At what point would we notice or be able to detect an object entering our solar system at 0.20 speed of light?
What's the maximum range we would be able to detect with certainty beyond a reasonable doubt (e.g. edge of Oort Cloud)?
And how much time would it take for that object to reach the inner solar system, or up to 5.2 AU (Jupiter's orbit), from the moment of detection?
For the sake of this question, the object's trajectory is not expected to intersect with any other stellar body in the system and its course is not expected to deviate (direct line). The speed is not expected to deviate, i.e. no external forces are expected to act upon the object to decelerate or accelerate it.
Let's say it's a:
- multi-generational ship,
- constructed from known metals and materials (nothing exotic),
- around 3km in length, with a 2km wide protective "asteroid-like" mass at the bow,
- weighing in around, say, 8.5 billion metric tons,
- with an albido of 0.5 - 0.8,
- it does not rotate,
- it would radiate in all the typical ways you would expect a ship of this nature to.