Kurt Feltenberger wrote:
 > If you wanted to observe a planet without being seen...

What does 'observe a planet' mean?
Continuous coverage of the entire surface needs multiple observers in
different positions.

Continuous coverage of a region requires a synchronous orbit, which is
pretty close to a planet by Traveller standards.

Etc.

 > assuming you had a stealthy enough ship...

Depends on the rule set and its tech assumptions.

For Traveller, electromagnetic masking with an overall volume of less
than 100 displacement tons seems a good starting point.

You are limited by how you want to observe the planet.
Electromagnetic spectrum only?
Neutrino emissions?
Gravity waves?
What resolution is required?

Is aperture synthesis possible to improve resolution?
How long are we allowed to watch the planet before integrating data?
(consider exposure time effects in astronomy).

Then: what sensor tech does the target have?

Sensor package + data storage + power supply (nuclear with battery
backup?) + propulsion system + robot brain + commo system (does the spy
phone home?) can be made pretty small at typical Traveller tech levels.

Rob O'Connor