Second, having all of humanity live on a single speck of rock puts us in a very precarious position as a species and introduces a single-point of failure to our civilization. Therefore, the development of technical capabilities to move freely around the solar system and eventually beyond is also an objective for humanity. Again, I admit, it is a long-term one without short-term urgency of say, getting rid of Ebola.

So the whole issue is essentially a question of the right balance of the amount of effort we invest in working on our short-term urgent objectives and the long-term ones.

With world GDP in excess of $70 trillion per year [1], spending 1 billion EUR ($1.25 billion) for a decade-long mission does not seem like extravagance.

[1] http://en.wikipedia.org/wiki/Gross_world_product

EDIT: To further put the cost of the mission into perspective: assuming the contributions for the mission came from about half a billion people (EU population) over a decade this translates into about 20 euro-cents per year. Seriously, this is not extravagance. In fact, given that the mission cost is over 20% of ESA's yearly budget it makes me think Europe is under-investing in its space capabilities and scientific research. Nitpick: ESA member states don't overlap with the EU exactly, but it is a good enough proxy. More accurate figure would allow for American and other contributions to the on-board instruments so would be even less than 20 euro-cents/year/person.

Military: $640bn (per year) War on drugs: $41bn (per year) Health-care overcharging & overtesting: $1200bn (per year)

And this is just the US, now imagine those costs worldwide.

So, in contrast, this is a first-time-experiment where we get to do something never done before, using a lot of tech in brand new conditions (for us) where thousands put to test their knowledge of physics, electronics and astronomy. In such light you might understand why you are being downvoted.

One of the initial motivations for this mission was to learn more about our solar system's origins by studying a comet up close. Many engineering challenges were also overcome in the process, which will lead to positive side effects for future space missions and related areas, but one of the primary scientific questions this mission attempts to answer is "why is our solar system the way it is?" Answers to that question will likely help us better understand our place in the universe, and possibly shine a light on other solar systems as well. Maybe even give us a better idea how likely we are to encounter life in other solar systems, and what that life might be composed of.

Ironically, compared to questions of cosmic significance like these, short term concerns like treating present diseases or improving the current economic environment are petty and irrelevant by comparison. These are issues that science and government can tackle on earth, and issues where throwing more money at the problem doesn't always help. It is a fallacy to think that because space travel requires money, that earthly needs will go unfulfilled. It's deeply short sighted, in fact.

Your comment seems to be rooted in a deep misunderstanding of the nature of science. Science does not proceed by first devising a list of improvements to human wellbeing that will result from an experiment (though the grant application process in universities sometimes encompasses this task--with a lot of handwaving as a result). Science is about expanding our sphere of knowledge. By definition, we don't know what will result, or if it will have a positive impact. But we are trying something that has never been tried before, which is deeply interesting, and which has the potential to greatly expand our knowledge of the universe. Science is fundamentally a research activity: you can't know in advance what you will find. Asking what we will find before performing the experiment is pointless. The "might find" category for a mission like this is enormous, however.

I'm not sure how Rosetta was run; it had a longer germination time than many missions.

But for a typical science-driven mission, there is a thing called a "Science Traceability Matrix", or STM, that has science goals down one side and measurements down the other side. Every science goal must be traced to one or more measurements that will achieve it, and every measurement must correspond to some goal. If the linkage is not clear, the measurement (i.e., instrument - one spacecraft typically has many instruments) will be booted.

For more: http://csc.caltech.edu/references/Grogan%20STM.pdf [page ~8 has a matrix]

The science goals, in turn, are arrived at through National Academies studies, typically Decadal Surveys, done once per 10 years (often with a midpoint course correction). Membership in the NAS is a very big deal that few scientists achieve; the expertise of these people is unreal. That is:

  state of science knowledge -> 
  decadal survey ->
  science goals ->
  traceability matrix ->
  specific instrument choices

http://www.nap.edu/catalog.php?record_id=13405

One of the outcomes of all the space funding over the years is that there is now a somewhat orderly process for attacking big questions systematically.

Thank goodness at least some of our money is spent on things which might benefit humanity in the long term, like discovering how the solar system was formed or whether life was formed on earth or elsewhere.

As for your long term benefits:

Great so we find out how the solar system is formed or where life was formed - now what? Those are really cool things to know but what do we do with that information? The reason I'm curious about this is that although I want to see more of these missions and I think it would be great to have answers to those questions I find it difficult to actually justify spending that money on answering questions. There must be some tangible benefit. Other responses to my question have provided me with answers to that.

Maybe they haven't been discovered yet. Application can lag theory or basic experiments for a very long time. Did the car come right after the wheel? Did the iPhone come right after Ohm's Law? Maybe this will aid comet/asteroid mining missions. Or optical image processing. Or orbit optimization for long solar system traverses. I think it's very shortsighted to pursue only research that has visible short-term benefits, because you could be missing out on countless innovations that are below the horizon. We can't predict the future. But I also think application is unnecessary, and that there is inherent value in understanding our world.

In any case, while I don't know how much the average European pays for ESA in taxes, I do know the average American pays something like a few bucks to fund NASA. So you're quibbling over the use of a tiny proportion of resources to answer some big questions.

Which you then contradicted by whining about not seeing the benefit of it; you're clearly not all for this mission or you wouldn't have derailed this top thread with your concern troll.

There doesn't have to be a tangible, short term benefit for these missions to be considered worthwhile by the majority of humanity, so your question is irrelevant to them. If you can't see the benefit to us of finding out when and where life was formed, I can't help you.

That's fine. There are plenty of other answers to my legitimate question (certainly wasn't trolling) and those were able to help me.

The LHC took over $13B to discover Higgs. It's a bit of a stretch to call it a discovery, it was really a confirmation of what we were already pretty sure of. But, that money didn't just disappear; it went into building a tech industry which is pretty valuable to have. And a better understanding of our universe has a funny way of being really useful down the road.

  Ernst Stuhlinger wrote this letter on May 6, 1970, to Sister Mary Jucunda, a nun who worked among the starving children of Kabwe, Zambia, in Africa, who questioned the value of space exploration. At the time Dr. Stuhlinger was Associate Director for Science at the Marshall Space Flight Center, in Huntsville, Alabama.