's yield is on the civilian level instead of whatever monsters they have in their era. As for their use, they're great for burning down forests, so long as your air burst is high enough off the ground. A sprinkling of a few Little Mans here and there and you're basically can drastically cut back on the logging equipment.
1. All RDA officials live on Earth. Sure they could hold each mutually hostage, but it would accomplish much
2. Also, using the ship as a weapon would be rather hard.
-The ship requires 6 months to arcelerate
-At those speeds, even a dust particle would blow up the entire ship. It has shields to avoid this, but since the defense systems relies on the first 3 shields being destroyed in the collision, just sending 2 pebble sized objects can destroy it.
-An integral part of the ships arceleration is a giant laser placed somewhere in the solar system. I'm pretty sure this one would be under governement control.
3. They wouldn't have got permission to build the ship without signing the contract. I'm pretty sure they made sure that the RDA wouldn't cheat on them.
4. Fusion only happens at high temperatures/high densities. In order to have a fusion bomb you'd need to use a fission bomb to set it off. As for using them to burn forests, why? The RDA doesn't want to destroy the forests, nor anger the natives, the general public or the United Nations if they don't need too. Over it's thirty year colonization, the RDA has cut maybe just enough forest to justify 3 warheads. (The mine, Hell's gate and Hometree). Nevertheless, they would still need to invest in logging equipment, if only to control the Jungle when it grows back. Blowing it up works the first time, but after that your own material gets in the way.
The only star they can reach is alpha Centauri, and that only at great expense. All power generation is done by fusion (excepts for the ships main drive, which is an antimatter/matter reactor). Unlike fission, fusion can not be brought to explode. Besides, since fission is barely used anymore, the amount of nuclear weapons would most likely be less, not more. Antimatter/matter reactors being as dangerous as they are, I expect them to be strongly guarded. The deuterium and he 3 required for fusionpower on Pandora are gathered from the gas giant around which the ship flies, using retired shuttles.
Fission doesn't explode either :/. It just generates an intense amount of heat in a very short time given a very specific situation. The explosion is from whatever material around the bomb superheating into plasma in an instant and expanding at an indredible rate due to how much heat is released in such a small time frame. A fusion bomb is quite possible. But having read your comment, nukes are once again the least of your worries. The fact that their engine uses antimatter is scary thought given how much power you have concentrated there, and how volatile it is. And so, if you trust a company with something this scary, you can probably trust them with some big bombs when they ask for some.
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Given the danger of anti matter weaponry, it would make sense for the governements of Earth to enforce strict rules and such. Maybe all higher RDA officials are forced to remain on Earth. Maybe there are failsafes build in in the engines. The particle arcelerators that produce the antimatter are most likely heavily guarded and governement controlled too. In the end however, why would the RDA ask for the bombs:
Nuclear weapons
Positive
-Neglible civilian use
-Should the natives somehow revolt and manage to beat our troops using stone age tech, we can nuke them
Negative
-Requires breaking a contract with the effects that all RDA's space bound operations would be cancelled
-Will make the natives revolt, immediatly
-PR death
Really, the costs completly outweight possible benefits
Yeah, so they could just have kinetic killed the tree. Or dug under it.
No time, No capable weaponry, no reason.
Now I'm curious if the gas giant Pandora was orbiting could've survived their ship hitting it head on at .7c.
Gimme a moment. This might be hard to calculate, since the ISV's are technically impossible.Well, the effects would be similair to one fourth (0.49/2) of the ships weight in matter/antimatter detonating.