Admittedly, it would only work for "intelligent" life, but seeing
luminescence on the dark side of the planet on a regular basis would
definitely float my boat.

-- Stinger

"Steve Dufour" <stevejdufour@yahoo.com> wrote in message
news:744cc401.0312020824.174dddaa@posting.google.com...

Seeing Extraterrestrials

By Seth Shostak
Senior Astronomer, SETI Institute
Space.com

China's Great Wall may, indeed, be a whale of a wall, but you can't
see it from space with your naked eye.

I made this point in my column of last month, "Can Aliens Find Us?"
where I considered whether sophisticated extraterrestrials could
easily discover Homo sapiens. My example was intended to show that
even from relatively nearby, the physical artifacts of human society
are difficult to detect. Our radio signals are far more conspicuous.

But a reader of that column, Fred Hapgood, wrote to say that, after
all, just because the constructions of an advanced culture would be
difficult to see directly, this doesn't mean that they're thoroughly
impossible to find, does it? Consider how much better the telescopes
of a civilization hundreds of times older than ours would be, Mr.
Hapgood suggests.

He's right, of course. We inevitably tend to envision the capabilities
of putative extraterrestrials as being similar to, or slightly more
advanced than ours. But what could a society that's many millennia
beyond us do? Could they ever map our world and see our ancient walls,
our cities, or even us?

A parallel question, albeit in less extravagant form, was posed by
former NASA Administrator, Daniel Goldin, shortly after astronomers
detected the first extrasolar planets around normal stars. On a 1997
PBS television show, Goldin enthusiastically exclaimed "Could you
imagine if in twenty-five, thirty, or forty years, we could take a
picture of a planet that's perhaps fifty light years from Earth, and.
if the resolution was high enough. to take a picture of oceans and
clouds and continents and mountain ranges - breathtaking!"

Indeed it would be. So let's consider this more modest proposal: to
map the mountain ranges on a world 50 light-years distant. What's
required?

Roughly speaking, you'd need to be able to see detail as small as
about 50 miles (this is the necessary pixel size, as digerati would
call it). With a bit of high school physics, you can work out that
this necessitates a telescope with a mirror two thousand miles across.
It wouldn't have to be a one-piece mirror of course: you could borrow
a technique in vogue with contemporary astronomers, and construct your
instrument out of smaller, widely-separated, individual telescopes.
This mammoth spyglass would have to be space-based, to avoid
atmospheric blurring; but after all, if you can construct a telescope
this size, you undoubtedly have the technology to heft it into space.

In the accompanying figure, I've plotted the diameters of some
existing and proposed mirror and lens telescopes, and not
surprisingly, you can see that they have become larger over time. If
you make the daring assumption that this growth curve continues into
the distant future, then we will be able to build a 2,000-mile
telescope in the middle of the next century. In fact, it's conceivable
that we will do this much sooner, since large telescope arrays will be
easier to construct than the filled aperture telescopes of the type
shown in the plot. Goldin's vision, as it were, is not an impossible
one; certainly not for clever extraterrestrials.

But could they up the ante? Could a civilization for whom massive
engineering projects are commonplace ever build an instrument that
could actually see the life on Earth? Could they have detected the
dinosaurs, for instance, simply by imaging them? No, I don't mean the
far simpler task of detecting the oxygen or methane in our air that
betray biology on this planet. I'm asking, could they actually see the
animals?

Making even a crude picture of a stegosaurus (or us) would require
pixels of about one foot in size. At 50 light-years, that demands a
500 million mile telescope, one that - if we built it - would just fit
between the Sun and Jupiter. Of course that's an instrument of
ambitious dimensions. But what's to stop an alien civilization from
scattering small telescopes throughout its solar system, thereby
achieving this impressive aperture size?

Probably nothing. However, there's another problem. Would enough light
from the hide of that stegosaurus actually reach an extraterrestrial
telescope? On a clear, sunny day, every square foot of dino skin would
reflect about 10 billion billion photons per second back into space.
That's a lot of photons, but they spread out. At 50 light-years, it
takes a mirror that's 100 thousand miles in diameter to collect even
ONE of those photons each second (and since dinosaurs move, you need
short exposure times for the photo).

Bottom line: such a dino-detector would need the equivalent of 100
million billion Keck-size telescopes, spread out over a half-billion
miles of space. And we haven't even talked about the difficulties
alien engineers would face in precisely combining the data from these
instruments. Nor have we considered the image-scrambling effects of
interstellar gas. This is a project that should boggle the brain of
the most ardent futurist.

So what we can say is this: finding mountain ranges isn't terribly
hard. But making pictures of extraterrestrial megafauna is.

Of course, there's another approach: send robot probes to worlds with
life. We'll consider that in a future column.