Protection of the Earth needs to take several forms - protection from internal dangers and external dangers.

The external danger that most of us worry about - if we worry about anything - is the threat of a meteor or comet hitting the Earth and causing an Extinction Level Event. The reason we perhaps worry about this so much is because it's been popularised by Holywood in such films as Deep Impact and Armageddon. How likely is an extinction level event? There's strong evidence to suggest it's happened before, and it will certainly happen in the future. The only question is when, and that's something that's very hard to predict. Astronomers are apparently doing well at tracking large objects in the solar system, but just the other week a meteor that could have destroyed a small city passed by Earth at a near miss, and the object hadn't been detected until it was really too late to do anything about it.

The internal dangers that most of us worry about are pollution and Global Warming. Some people may also cite the hole in the ozone layer, but that'll be gone in fifteen years anyway.

Is Global Warming a threat? It certainly looks like it, although we must also remember to take into account standard climate variation that the Earth undergoes anyway - the planet's been put through a lot worse than our hypothetical global warming scenario is/will putting/put it through without our help!

Another danger we think about a lot is the threat of nuclear war. Unfortunatly, this threat is totally in the hands of human beings. If I could think of anyone to give a nuclear arsenal, it wouldn't be human beings. Thankfully, political tension seems to be decreasing rapidly now, and with disarment in process this threat should dwindle. The possibility of us destroying ourselves in some other elaborate way will remain the same, of course.