"Martin Andersen" <martin@al-data.dk> wrote in message
news:41ab3773$0$156$edfadb0f@dtext02.news.tele.dk...
[.....]

Such information could be retrieved by sending a probe spaceship, capable

of

observing, doing calculations and communicating. A probe like this could
be smaller
than a grain of sand !

Interesting ideas, but there are a few theoretical limitations to
miniturization.

First of all, computation rate and memory have some limitation in space.
Scientific American actually had an excellent article about these
limitations
in this November 2004 edition.
Memory seems to be limited (and this is somewhat surprising) not to
the volume in which it is confined, but to the surface area....
Computation rate squared seems to be limited to the memory limit, so this
is also putting a bound on the area in which any intelligent probe needs to
operate. Now, the limitations are nothing to worry about, since a grain of
sand theoretically has enough surface area to store more info and compute
faster than all current earth's computers combined.

Second limitation is a bit more important : Assuming the probe wants to
do some observations, then the resolution is limited approximately
to the following formula :

  resolvable-length = R*w / r

where R is the distance from which the observation is done,
r is the diameter of the observer (aparture of the telescope used)
and w the wavelength of light used.

If the probe is sipping along 500km above the planet, and it wants to
resolve details of 10cm (4 inch) on the planet surface, then it will
need to be at least 5 meters wide.

Incidentally, having an extremely small probe will not only have blurry
vision (as explained above), but it will also be almost blind :
In order to capture photons from far-away semi-dark objects you need
some  capture surface. Sensitivity increases with surface area.
We should at least expect some reasonable size telescope on
board the probe.

The last limitation is probably the biggest problem with small probes.
Assuming that they need to communicate with their home-planet
the probe needs a transmitter. The transmitter operates on some
frequency. If that frequency is microwave, then the probe needs a
gigantic antenna (think bigger than Arecibo) to create a beam that is
narrow enough to drop a detectable signal thousands of lightyears
away. If the transmitter is in optical, then the probe needs a 10meter
telescope to beam laser pulses (or some other form of slow data
communication) to a telescope thousands of lightyears away on the home
planet. It is NOT an option to downsize the transmitting telescope.
The same resolution formula applies as the one above, and any
energy outside the resolution limit will be wasted energy.
Either way (radio or optical) the probe must be large : in the range
of 10 meters across.

So a grain-sized probe is extremely impractical : It is deaf, blind
and can't communicate back it's findings to the ET that sent it.
The only thing that it CAN do is compute...