atmosphere
Has anyone watch "Live From Space" on Channel Four recently? Or the film "Gravity"?
The over-riding impression gained from these features is how fragile and guarded our atmosphere is from the ravages of space. The air we breath is, roughly, only available up to 12km above sea level before we start running into problems.
I've expended much energy trying to convince people that our petty political borders and economic barriers are not visible from space. It would be interesting to get Nigel Farage or that dickhead from the BNP, Griffin, into a spacecraft and ask them where things start and stop.
The more pressing problem is our biosphere may protect us from the vacuum of space, but it also keeps in all our pollution and crap. It'd be interesting to measure how much rubbish we actually breath in now. We're all hypocrites. Some of us like to expound how we need to keep greenhouse gasses to a minimum, yet get in our cars, or run every possible electric appliance in our homes.
There's a fine line between the Earth's natural balance and the garbage the human pumps into its delicate atmosphere. Stuff an alien invasion destroying the planet, we're doing a damned good job of it ourselves.