This paper discusses the motivation and general design elements of a new fundamental physics experiment that will test relativistic gravity at the accuracy better than the effects of the second order in the gravitational field strength, G². The laser astrometric test of relativity (LATOR) mission uses laser interferometry between two micro-spacecraft whose lines of sight pass close by the Sun to accurately measure deflection of light in the solar gravity. The key element of the experimental design is a redundant geometry optical truss provided by a long-baseline (100 m) multi-channel stellar optical interferometer placed on the International Space Station (ISS). The spatial interferometer is used for measuring the angles between the two spacecraft and for orbit determination purposes. In Euclidean geometry, determination of a triangle's three sides determines any angle therein; with gravity changing the optical lengths of sides passing close by the Sun and deflecting the light, the Euclidean relationships are overthrown. The geometric redundancy enables LATOR to measure the departure from Euclidean geometry caused by the solar gravity field to a very high accuracy.

LATOR will not only improve the value of the parametrized post-Newtonian (PPN) parameter γ to unprecedented levels of accuracy of 1 part in 10⁸, it will also reach the ability to measure effects of the next post-Newtonian order (G²) of light deflection resulting from gravity's intrinsic nonlinearity. The solar quadrupole moment parameter, J₂, will be measured with high precision, as well as a variety of other relativistic effects including Lense–Thirring precession. LATOR will lead to very robust advances in the tests of fundamental physics: this mission could discover a violation or extension of general relativity, or reveal the presence of an additional long range interaction in the physical law. There are no analogues to the LATOR experiment; it is unique and is a natural culmination of solar system gravity experiments.