For most of recorded history, Earth has looked like a nice, comfy place to raise a human race. Not too cold (mostly), not too hot (mostly), it was just what Goldilocks ordered. And barring the wrath of angry gods*, it didn’t look like anything could ever destroy this big blue marble – or, more importantly, us.

We know better now. The Universe is a pretty scary place, and our tiny mudball of a planet is constantly on the verge of a nasty encounter with something from the deep, cold darkness out there.

This is how the universe is planning to kill you. Be afraid. Be very afraid.


Asteroid Impact: Good enough for the dinosaurs…
There are millions, maybe billions of rocky asteroids zipping around our solar system, and occasionally one reaches Earth. Small ones burn up in our atmosphere, forming shooting stars. But once in a while a larger one makes it to the surface, touching off a mini-explosion. And sometimes really big ones hit our planet – ask the dinosaurs. And if a big one were to hit us now…

How it goes down:
A suburb-sized rock comes screaming in from space and smashes into Earth. If it crashes on land, the shockwave wipes out anything within a few hundred kilometres and demolishes a few thousand kilometres more, raises dust and debris that blocks out the sun, killing plant life and disrupting food webs. If it meets a watery end (more likely) it causes massive tidal waves that demolish our coasts for hundreds of kilometres inland, causes severe earthquakes and ends most oceanic life. Either way, we’re not happy.

What are the chances?
Pretty good in the long run (there's a massive meteor impact every couple million years or so, though mass extinction types are a hundred or so times as rare), but only about 1 in 700 000 for your lifetime.

What can we do?
There are actually several plans for dealing with attacking asteroids, besides sending Bruce Willis up there to Die Hard them to destruction. Rockets could push them off course, nukes could blow them, or directed focused solar energy could change their momentum. But if we only start dealing with it when it’s a million miles away or so, anything we try will probably just cause more problems.
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Gamma-Ray Bursters: Meet the Death Star
Incredibly high-energy Gamma Ray Bursts have occurred in other galaxies, lasting for a few milliseconds to several minutes, followed by a longer, less energetic “afterglow” (ultra-violet, x-ray, etc.). Most of these bursts appear to be caused by hypernovas – exploding stars thousands of times bigger than our sun, which collapse to form black holes with two rays of radiation projecting out of them, like massive laser beams. If one occurred in our own galaxy, and the earth was hit by a ray…

How it goes down:
You’re playing a gruelling game of touch rugby (or American football, or ice hockey, or whatever you do to pass the moments of your meaningless existence) with some mates. Suddenly you feel a swift, searing pain that lasts for a fraction of a second. And then it’s all over. The ozone layer has been blasted away, Earth’s crust has been seared to a crisp and set on fire, and everybody on this side of the planet is dead. The rest of the world has to cope with lots of UV radiation, reduced atmosphere, X-rays, and just generally a very horrible, painful and brief existence.

What are the chances?
We just don’t know - they're hard to detect, and the ones that are easy to detect are kinda dangerous. The best estimate is around 1 in 15 000 000 or so for any particular lifetime (assuming threescore and ten, I mean).

What can we do?
Not a damn thing. But we won’t even know about it until it hits, so there’s no point worrying about it. Just live every minute like it could be your last. Dance like it hurts. Work when people are watching. Love like you need the money.
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The Death of the Sun
Stars are big nuclear fusion reactors, and our sun is no exception, driven by Einstein’s famous equation, E = mc^2. Roughly speaking, this equation says that matter and energy are interconvertible: when the helium in the sun’s core fuses into hydrogen, it loses some mass as energy (and lights up our solar system). In a few billion years, the hydrogen will run out, and the sun will start fusing hydrogen in to carbon and nitrogen. This will make the core hotter and cause the sun to expand into a red giant, over 200 times its current size. This could cause a few problems.

How it goes down:
When one day Sol goes all red giant on us, there are two possibilities: either it expands to consume the Earth (in which case we all die, obviously), or it consumes Mercury and Venus and stops short of us. If that happens, massive amounts of solar radiation strip away the Earth’s atmosphere and fry the planet’s surface, boiling away the oceans and crisping all life.

What are the chances?
Inevitable. But it’s about five billion years away, so the chance of you experiencing it if you’re reading these words is basically zero (and if you are reading it while the sun is going kablooie, you really need to re-examine your priorities - but hey, thanks for the endorsement).

What can we do?
Nothing much, except not be on Earth when it happens. But if our descendants haven’t left Earth in a few billion years’ time, they can’t exactly say they weren’t warned.
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Alien Invasion: Bugs from outer space
Scientists disagree about the probability of life on other planets, but most think it’s pretty likely. Organic molecules synthesize fairly easily, and we’ve found them in comets and meteors. And life evolved on Earth nearly as soon as it was cool enough. But if life is out there, it’s probably in the form of bacteria-like little cells, or viruses. And they’ll be wanting something to infect when they get here…

How it goes down:
A buggy meteor touches down in your backyard. Two days later, you’re feeling sick, and you start vomiting. At the hospital, doctors are baffled and treatments are ineffectual. A few more cases of this mysterious illness are reported over the next few days, and then suddenly it’s everywhere. Millions of people are dying in Moscow, London, Egypt, and nobody knows what to do about it.

What are the chances?
The Drake equation lists several probabilities that help us tell whether or not there’s life out there. Unfortunately, we don’t know the actual numbers involved – like how many planets there are out there, or how long advanced civilizations usually last - or even how to define “alive” (the Drake equation is also incredibly limited, essentially looking for aliens that are exactly like us in almost every way, and us at a very particular moment in history at that). And we have no idea how extraterrestrial bugs could affect us.

What can we do?
There’s no way of telling whether a meteor has a little bug in it before it actually gets here. But once it’s here, we can isolate the carriers and bring the full force of modern medicine to bear on it. Sadly, that might not be enough.
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In the End

Simply put, to paraphrase a Wizzard from another world (a Disc-type-World, for those not in the know) the Earth may be a nice place for a holiday, but it's not a place for anything long-term.

Species come, and they go – mammalian species endure for a few million years, and then are no more. We've only been around in our current form for a few hundred thousand years, and things are looking pretty grim already – overpopulation, global warming, the looming spectre of nuclear annihilation. If we avoid all that, there’s still meteor strikes, volcanoes, drifting continents. Avoid all that and one day, a few billion years from now, the sun will simply swell up and roast its little satellites.

So yes, one day the sky is going to fall on our heads, and it won't be pretty. But there's pretty much nothing you can do about it, so don't give it another thought. Just go about your day-to-day life and do the dishes and balance your budget and kiss your kids and be happy, okay? And whatever you do, don't look up.

Oh, yeah - and happy Earth Day, folks!

Also: That pretty pic up top is NASA's Image of the Day (http://apod.nasa.gov/apod/astropix.html)...It's NGC 7635, also known simply as The Bubble Nebula, and it's awesome...