An analytical study of the asymptotic behaviour of descending spikes is carried out for the idealized limit of an inviscid, incompressible fluid without surface tension, bounded by a vacuum. A self-similar solution is obtained for the shape of the free surface at the spike tip, yielding the evolution in time of the surface curvature there. The approach to free-fall acceleration is shown to follow an inverse power law in time. Results are given for both planar (two-dimensional) and axisymmetric spikes. Potential areas of application include ablation-front dynamics in inertial-confinement fusion.