In message , Hobbs aka
McDaniel writes
Even if ET have never sent a probe to our solar system,
they may have sent fast moving probes to other stars
and systems and those probes could have since drifted
into our system. The challenge is how do you find an
alien probe that's been inactive for perhaps eons?
Especially since we have no clue what such a probe
should look like either when it was launched or after
it has been pelted by meteorites for eons.

I think one relatively easy way to search for these
hypothetical probes would be to use a combination of
radar, lasers and visual spectrum telescopes. If
a probe has a large radio dish then when it is in
a certain orientation to earth (say within 40 degrees
of pointing at earth) it's radar signature will be
louder than that of many random shapes the same
size.

I suspect that the problem is that there are hundreds of thousands of
objects in the solar system the same apparent size (say 100 meters) as
your hypothetical probe. The fourth-power rule determining the return
signal from an object at an unknown location makes radar a very poor
tool for searching for such objects, which is why most searches for NEOs
use visual methods.
Radar has been used to map asteroids at known positions, and to look for
relatively nearby terrestrial probes when their status was uncertain
(such as SOHO), but needs enormous output power.
And the return signal is the only way to tell the size of an object
unless we already know its albedo, or it has some unusual polarisation
properties.
Anyway, I suspect that if ET is in the solar system he is hiding in the
anti-Earth Lagrange point, shielded from view by the Sun!
--
Save the Hubble Space Telescope!
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