  All powered spacecraft experience residual systematic acceleration due to
anisotropy of the thermal radiation pressure and fuel leakage. The residual
acceleration limits the accuracy of any test of gravity that relies on the
precise determination of the spacecraft trajectory. We describe a novel
two-step laser ranging technique, which largely eliminates the effects of
non-gravity acceleration sources and enables celestial mechanics checks with
unprecedented precision. A passive proof mass is released from the mother
spacecraft on a solar system exploration mission. Retro-reflectors attached to
the proof mass allow its relative position to the spacecraft to be determined
using optical ranging techniques. Meanwhile, the position of the spacecraft
relative to the Earth is determined by ranging with a laser transponder. The
vector sum of the two is the position, relative to the Earth, of the proof
mass, the measurement of which is not affected by the residual accelerations of
the mother spacecraft. We also describe the mission concept of the Dark Matter
Explorers (DMX), which will demonstrate this technology and will use it to test
the hypothesis that dark matter congregates around the sun. This hypothesis
implies a small apparent deviation from the inverse square law of gravity,
which can be detected by a sensitive experiment. We expect to achieve an
acceleration resolution of $\sim 10^{-14} m/s^2$. DMX will also be sensitive to
acceleration towards the galactic center, which has a value of $\sim 10^{-10}
m/s^2$. Since dark matter dominates the galactic acceleration, DMX can also
test whether dark matter obeys the equivalence principle to a level of 100 ppm
by ranging to several proof masses of different composition from the mother
spacecraft.
