The first telescopes were toys, charming amusements. Sure, there were a few practical uses, such as observing distant ships coming into harbor. Doing so allowed merchants to hurry down to the docks ahead of their telescope-free competitors, and snag all the better goods. Military commanders occasionally found telescopes handy as well. And when they weren't being used for commerce or conflict, these simple devices were undoubtedly helpful for checking out the personal parameters of careless neighbors.
In 1609, Galileo turned a telescope skyward -- a move that no one else seems to have considered. His instruments had lenses about the size of a half-dollar coin, and magnifications that were only about 20 times. Their simple optics had more aberrations than Vlad the Impaler.
Today, you wouldn't give a kid a telescope this lousy, unless you're inspiring her to forsake science in favor of a more lucrative occupation, like starching shirts. But these low-grade constructions were good enough to see the bigger moons of Jupiter, the craters of the moon, and stars making up the Milky Way. They were, despite their pitiful specifications, arguably the most important astronomical telescopes of all time.
Modern researchers would find Galileo's 'scopes useful only for batting Whiffle balls. They've moved on to bigger and better, and today are building some truly impressive instruments: a new generation of titanic telescopes that sport primary mirrors larger than tennis courts. These will snag a million times as much light as Galileo's instrument, which is really the motivation for their construction. But, thanks to an ability to undo a lot of the distortions caused by Earth's shuddering atmosphere, these new outsized 'scopes will be about as hawkeyed as the famed Hubble instrument -- able to see detail at a level of about 0.1 seconds of arc. That's enough to just make out a dime a dozen miles away.
Impressive, yes, but no one cares about examining far-off dimes. What about inspecting worlds around other stars, the so-called exoplanets that dominate a lot of astronomy news these days? Well, with these new giant telescopes, any Earth-size exoplanet would be smaller than one pixel in size. It would be a thoroughly unresolved pinpoint of light.
Useful, but not entirely gratifying.
I think it's fair to say that, given your 'druthers, you'd want an instrument that could map exoplanets in the kind of detail you get with Google Earth, with enough resolution to actually see the Great Wall of the Klingons, in case they've built one.
Could we construct such a telescope ... ever?
Here's what it takes: Let's assume that all the alien worlds you wish to view up close and personal are no more than 100 light-years away. That might sound pretty cramped to astronomy nerds, but there are probably several hundred thousand planets within that distance - enough to gratify even the most spirited voyeur.
At 100 light-years, something the size of a Honda Accord -- which I propose as a standard imaging test object -- subtends an angle of a half-trillionth of a second of arc. In case that number doesn't speak to you, it's roughly the apparent size of a cell nucleus on Pluto, as viewed from Earth.
You will not be stunned to hear that resolving something that minuscule requires a telescope with a honking size. At ordinary optical wavelengths, "honking" works out to a mirror 100 million miles across. You could nicely fit a reflector that large between the orbits of Mercury and Mars. Big, yes, but it would permit you to examine exoplanets in incredible detail.
The down side is obvious: Who could ever construct such a thing? Well, fortunately, no one has to. Instead, you could field a phalanx of small mirrors in space, spread out over 100 million miles. They wouldn't even have to maintain a fixed pattern, as long as you could accurately keep track of their relative positions.
No huge mirror: just a manageable number of small ones. The ability to see detail would be the same. And, of course, it's a heck of a lot easier to turn an array of small instruments to different places on the sky than to pivot a 100 million-mile monstrosity.
Of course, there are a few small problems of principle here. You need to collect enough light to make the imaging possible, and correct for the fact that the target exoplanet is both rotating and sliding across the sky. Both problems can be dealt with, at least in theory -- which suggests that they can also be dealt with in practice, given sufficient effort.
But think of the implications. There's a lot of talk about interstellar travel, and whether we will ever be capable of rocketing to other stars. It's a tough thing to do.
However, if the type of telescope described here can be built, then the tyranny of distance is vanquished. You can forget deep space probes and their long travel times. We could explore alien worlds in the comfort of our own homes, as our laptops scroll and zoom through data sets collected by a mammoth, space-based telescope array.
It would also, quite obviously, be a whole new way to search for extraterrestrial life ... just look for it, or its artifacts (like cities).
This is, to my mind, the ultimate telescope. It's not for our generation to build, or even the next two. But after that ...?