  The proposed Satellite Test of the Equivalence Principle (STEP) will detect
possible violations of the Weak Equivalence Principle by measuring relative
accelerations between test masses of different composition with a precision of
one part in $10^{18}$. A serendipitous byproduct of the experimental design is
that the absolute (common-mode) acceleration of the test masses is also
measured to high precision as they oscillate along a common axis under the
influence of restoring forces produced by the position sensor currents, which
in drag-free mode lead to Newtonian accelerations as small as $10^{-14}$ g.
This is deep inside the low-acceleration regime where Modified Newtonian
Dynamics (MOND) diverges strongly from the Newtonian limit of General
Relativity. We show that MOND theories (including those based on the
widely-used $n$-family of interpolating functions as well as the covariant
Tensor-Vector-Scalar formulation) predict an easily detectable increase in the
frequency of oscillations of the STEP test masses if the Strong Equivalence
Principle holds. If it does not hold, MOND predicts a cumulative increase in
oscillation amplitude which is also detectable. STEP thus provides a new and
potentially decisive test of Newton's law of inertia, as well as the
equivalence principle in both its strong and weak forms.

General Relativity and Quantum Cosmology