"For example, there are millions of asteroids of different sizes and composition flying throughout space. One category, known as S-type, is composed of iron, magnesium silicates and a variety of other metals, including cobalt and platinum. An average half-kilometer S-type asteroid is worth more than $20 trillion."

http://www.googlelunarxprize.org/lunar/featured-article/spac...

No it is not - because if that much were available it would drive down the price...

Well, it's not like you are just going to grab the whole thing and bring it back. But I agree, a new source of rare metals would change the pricing. But, platinum is an interesting metal -- engineers could do a lot fun things with it if it wasn't so expensive.

In one asteroid is worth $20 trillion, how much are 5000 asteroids worth?

I expect if you had 5000 asteroids worth of iron, you'd have to pay people to take it off your hands.

An 800m iron asteroid would make a rather nasty weapon.... (about 75000Mt, probably considerably more than all of the nuclear weapons that have ever existed).

Great response. Googling around led me to order this book on the matter: "Mining The Sky: Untold Riches From The Asteroids, Comets, And Planets"

It's a bit old but looks interesting...sadly not available on Kindle.

http://www.amazon.com/Mining-Sky-Untold-Asteroids-Planets/dp...

Also: The Universe is awash in energy; it is young, and full of hydrogen, much of it pre-packaged into natural fusion reactors for us. Most of it is available only on not-Earth. If it were actually feasible to put things in space we could use more of it without the issues arising when we try to do it in our biosphere.

Space is dead. Anyone who gives a couple moment's serious thought will realize that there is really no such thing as environmentalism in space; at best, there are aesthetic concerns. (Many people are not willing to be even that thoughtful, but one can only try so hard.) Some of the things we do on Earth would be better done in space, where the byproducts hurt nobody.

Earth does not have enough of certain resources for a really green society, based on the fact that many green technologies require various rare earths. Getting more palladium or platinum from space will enable us to build more fuel cells and such things. For bonus points, combine with the above and refine in space.

The problem isn't that space is useless by any means. It's not even useless to "the poor". It's just that there is a bit of a gulf between where we are now (communications satellites) and where it becomes really useful, even putting aside the technological advances that will arise in response that will be useful even on Earth (a lot of environmental systems work and such).

There's also the not having all our eggs in one basket, which is getting increasingly important. I do fear in the race between anybody's brother being able to build a biological uberkiller and having humans in space the former will win by a decade or four.

Humans are going to space. Those that do are going to be awash in riches far more vast than the Europeans found in the New World. America today is a great place for innovation, startups, and living comfortably. We got here by risk-taking, fighting, exploring (geographically and other ways). The people who continue to do that will be the new America. I'd like the old America to be the new America.

Space Indian genocide!

The massive amounts of resources in space are, of course, a plus... but I mostly like it for the long-term survival of the human race.

Setting up self-sustaining human presences in colonies on other planets/moons makes it vastly less likely that some random asteroid that we didn't see coming will wipe us all out. (Other potential disaster scenarios involve superviruses, grey goo, that giant volcano under Yellowstone, and so forth.)

4) so we can build a defence system against incoming asteroids 5) to provide a hedge against a planet-wide disaster (aforementioned incoming asteroids, global nuclear war, massive pandemic, runaway greenhouse, take your choice) 6) to get at the massive resources available in space - this one needs expanding. Yes, with current technology those resources simply aren't cost effective compared to what is available on Earth. But imagine a world where space-gun launches or a space elevator reduce launch costs down to tens of dollars per kilogram in orbit. That changes everything in the cost benefit analysis of space resources. 7) Science. We'll discover things that we never even imagined could exist. Who knows how those discoveries will aid humankind in the future?

Those are just some reasons that I could come up with quickly, but I'm sure there are others

8) Expansion (possibly covered by 5) 9) to preserve the Earth: if we can move the bulk of the human population off planet before we totaly destroy it, we could turn the Earth into a planet-sized nature reserve.

"that threat doesn't exist anymore"

I know everyone believes that the threat of nuclear war has completely gone away - but there are still an awful lot of ICBMs and IRBMs in the world.

for the same reason that after the age of 14-21 normal people don't live home with parents.

One you left out: Because space-based remote sensing can tell us about Earth. See:

http://science.nasa.gov/earth-science/focus-areas/

(There are others.)