As others have pointed out there are 50 thousand assumptions being made here, but one of the biggest is why you assume technology can ever be efficient enough f

As others have pointed out there are 50 thousand assumptions being made here, but one of the biggest is why you assume technology can ever be efficient enough for this? The entire universe is limited to the periodic table. You would need to have a device made entirely out of material that has a half life greater than millions of years, and a battery that can last that long without sunlight. I'm not saying it's impossible because technology is often surprising, but that's a fundamental limitation that so far we know cannot be overcome. Edit: Even if the robot behaved in a self-sustaining manner, this further assumes a perfect, unobstructed trajectory and asteroids full of essential materials are on the route to earth.