  Do the laws of quantum physics still hold for macroscopic objects - this is
at the heart of Schr\"odinger's cat paradox - or do gravitation or yet unknown
effects set a limit for massive particles? What is the fundamental relation
between quantum physics and gravity? Ground-based experiments addressing these
questions may soon face limitations due to limited free-fall times and the
quality of vacuum and microgravity. The proposed mission MAQRO may overcome
these limitations and allow addressing those fundamental questions. MAQRO
harnesses recent developments in quantum optomechanics, high-mass matter-wave
interferometry as well as state-of-the-art space technology to push macroscopic
quantum experiments towards their ultimate performance limits and to open new
horizons for applying quantum technology in space. The main scientific goal of
MAQRO is to probe the vastly unexplored "quantum-classical" transition for
increasingly massive objects, testing the predictions of quantum theory for
truly macroscopic objects in a size and mass regime unachievable in
ground-based experiments. The hardware for the mission will largely be based on
available space technology. Here, we present the MAQRO proposal submitted in
response to the (M4) Cosmic Vision call of the European Space Agency for a
medium-size mission opportunity with a possible launch in 2025.
