  Determination of the high density behavior of the symmetry energy through the
simultaneous measurement of elliptic flow excitation functions of neutrons,
protons and light clusters is proposed. The elliptic flow developed in
relativistic heavy ion collisions has been proven theoretically and
experimentally to have a unique sensitivity and robustness in probing the
symmetry energy up to around $2 \rho_{o}$. The knowledge of the density
dependence of the symmetry energy in a broad range of densities will provide a
missing link for astrophysical predictions of the neutron star mass--radius
relation. In particular, the data colud provide tighter constraints on the
slope parameter L and entirely new limits on $K_{sym}$, the currently poorly
constrained symmetry energy curvature parameter.
