Except we can defend the earth from an asteroid for probably five orders of magnitude less money than establishing a self-sustaining colony somewhere else. And if self sustaining is all you need, setting up a self-sustaining colony a mile underground is also five or six orders of magnitude cheaper.
I can't think of any risk to humanity where going to the moon or Mars makes sense as insurance.
> Except we can defend the earth from an asteroid for probably five orders of magnitude less money than establishing a self-sustaining colony somewhere else.
If we find out about it decades or centuries out. If it's a year out when we spot it, there's very little we can do with current technology to head it off (asteroid disaster movie plots notwithstanding).
> And if self sustaining is all you need, setting up a self-sustaining colony a mile underground is also five or six orders of magnitude cheaper.
It's also much more accessible by desperate refugees trying to get in as the planetary disaster approaches.
"Five orders of magnitude less" than the cost of the Apollo program is $20 million [1]. Current large space telescopes and underground labs cost of order $2 billion, only 1/100th of the cost of the Apollo program, so I don't think your suggested alternatives are as cheap as you think.
What makes SpaceX important is that they've rethought the design and manufacturing process to make their rockets far cheaper than others to date. If you bend the cost curve enough, surprising things become reasonable.
You aren't going to get a self-sustaining colony on Mars for the cost of the Apollo program. Just getting a biosphere-level of sustainment would be several orders of magnitude more expensive. If "self-sustaining" is meant to mean an insurance policy, then it requires more than just generating food and water; it will require a whole manufacturing base. I don't think 10+ trillion dollars, about three orders of magnitude more than NASA's budget either now or during Apollo, is unrealistic at all.
For comparison, the space station cost $150 billion 2010 dollars (according to Wikipedia). It is in low-earth orbit and satisfies no conditions of self-sustainability. With a factor of 100 on that, which is impossibly optimistic, you are already at $15 trillion.
I can't think of any risk to humanity where going to the moon or Mars makes sense as insurance.