This is what I was thinking. Lots of “salted” nuclear weapons, buried liberally across the planet. Perhaps with different yields available so we can have options between “large ‘dirty’ explosion at landing site” and “500 megaton cobalt bomb destroys hemisphere.” If we make it clear enough that we’re able and willing to destroy the planet rather than let it be taken, the aliens might call off their invasion.
Skip on the pandas: they have about the worst sex life of any species on Earth and their reaction to it is “meh, has anybody seen my bamboo shots?” Better focus on the actual bearie bears.
I’d bet that a race capable of managing the power requirements of interstellar travel would have little trouble inducing Sol to go nova. Or at least kick out some truly devastating solar flares.
A little gravitational lensing and we’re sitting in the crosshairs of the solar system’s largest x-ray laser.
In reality we most likely wouldn’t have a chance at all due to the sheer magnitude of difference in technology level. I recently read ‘Forge of God’ by Greg Bear and in that book the aliens:
Drop some exotic material in an matter/antimatter pair that penetrate the Earths crust and interior like they were water, as well as running some psychological operations, Earth is destroyed and humanity only survives at all because of the help of some benign aliens, the sequel ‘Anvil of Stars’ is pretty good as well
I do think though we give ourselves too little credit in the whole warfighting stakes, humanity really does have some snazzy death-dealing technologies out there which any alien species on a somewhat equivalent technology level may not have.
I once read an interesting short story with this premise where humanity managed to fight back because the aliens had never invented stealth technology so they had a blind-spot where that was concerned.
The weakness is discovered after F-117 stealth planes are the only aircraft able to get into firing range to attack the alien landing craft, note they were still destroyed but they managed to get within attacking range, the powers that be send a badly-injured stealth pilot who survived ejection into orbit to attack the mothership in a hastily constructed stealth-spacecraft with multimegaton warhead…while it could have been fully automated they use a human pilot just in case there are any unforeseen eventualities. Which there are as the mothership detects his craft earlier than expected and so he triggers the warhead manually which does the job.
Can’t remember the title or author but it was a good-story, an American pilot flying European technology with a Russian warhead and carrying into battle a rose given to him by a Japanese pilot who flew an aborted Kamikaze mission in the second-world war. 
We could only hope that in an alien invasion the aliens aren’t completely out of our technological range and are fighting for limited objectives, ie: they don’t actually want to wipe us out.
Good luck with that.
If they don’t when they get here, watching commercial TV will change their minds.
Perhaps they will think Star Trek TOS are “The Histories” and run like hell.
Now there’s a thought.
We start running Independence Day and Mars Attacks on the History Channel with descriptions that they’re documentaries.
Safe assumptions:
- Their sole intention is not to annihilate us. The warning stated it would be an “invasion” which implies that the aliens are arriving with the intention of occupying or acquiring something. (Besides, what would be their motivation to simply destroy us? We don’t even know where they are. aren’t capable of mounting an interstellar attack anyway, etc. so we are obviously not a threat to them.
We can reasonably assume they want something that:
a) is not commonly found on other celestial bodies. Otherwise they likely get whatever they wanted somewhere else- and not bother us and;
b) they believe may/will be necessary to take by force. They would not arrive with an invasion force otherwise. They want something that they believe we are not likely to give up freely. Something we use or something we value.
We can reasonably assume that:
- They wish to inhabit this location because Earth is an uncommon, life-supporting planet.
- They wish to take resources uncommonly found on other celestial bodies that we ourselves use (or value.)- These could be natural resources or us (enslavement, harvesting, whatever.)
Poison Pill option.
In order for it to reliably work, we would have to have prepared a means to trigger a process that, once triggered, cannot be stopped by the aliens) and that will:
a. make Earth permanently or semi-permanently incapable of supporting life and;
b. make Earth permanently or semi-permanently devoid of uncommonly found resources that we value.
Problems with Poison-Pill as solution:
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We are nowhere close to having the means of accomplishing either one, much less both. We’d almost have to be able to literally destroy the entire Earth at the push of a button. It’s not feasible. It would be next to impossible.
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Even if it were, the option relies on the Aliens deciding not to call our bluff. If they do, we cease to exist. So it’s not fool proof there, either.
