It's not a stupid question. Our limited understanding of the quantum world has led to many such questions, some with surprising answers!
To your question, and to address another point made here, the vacuum is not, unfortunately, a simple emptiness. It is actually incredibly complicated with (sometimes virtual) particles popping into and out of existence all the time. Quantum field theory deals with this, and if you want to crack into it, try Peskin & Schroeder's book.
You do not need an observer for phenomena to occur, and an asteroid in empty space will not superpose with another (assuming there were actually 2). Quantum field theory is still an evolving theory that is sadly incomplete, but in this area (QED and QCD) it is fairly well developed and enough results from CERN, Fermilab and other experiments have shown a substantial increase in the understanding of the standard model, which describes interactions of ordinary matter.
More to the point, every particle in the asteroid is interacting with other particles in the same asteroid. You cannot correctly expand quantum theories to macro objects because of the incredible number of emergent properties that come from collections of particles. Also, there is no reason to believe that a collection of particles with behave like a single large version of its constituents. Condensed matter physics is an area that deals with many of these emergent properties. Phonons are a great example of emergent physics that don't exist at the fundamental level. Schroeder has an amazing book on thermodynamics.
I hope this helps answer your question and gives you some food for thought.