Because pretty much everything that was possible to discover with earth based telescopes has been discovered. Sure, you can always do "science" with earth-based telescopes - reconfirming existing theories, making sure theoretical data matches up to real observed data, but there's a limit to how much "discovery" can actually be done.

Extra-planetary experiments like the deep impact mission allow scientists a glimpse at new experimental data that can confirm theories that there is no other way to confirm. Sure, you can observe a comet's tail to figure out its surface composition - but that has limited resolution and many elements simply can't be detected that way. This isn't even taking into consideration all the sub-surface elements that would be utterly impossible to detect from observation alone.

In essence, even if the Deep Impact mission mostly failed and scientists only gathered a limited amount of data, it is still very much worth its value when the alternative of building permanent terrestrial telescopes does nothing more than confirm data that has been confirmed countless times in the past.

> even if the Deep Impact mission mostly failed and scientists only gathered a limited amount of data

I don't think you meant it that way, but I'd like to clarify that Deep Impact's mission most definitely did not fail. The spacecraft survived for eight years after it completed its primary objectives in 2005.

"Because pretty much everything that was possible to discover with earth based telescopes has been discovered."

That's outright wrong.

Is the Large Synoptic Survey Telescope there just to look pretty and reconfirm existing theories? Why build the Giant Magellan Telescope when apparently there's no more discoveries to make? Or the European Extremely Large Telescope? The Thirty Meter Telescope?

All the primary missions were completed by 2010, so you got your money's worth. And 267 million is nothing for a mission with that level of complexity, for comparison, producing GTAV was between $200 and $250 million!!!

Considering that Earth based telescopes have never been in a better position and are improving at an incredible rate from one decade to the next I find it hard to believe that money spent on a unique and unprecedented scientific endeavor would have been better spent on Earth-based telescopes.

The simple fact that Hubble was launched twenty years ago should give you some clues as to why we aren't doing that. Even twenty years ago, land-based telescopes were already too limited.

But we are doing that.

Hubble was started and launched before adaptive optics. That's revolutionized ground-based telescopes. The newest generation of extremely large telescopes, like the Giant Magellan Telescope, will have better angular resolution of Hubble, in visible light.

Of course, the relevant comparison should be to the James Webb Space Telescope, but it's estimated cost of $9 billion is rather a lot higher than the $800 million for the GMT.

I'm emphatically not saying that we shouldn't have space telescopes. There's no way to have a ground-based X-ray telescope, as an obvious example.

I'm only pointing out that the evidence is that land-based telescopes are not, broadly speaking, "too limited" to do new science.

Smart comment. Adaptive optics have rearranged the fundamental drivers for visible-light astronomy.

And a lot of physics experts who were working on high-resolution space-based telescopes, telescopes that are in many cases are now replaceable by ground telescopes, have paid a heavy price for this fact. Technology can be a tough field sometimes.

If you want to see what is likely to be built in the next decade, you can start at:

http://sites.nationalacademies.org/bpa/BPA_049810

Go to the last report on the web page ("2020 Vision") and go to page 17 of the PDF. It's all there.

Thanks!

"What's likely" is, sadly, optimistic. I see that LISA's isn't likely to be built by the 2020s.

Reviewing the list of other space telescopes, WFIRST is now WFIRST-AFTA, with a change to use a second-hand NAO telescope.

NuSTAR and IRIS are two launched Small Explorer missions since 2010. (Oddly, they used an artist's concept of WISE to highlight the concept. Odd, because WISE was launched in 2009, so a 2010 publication should not have needed a concept image.) I haven't figured out what the new missions are/might be, but I didn't look too hard.

The International X-Ray Observatory has had a "reboot", to ATHENA (Advanced Telescope for High Energy Astrophysics). NASA withdrew from IXO.

Awesome! I've dreamed of adaptive optics (not specifically, just how it might be achieved), I didn't realize they already existed. Thanks for the information.