They wouldn’t be up there if they didn’t have a high velocity to start with. They’re pieces of orbiting junk.
Was that the same documentary said that an army with inter-planatary travel can be defeated by angry teddy bears?
I say we spend 30 yeard breeding disgruntled pandas, grizzlies, and polar bears.
Jaaz. Think about it. These pieces are orbiting the earth once an hour at most. Orbital speeds are not ‘fast’ compared to the diameter of the planet. They are ‘ambling’. Just watch a satellite go over. To stop landing craft getting through you have to have time gaps between the pieces shorter than it would take a craft to go through this ‘shell’. Re-entry speeds are 15k mph plus. Therefore you either have to have a continuous stream of ‘junk’ with practically no gaps or lesser numbers of junk pieces orbiting much, much faster than orbital velocity.
And they have to leave no gaps over a sphere the size of the Earth + their orbital height.
For your scheme to work with pieces orbiting at orbital speeds it would have to blanket the entire sky with pieces orbiting practically nose to tail.
Blocking the sun.
Wovoka, is that you?
I don’t think so. We spend a lot of time tracking the space junk already up there, and there have been near misses already. It would take much less than blanketing the sky to make re-entry impractical without heavy shielding. But I’m not interested in this approach any more. I think the big blue space balloon will work. Just have to figure out what we do once they start chasing the wrong target. I’m think a big wooden space creature of some kind would help.
Tie it to some rockets, and the aliens will chase it when we light the fuse. 
Why wouldn’t these magical alien defenses, which apparently can destroy incoming asteroids, missiles with nuclear warheads, and “all forms of orbital bombardment”, work against invading ships? What kind of dumbass alien allies do we have?
Our only chance is to take these defenses over and make them work in a not so dumbass fashion.
- Throw water on them
- Say their name a few times (but not into a mirror)
- Sneeze on them
- Hug a kid in their presence and make them feel something in the sub-cockles of whatever heart they may have
- Call for a doctor
- Nuke them from orbit. It’s the only way to be sure
- Build large walk-in freezers to toss them into
- Put on some Slim Whitman yodeling records
- Transport yourself back to their planet and cause a nuclear holocaust
- Lull them into complacency with Reeces Pieces
- Call in the New Hotness
- Destroy the massive broadcasting towers with hypnotic signals that disguise the aliens as humans.
- Train a group of 6 year olds in battle techniques to lead a counter invasion on the aliens’ home planet
- Just accept your defeat gracefully.
No. 8 should wipe them out to a man…er, tentacle, but there would be lots of friendly fire.
What about landing biological weapons? Or would that be counted under orbital bombardment too? A highly contagious, hard to detect, slow to kill disease specifically engineered to kill humans would be very hard to deal with. Hiding in bunkers would probably help, but you’re not going to be able to get 7 billion people to safety. Any such plan would probably involve sacrificing at least 90-99% of the population.
Guerilla warfare of some kind would probably be our best bet. Even with plenty of advance notice preserving civilization as we know it would probably be impossible. The best defence would be to not be seen, so they’d never come here, but that’s already impossible in the premise.
Oh, hey… that worked in The Matrix, right? Sort of?
Seriously ? With no orbital bombardment it means they have to land (either ships or troops). Despite the movies, I find it seriously hard to believe anybody is totally immune to a 100 Megaton H-bomb (like tsar bomba). So build as many of those as we can, and blow them every time the enemy makes a landing. So we just have to blow ourselves to smithereens and we get to keep the rubble :).
Dumbass long-extinct alien allies. They are dead for a reason
System Defense Boats. LOTS of 'em. Also monitors.
Best of all, a forward defense. Build huge space fleets and go out and fortify the nearest stars. And likely jump nodes in empty space. In fact, build up a huge earth-concentric Imperium, (call it the Ramshackle Empire) so they can’t approach us at all.
That always works…
If you talk about a stellar empire, we need to get the Vulcans and Klingons on our side.
30 years? Our best defense is going to be a poison pill. Be prepared to destroy ourselves and whatever they are invading us to acquire. And of course, make sure they know someone’s finger is on the dead man switch that will trigger it.
If their goal is our annihilation then we’re screwed. All they’ll need to do is scoop a few dozen XXL rocks out of the asteroid belt and lob them in our direction.
Why didn’t I think of this before? We can just go hide on Earth 2. The aliens will never know it’s there because it’s on the opposite side of the sun. What a bunch of maroons.
The Vulcans will join us (reluctantly, and with much criticism) because it’s logical. We can get the Klingons by promising glory.
(“When a Klingon speaks of ‘honor,’ substitute the word ‘renown.’”)
Or we can all scatter back in time to previous eras of history and blend in there.
I fear that may the best we could do too. The likely difficulty of interstelar travel would make sure anyone who could bridge the gap would be leagues ahead of us in technology.
The alien defence sats in the OP would prevent astroid strikes and more, but they’d do nothing against the invaders on the ground.
Didn’t Bean game out this scenario in Ender’s Shadow and decide that it can’t be done without taking the fight to the aliens first?