Passive Sensor questions

[Gump (OP)]

If I have a passive sensor that can detect a strength 1000 emission at 10mkm, at what distance could it detect a strength 500 emission, or a 2000 emission?

As I understand it thermal sensors will detect a ships/missiles engines, and an EM sensor will detect active sensors and shields.  Which sensor type would be best for detecting colonised planets?

[Erik L]

Either should work. The colonies put off both EM and Thermal signatures.

[AL]

The formula for detection distance is (sensor strength/sensitivity) * (signature strength) * 1000km
The simple way to work out your detection distances in this case is to note that detection distance is directly proportional to the signature strength.  So with your sensor able to detect a 1k signature at 10m km, it will also be able to detect a strength 500 signature at half that range (5m km) and a 2k signature at twice that range (20m km).
There's more details on the wiki page for sensors if you're interested.

[MarcAFK]

That's for passive sensors of course, active sensors are somewhat more complicated, the detection range drops significantly when looking at objects smaller than they're designed for, and range is of course capped with no increase for larger ships.