A subject which has been fascinating me recently is the nature of interstellar space exploration at some far off point into the future. It would seem to me that even if one day decades or centuries from now interstellar spacecraft (unmanned or manned) become a practical proposition, the most efficient way of exploring the galaxy around us will continue to be bigger and better telescopes in space, on the Moon, on Mars, etc.

While there is no substitute for being on the spot, it will certainly be many centuries before we have the technology and resources to visit planets and fully explore them more than a few light years in any direction from Earth. But we already know, in principle, how to build ginormous space telescopes that can capture the trickle of reflected photons from exoplanets many light years away, and the odds are we will have the ability to do so within decades, not centuries.

It's all very interesting speculation, though since I am but a mere amateur, I have a few questions about the practicalities and limitations of such an enterprise, assuming the funds and technologies are available. Remember, we're not talking about the near future here. Assume we already have a dozen space elevators around the Earth, colonies on the Moon and on Mars, and regular missions to the asteroids, Jupiter, Saturn, and beyond. Perhaps 200, 300, even 400 years hence.

- Is there a point where bigger or sharper scopes won't help resolve further detail in the images of exoplanets?
- At what point will we have to send the telescopes into the outer system beyond the effects of zodiacal light?
- How far will we be able to probe before interstellar gases and dust become a problem?
- Any guesses as to the absolute limit of reliable observations where we can examine the nature of a planet and its atmosphere--i.e. enough to detect signs of life and civilization--(excluding chance events like gravitational lensing)? Is it tens, hundreds, or thousands of light years?
- If there are planets in, say, the Beta Centauri system, how much detail on their surfaces could we potentially resolve. Could we ever see Beta Centauri b as well as we can Mars from Earth through a thousand dollar telescope?

There are some very interesting mission designs already on the drawing board for the decades ahead, but how much further can we go in exploring the galaxy around us before we would have to leave our own solar system to continue the work?

The most amazing project I ever read about is the Exo Earth Imager which is a brainchild of Antoine Labeyrie. It's made up of 10000 mirrors each with a diameter of 3 m which orbit at a Lagrangian point and form a giant sphere with 400 km diameter. At the center of the sphere sits a satellite which processes all images and send data back to Earth

http://www.otherworldslecture.org/bio3.html
http://www.oamp.fr/lise/seminaires/imagesLabeyrieGlasgow.pdf

"Amazing" is a deep understatement, Phil! I love it, but has anyone run a cost estimate (if that's even possible?) The launch costs alone seem truly terrifying...

I'd guess development & deployment at around US$100 billion, which by the time it can launch would be like five Euros...maybe it is feasible!

In the forseeable future, perhaps such a project is undoable. But what about in 100 years, or 200? Once we've been to every corner of our own solar system, what's left but to explore beyond it? For that we have only two choices--launch ourselves (or, by proxy, unmanned probes) into interstellar space or build bigger telescopes all the better to see with. Which one of these endeavors will be more practical first? I would be very surprised if it's the probes. The return on investment, no matter howdaunting, would be far greater from launching a mega-space telescope that could instantly observe hundreds of worlds than from launching a probe that would take decades to get to one.

Thanks to tacitus for this thread, and to Phil for the amazing link about "Hypertelescope" project!
Believe or not, some years ago I was playing with an even more crazy idea: a billion huge telescopes (in the order of 30-100m diameter) making an interferometric array in the anti-sun Lagrange point of Jupiter or Saturn!
This would allow to work in a very dark involvement (contrary to Earth, the eclipse in the giant planet's L2 point is total and this region is about 50000 Km wide!) outside of most zodiacal light. Moreover, resolution and sensitivity would allow to image nearest exo-planets with a few Km resolution!!!
Obviously, this was a mental exercice and I know that is way ahead present technology; perhaps, we could realize in 100 years.
If I find time, I will make/publish the draft project in the forum...

I am not sure if telescopes can be build as large as someone may think. There is a crucial problem with the exit pupil which gets larger with the mirror diameter. Additional, there are some limiting effects due to the wave nature of light and the segmentation of the primary and secondary mirror.

Interestingly, a quadruple of four 50 m telescope arranges in orbits around the sun in Saturn distance (to habe a very large baseline and to avoid inner system zodiacal light) giving a large tetrahedron at each time would allow instantly parallax mesurement of nearly all visible stars of the galaxy with two or three telescopes at one time and allow precise tracking of double stars and suns with planets.

Planets of Alpha Centauri could be seen with that, too, and other planets also. These picture will not allow to see if there is any civilization, but it would be the first step to get useful data for a probable interstellar probe of coming centuries (but: if manned spaceflight will evolve ever like now, it will take millenia).

One of the problems with the Hipparcos satellite was, that doing such measurements only from one site at a given time does not allow to get precise parallax and position data of double stars.

That's actually true of all the planets beyond Earth -- even Ceres - although for Mars it's barely so. (In fact, given how elliptical Mars' orbit is, I'd expect libration to spoil the eclipse.)

--Greg

Well, if Exo Earth Imager could be build, it would look like this (note satellite in the middle):

Nice image, Phil!

Got me thinking, too. Maybe this would become much more practical--and affordable-- if the core satellite and maybe 10 mirrors could establish an initial operating capability al a TPF; not full-sky parallel coverage as depicted in the final version, but maybe enough slewing agility to examine nearby systems & do deep-sky Hubble-style work (though it would be an order-of-magnitude leap above Hubble's performance already.) Over the next few decades, more and more mirrors are added, and replacements are also launched for mirrors that have failed or run out of fuel (probably the coresat would be replaced as well a few times for the same reason, as well as for system upgrades).

Bottom line is that this seems like an ideal concept for very long-term spiral development rather then directly committing to fielding the whole shebang. Might take 50 or 100 years from start to finish, but it'd get better all the time.

I love the idea, but feel that the major challenge would be keeping all the elements cohesive.
The sun's radiative pressure would make the whole assembly flex like a soap bubble (or a planet's magnetosphere). When today's mega-mirrors are affected by an imperfection of a few nanometres, this telescope would spend it's entire budget on station-keeping propellant.

I still like the proposed TPF / Planet Imager with its 6000km baseline. If it is scalable to include more telescope arrays, all the better. This would show 25 pixel images of "nearby" Earthlike planets.

http://www.daviddarling.info/encyclopedia/...anetImager.html

What are the wave nature of light limitations - Harkeppler? Do they prevent making telescopes beyond a certain size?

Or maybe not. For some interferometric applications, precise knowledge of the baselines is needed, but control of the baselines can be rather less exact. That is, if you have adequate compensation techniques (such as "optical trombones"). drift can be tolerated as long as it is known in real time exactly enough for compensation. And some of the exoplanet interferometer projects have already counted on using differential sunlight pressure to assist with attitude and stationkeeping, since it can be controlled at a level much finer than anything else available in deep space (where the geomagnetic field isn't available; I gather the solar wind field is too unsteady for this).