This won't have much effect other than gravity, as the space between stars makes the chances of any collision almost nil.

Well, I don't know. Humanity has always had its problems, so I don't think we can realistically wait for the creation of a worldwide utopian society before even thinking about studying space.

Yeah, for 2007, it's about 16 billion dollars, and I do think that's not enough. 16 billion dollars is a lot of money, but relatively to the rest of the US budget it's almost nothing, and I honestly don't think it would make any notable positive difference -- neither in the long run nor short term -- if that money was diverted somewhere else.

This site mentions that ''it has been conservatively estimated by U.S. space experts that for every dollar the U.S. spends on resarch and development in the space program, it receives $7 back in the form of corporate and personal income taxes from increased jobs and economic growth.'' I don't know if that's true, since ''estimated by space experts'' isn't exactly a proper reference, but who knows. There's also a list of some NASA science projects that do benefit those of us that stay on Earth.

That said, I do think that the money NASA is given could probably be better spent. For instance, I'm not really sure about the usefulness of manned space flights at this point; it'd probably be better to concentrate primarily on R&D to make space flight much safer and more economical. As I understand, they plan for ''humanity to return to the Moon'' by 2018, and I'm not really sure if there's any good reason for that, or if it's just for kicks. Though I'm no expert.

About asteroids: yes, we can. It should be possible to change the trajectory of an asteroid, using either current technology or technology that could be available in the coming decades -- if we discover the object on collision course early enough (at least ten years or more before impact). That's not impossible either, although one problem is the glare of the Sun, which creates a blind spot (I'm not sure how large) in which nothing can be detected. That can be solved by using an extraterrestrial telescope (say, on Mars or something).

I read on Wikipedia (lol) that it's estimated there are ~1500 large objects capable of crossing Earth's orbit, and that within a 1000-year period, some of them are going to hit us. It's impossible to tell if they'll start a new ice age or wipe out a city or two with tsunamis, or just hit some area in Syberia that no one cares about, but either way, the possibility or preventing it rises greatly if we keep up research and development of astronomy as much as we can.

Preventing impact with a comet would be more difficult, as our advance warning would be shorter, but with enough research it's not impossible.

Now, if something like a type Ia supernova occurs somewhere near us (near being closer than 3000 light years or so), then I have to admit we truly are completely fucked, whichever way you look at it. They are almost impossible to detect, and for all I know, gamma rays from a 1000 year old supernova could destroy the Earth's ozone layer before I finish writing this post. There is an unproven theory that a supernova 6000 light years away caused the death of more than 50% life on Earth ~450 million years ago. Ways of avoiding something like that are so far off that it's basically just science fiction, but luckily the probability is also extremely low.