We construct 2-surfaces of prescribed mean curvature in 3manifolds carrying asymptotically flat initial data for an isolated
gravitating system with rather general decay conditions. The surfaces in question form a regular foliation of the asymptotic region
of such a manifold. We recover physically relevant data, especially the ADM-momentum, from the geometry of the foliation.
For a given set of data (M, g,K), with a three dimensional manifoldM, its Riemannian metric g, and the second fundamental
form K in the surrounding four dimensional Lorentz space time manifold, the equation we solve is H+P = const or H−P = const.
Here H is the mean curvature, and P = trK is the 2-trace of K along the solution surface. This is a degenerate elliptic equation
for the position of the surface. It prescribes the mean curvature anisotropically, since P depends on the direction of the normal.