A solar flare or a gamma ray burst might knock out satellites and electrical equipment and damage earth's atmosphere. In the worst case, they could kill a few billion people. But Earth will still be more hospitable than Mars.

>Same reason people crossed the Atlantic, to take part in something greater than themselves.

People crossed the Atlantic because they wanted gold, or slaves, or timber, or land, or because they wanted to escape from religious persecution, or because they were slaves or prisoners transported against their will (by someone who wanted gold, or timber, or land). They received resupply, not as charity, but from companies which expected to receive gold, tobacco, timber, or other goods in return from their investment.

Colonization is driven by economics. That's why thousands live permanently in Norilsk, Yellowknife, La Rinconada,and Kalgoorlie, but nobody lives permanently at Amundsen-Scott, under the sea, or in LEO. If history is any guide, "how will this turn a profit" is the question any Mars colony needs to address if it is to have any hope of prospering.

> People crossed the Atlantic because they wanted gold, or slaves, or timber, or land, or because they wanted to escape from religious persecution

That was before the industrial revolution - the new world offered relatively mild opportunities; nowadays people still cross the ocean - but these are all people from third world countries, or countries destroyed by civil wars; i think that in order to move people from the first world you would need to promise to make them really rich.

This conveniently ignores the fact that people will do it just for the sake of it. Don't always need economic incentives.

For instance: I'll try to do it for the sake of it. Sampling folks around me, a dozen would. It's not that crazy or rare a proposition. Arguably my environment doesn't represent an unbiased share of the population, but it's still a certain amount of people.

These natural events are rare enough that, to me, they don't create a sense of urgency that we need to get off the planet in the next decade.

This is the big question to me... when is the right time to go? If we wait 50 years, wouldn't the technological progress make such an endeavor cheaper, safer?

And the counterpoint is, well... it's been 50 years since apollo and still we don't have the ability to get to mars, so if there's nothing driving the technology then it doesn't just magically happen, and there's nothing to say that it will magically happen in another fifty years either.

To me, it's not the natural extinction events that are driving this, but the thinking that there is a short window of perhaps 200 years between technological enlightenment (particularly in nuclear and biological engineering) where the risk of self-annihilition is considerably increased, and the only way to decrease it is to add the redundancy.

To be sure, we will not achieve redundancy during the first decades or even perhaps centuries of this colonization. I think that terraforming needs to happen first

We don't need to invent new physics to do this. The challenges are very real; by various definitions the Saturn V was the most powerful machine ever constructed, and Musk proposes a rocket with several times the thrust. Mankind has not stopped developing spacecraft and rockets since the Apollo era. Whether or not we colonize Mars, we have to start by getting there. I'm sure waiting for a space elevator would be ideal, but if we can do this economically with rockets there's no particular reason not to. We've done this "build a big rocket" thing before. The only thing really standing in the way of it happening again is sufficient motivation. It's not yet extremely politically useful for US politicians to do more than talk about going to Mars, but so far aerospace has proven to be a pretty good source of pork and high-paying tech jobs, and NASA still has a pretty good public image. The only thing we've really been missing is a popular, charismatic leader who can get common people excited about space again. A national threat wouldn't be bad either, but those are pretty easy to gin up.

>This is the big question to me... when is the right time to go?

Could a natural extinction event happen in the next 50 years?

If the answer is 'yes', I think we should go as soon as it is possible.

Humanity has been much closer to self-annihilation in the past. That gives me hope that we wont nuke ourselves before a giant rock falls from the sky ;)

Add one kickass meteor impact to your list of things that could bring us to extinction.

Old, but gold: http://www.exitmundi.nl/exitmundi.htm