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If we had the ability to essentially destroy the earth instantly, it would better be applied to destroying the aliens. Which, incidentally, would be far more feasible.
Self-Defense -
Personally, I’m not convinced that their ability to launch an interstellar invasion necessarily means they are far more advanced than us. The aliens’ attack will begin in 30 years. That could be how long it takes them to get here from there. Wherever there is. Our scientists may be able to eliminate their home planet from existing within a certain radius from Earth so that we could safely assume they are at least X distance away. Depending on where X is, 30 years may not even be that fast. We have the technology- in theory- to send life forms through space, too. They are obviously at the point that they can send an attack force, but that doesn’t mean that their technology is fundamentally better.
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Also, even if they have fundamentally better technology re: traveling through space doesn’t mean that they have superior weaponry. It’s quite possible that we’ve invented stuff they didn’t. They likely had different routes of discovery than we did. They may not have ever developed nuclear power on their plant, or may not have even have the plutonium (or whatever) on their planet to do so. Perhaps that’s why they’re coming to ours.
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The may not even know what they’re getting into. We can reasonably conclude that they have some information about Earth. Since they had to have based their decision to invade our planet off of something. But we don’t know how much they know. We can conclude that it’s likely that they think they’ll have the advantage. Or have no idea, but are so desperate for resources/living space that they’re willing to take a risk on losing their invasion force.
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They may not know that we know they are coming. They may have assumed a certain level of readiness on our part but, after 30 years anticipation, we will be many times more ready than they thought. (Meanwhile, their fleet (if it takes 30 years to get here) may be stuck with the technology they had 30 years ago by the time it gets here. Since, even if they developed the technology on their home planet(s), they may not have the resources in space to implement it.
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It’s also our world. We evolved here. It may not be as hospitable for them as it is us. Perhaps our ambient temperatures are too much (or too little) for them. They may not be used to operating in our conditions - or their weapons may not be equipped to work well within them. Will their vehicles be attuned to be used in our gravity situation? Etc. Their technology may be based on what works well in their world.
Whatever the case, I’m not at all convinced we can’t take these aliens. There is a good chance that when they get a load of our defenses, they’ll hop back in their fast little spaceships and hightail it with their tails (perhaps literally) between their legs. Hard to say. But that needs to be the focus.
(Granted, we should prepare poison pills for the resources (such as human beings) that we feasibly (within 30 years) can, on the chance that those are the ones they want and we can use it as a bargaining chip. )
Self-Defense:
But the world should focus virtually all of it’s human resources on the scientific pursuit of weapons- and the training of all members of our population to use them effectively. Maybe we’ll get a breakthrough that allows some weaponry that we can’t predict.
But we should assume, worst case scenario, that our most powerful weapons will still be based on nuclear technology. We should focus on developing them in a way that allows a more localized effect. (Since the way that they are generally used now are large AOE’s that are at least as likely to kill us as kill them.
The primary focus on weapon development/production should be on controlling the skies.
Why not focus our defense on taking them out in space? We can- to an extent. But that is where they will be at their strongest- and us at our weakest. They are used to traveling in space- and have probably developed a number of things we may not be able to anticipate re: battling up there.
(Still it wouldn’t hurt to throw some of the space debris if it can be done with minimal cost/effort. Incidentally, we have to assume our satellites cannot be saved. Which means we need to readopt the way our communication systems, etc. work.)
Why focus on Air Superiority:
- For the same reason that it’s so important in human vs. human conventional warfare.
- The aliens may be birds and we’ll need to be able to shoot them down.
- It’s possible that their primary method of attack is to parachute or fly to the ground and then attack. But, at the very least, they will have to use some flying mechanism/vehicle to get them safely to the ground. We should focus on developing a system to blast as many of [whatever they are] out of the sky as we can.
- The battle for control of the air is where they will likely be at their weakest and where we will likely be at our strongest. Will whatever technology they are using be attuned to our gravity, winds, barometric pressure (or whatever- insert science stuff), whatever? Possibly not. They’ve never flown here before. They may make very easy targets.
