I think it's strange to believe that a starship could enter the solar system without being noticed due to the requirements of:
- velocity --- it would need to be moving quickly and would have to decelerate (requires energy) to do more than pass through
- life support/technology requirements and attendant heat signature --- interstellar space is _cold_ and not conducive to either, so requires on-going energy output
- size requirements --- basically, an entire ecological system needs to be moved around --- how many trees does one person require to produce sufficient oxygen for them to breathe? (a quick search has an answer from 1--8)
&c. See Kim Stanley Robinson's recent novel _Aurora_ for a well-researched examination of this.
- velocity --- it would need to be moving quickly and would have to decelerate (requires energy) to do more than pass through
- life support/technology requirements and attendant heat signature --- interstellar space is _cold_ and not conducive to either, so requires on-going energy output
- size requirements --- basically, an entire ecological system needs to be moved around --- how many trees does one person require to produce sufficient oxygen for them to breathe? (a quick search has an answer from 1--8)
&c. See Kim Stanley Robinson's recent novel _Aurora_ for a well-researched examination of this.