  This paper discusses the Laser Astrometric Test Of Relativity (LATOR)
mission. By using a combination of independent time-series of highly accurate
gravitational deflection of light in the immediate proximity to the Sun along
with measurements of the Shapiro time delay on the interplanetary scales (to a
precision respectively better than $10^{-13}$ radians and 1 cm), LATOR will
significantly improve our knowledge of relativistic gravity. The primary
mission objective is to i) measure the key post-Newtonian Eddington parameter
$\gamma$ with accuracy of a part in 10$^9$. $(1-\gamma)$ is a direct measure
for presence of a new interaction in gravitational theory, and, in its search,
LATOR goes a factor 30,000 beyond the present best result, Cassini's 2003 test.
Other mission objectives include: ii) first measurement of gravity's non-linear
effects on light to $\sim$0.01% accuracy; including both the traditional
Eddington $\beta$ parameter and also the spatial metric's 2nd order potential
contribution (never been measured before); iii) direct measurement of the solar
quadrupole moment $J_2$ (currently unavailable) to accuracy of a part in 200 of
its expected size; iv) direct measurement of the ``frame-dragging'' effect on
light by the Sun's rotational gravitomagnetic field to one percent accuracy.
LATOR's primary measurement pushes to unprecedented accuracy the search for
cosmologically relevant scalar-tensor theories of gravity by looking for a
remnant scalar field in today's solar system. The key element of LATOR is a
geometric redundancy provided by the laser ranging and long-baseline optical
interferometry. We discuss the mission and optical designs of this proposed
experiment.