- Or perhaps their technology works well regardless of the conditions. Perhaps they will use the air to mount their attack. (For all of the reasons we consider air power to be a very important element to mount attacks.) All the more reason to focus on air defense.
That’s some of my thinking on the subject.
Or… or… we could tell them that they need a permit to invade the earth and hand them over to a bureaucratic hell.
A lot of annoying typos up above. Tried to fix them but my 5 minutes elapsed. OK, I answered “What can Earth do to prepare?”
**“What happens?” **- I think there is a reasonable chance that we win for some of the reasons I mentioned. They are not gods. They have limitations. After all, they aren’t so advanced they don’t need something Earth has. The laws of the universe will presumably apply to them as much as they do us. If it bleeds, we can kill it.
And, if that 30 years is the amount of time it takes their invasion force to travel here, we may have 30 years worth of intense research and development focused on what counts (weapons to defend Earth to) to catch up to/or exceed their battle technology (since they may be more or less stuck with the 30-year old technology they originally sent.) 30 years of intense, focused preparation can make a big difference.
But most importantly, our biology and weapons were evolved to work best not in space or conditions to which the aliens may be accustomed. How well would we/our technology work on a planet where we don’t have the benefit of our satellites, cell phone towers, radar dishes and other land-based structures to help? Or on a planet we’ve never actually been to- with a different gravitational pull, different mixture of air, two suns, three moons, whatever.
It may be like in the hundreds of years ago an invading force from a tropical island invading a land-locked nation living in a frozen tundra because they ran out of room to live- or they want the trees for wood. The land-locked nation never developed boats to travel long distances. The islanders, surrounded by water, did. The land-locked nation, surrounded by other nations, developed much more sophisticated weaponry. Or had to learn to hunt with bows and arrows for food, while the islanders only developed fishing poles.
The islanders would arrive in their fancy, technologically-advanced boats they used to traverse the ocean. But they’d be wearing loin clothes and carrying clubs- no skies, no snow shoes, no ability to hunt animals to live off the land, no ability to stay warm. It would be no contest. And if the land-locked tundra country had a year to prepare for the invasion, the islanders would simply be slaughtered.
Which brings up one other thing. The aliens, unable to be in our environment for long and/or not knowing how to meet their biological needs on Earth, may be forced to return to an orbiting “mother ship” to sustain life/recharge. So we need to be able to launch nukes into space at their “mother ships”. Then we just wait until they all die.
I think there’s a good chance that, when the aliens come, Earth wipes them out or the aliens simply can’t survive. Very hard to say what would happen. But if I had to guess. Earth wins - Aliens lose.
"What would be the best outcome for the people of Earth?"
We annihilate their invasion force, reverse-engineering as much technology as we can and learning from any of their survivors (once we figure out how to communicate with them). We also learn where their home planet(s) are.
We prepare an invasion force of our own and wipe them out, enslave them, steal their technology, living space, resources. Then find other alien planets to conquer. Or, we learn to peacefully coexist, sharing resources, technology, recipes, and laughs.
I know, its unlikely, but it beats the thought of us impotently shaking our collective fist at the sky while said alien bastards drop rocks on us!
Yet another good reason to pour funds into space exploration…
And the Aliens movie, after all they have sharp sticks, although we’d better leave out the part where the marines get their ass kicked…
Hell just point Sigourney ‘Ripley’ Weaver at them and they’d surrender to us. 
That would make them put mud guards on the bottom of their space ship.
Gotta be careful about this…
I doubt we could even hide from them.
If the invaders are even a mere 100,000 years of human-comparable development ahead of us (a mere blink of an eye compared to how long, say, life has been on this planet), they’ll essentially be godlike beings from our point of view.
So the best preparation might be just to get ready for the reality that our fates will be in someone else’s hands from now on. I know this is defeatist, but really this is a far more one-sided situation that we can even conceive of.
OTOH in the unlikely case that their development is close enough to ours to get an Independence Day scenario play out (with technology that we can relate to like visible ships and lasers) we’re still screwed. The ID tactics sucked, all they needed to do was drop a few asteroids and then wait to mop up what few humans remained.
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The faithful will be rewarded by feeding our young.
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