  Physical theories are developed to describe phenomena in particular regimes,
and generally are valid only within a limited range of scales. For example,
general relativity provides an effective description of the Universe at large
length scales, and has been tested from the cosmic scale down to distances as
small as 10 meters. In contrast, quantum theory provides an effective
description of physics at small length scales. Direct tests of quantum theory
have been performed at the smallest probeable scales at the Large Hadron
Collider, ${\sim} 10^{-20}$ meters, up to that of hundreds of kilometers. Yet,
such tests fall short of the scales required to investigate potentially
significant physics that arises at the intersection of quantum and relativistic
regimes. We propose to push direct tests of quantum theory to larger and larger
length scales, approaching that of the radius of curvature of spacetime, where
we begin to probe the interaction between gravity and quantum phenomena. In
particular, we review a wide variety of potential tests of fundamental physics
that are conceivable with artificial satellites in Earth orbit and elsewhere in
the solar system, and attempt to sketch the magnitudes of potentially
observable effects. The tests have the potential to determine the applicability
of quantum theory at larger length scales, eliminate various alternative
physical theories, and place bounds on phenomenological models motivated by
ideas about spacetime microstructure from quantum gravity. From a more
pragmatic perspective, as quantum communication technologies such as quantum
key distribution advance into Space towards large distances, some of the
fundamental physical effects discussed here may need to be taken into account
to make such schemes viable.

Quantum Physics;General Relativity and Quantum Cosmology;Space Physics