The Bakerian Lecture, 1986. Ships from Space

Abstract

Narrow V-shaped wakes extending some 20 km behind surface ships were first found on Synthetic Aperture Radar (SAR) images from SEASAT in 1978. The V-wake geometry differed strikingly from the traditional Kelvin wake geometry consisting of divergent and transverse wave components generated by a travelling pressure point. The SAR images can be accounted for in terms of Bragg scatter from relatively short waves generated by the surface vessel. An essential ingredient of this hypothesis is that the wave generation is by an intermittent rather than a steady point source. Optical images from a hand-held camera on a 1985 space shuttle mission revealed many V-like wakes behind surface ships. There is no Bragg scattering from the ocean surface at optical wavelengths, so an alternative hypothesis is called for. We can interpret the observed features in terms of sun glitter from the tilted facets of a Kelvin wake. An essential ingredient is the generation by complex sources rather than by a single point source. We regard the present study as a step towards the interpretation of many unexplained naturally occurring features at the edge of the Sun glitter.