> The difference being that there is nobody living on a planet like say, Mars.
Stop right there. You don't know that there isn't anyone living there. We've sent a couple rovers, dug down a few inches, and looked at it from telescopes. There are millions of dollars poured into the search for life on Mars every year, as there should be. But we don't know very much yet!
I am totally in favour of terraforming and colonizing Mars if we can (reasonably) prove that nothing is alive there. But we aren't anywhere near that point. The greatest crime the human race could ever commit would be the annihilation of the (perhaps only?) living ETs found, even if by "accident" as we moved in there.
Don't think that Mars is your dead playground for you to do as you wish. Treat it and its unknown past with deep respect. There's a small chance that something, somewhere, is living there, perhaps under the surface, perhaps in caves.
Don't assume we know everything about other planets just 'cause we can see them with our telescopes.
I am in favor of doing with Mars as we wish. When Western Civilization had "first contact" with various "alien"[1] native societies around the world, most of the impact flowed from civilization to the "aliens". Civilization continued on and most of the alien societies became footnotes in history. Might makes right, and unless the Martian bacteria are crafting starships in their caves, I think we should take samples for scientific study and then open Mars to homesteaders. The Martian lifeforms have had just as long to do something productive with Mars as we have had with Earth. It is not our fault if they are still just bacteria.
I don't think it's that big of a deal. Considering the issue of wiping out a species, we do that all the time here on Earth (no that doesn't make it okay, but we do it all the time nevertheless, I don't think it's humanity's greatest crime). Considering the issue of wiping out an ET species, that makes me more worried for Great Filter concerns than about how we'll look back at ourselves in history. If there's life on Mars, or if there was life on Mars, either case is bad news for us.
nowhere to go? really? last time i checked most of the earth's surface was uninhabited. why do people want to go to mars when the land can be had in the mojave desert for practically no cost? its just as dry but has an oxygen atmosphere, and there is virtually no demand for the land
or forget about the mojave. northern canada has a population per square km that is effectively zero. pretty mars-like, but you can hunt for dinner if you're smart. and once again, the land is basically free.
oh i'm not even mentioning a half dozen inhospitable places that no one occupies....why on earth would these people need to go to mars again??
A) Because every dollar invested in the US space program since inception has created about $7 of economic benefit to the consumer economy.
B) (Related to A but worth mentioning separately:) Because the space program has led to major advances in medicine that have improved length and quality of life on Earth. [1]
C) To gain access to essentially limitless energy resources via space-based solar.
D) To gain access to material resources (e.g. minerals) that are scare or absent on Earth. [2]
E) To create an off-Earth breeding population of humans so that one asteroid / plague / nuclear war / whatever cannot wipe out the entire species.
[1] Here's a simple example: When the Hubble telescope was initially launched, it had a small lens defect that blurred the pictures slightly (the pictures were still better than any Earth-based 'scope, though). Image processing software was developed to clean up the pictures until a replacement lens could be deployed. That software then migrated over to MRI machines where it is now used to detect tumors much earlier than used to be possible.
[2] These resources are better accessed from the asteroids than Mars, but going to the latter gives you access to the former.
There were myriad new materials and other technologies developed to defeat the challenges of getting into and surviving in space, many of which launched companies and industries.
Some commonly cited examples: memory foam (TempurPedic Mattresses), better water filters, freeze-dried food, cordless power tools, various plastics, useful solar cells, carbon fiber epoxy.
On top of that, there are all of the benefits of having satellites (telecommunications, GPS, satellite TV, etc).
you raise a bunch of economic issues, but the poster to which i replied simply stated that there was "no where else to go", which is absurd
the rest of your points are dubious. your point B) has been proven false since the launch of the space station...this was one of its apparently initial goals, zero-g bioresearch...except that ended up being replaced with simulating everything in computers with bioinformatics on earth much more easily
C) also has no meaning as we could provide 100% of our solar needs by simply covering a part of any of our major deserts with solar thermal.
I second wunderfool's comment. Having driven from Richmond, Virginia, to Anchorage, Alaska, a couple of times, I can attest first-hand that most of North America is unoccupied, empty wildlands. And, don't tell me that I was confused by giant farms in Saskatchewan or something like that. I'm not counting them. Most of the land is simply empty and devoid of humans. (This fact got a little worrisome when it was night, I was low on gasoline, it was snowing, and I was still a good distance from the next "town" on the map.)
Last I checked, there are a whole bunch of people with nowhere else to go on Earth as it stands.