So much has been written, both of fact and fancy, about satellites and space travel in the past few years that the selection of material suitable for presentation to the members of a learned Society concerned mainly with purely aeronautical matters is a hazardous and difficult task. I propose in this paper to tackle it by restricting the subject severely to a description of the general characteristics of near earth satellites and the scientific exploration made possible by their use as observing platforms. The later sections of the paper have a bearing on the possibilities and engineering problems of space probes and space travel; I leave it to others to speculate on the likely time scale and types of such activities and attempt instead to make sure that the basic factors involved are made reasonably clear. To set the background it is important to review briefly the actual practical achievements up to the present time.