The impact of space experiments on our knowledge of the physics of the universe

With the advent of space experiments it was demonstrated that cosmic sources emit energy practically across all the electromagnetic spectrum via different physical processes. Several physical quantities give witness to these processes which usually are not stationary; those physical observable quantities are then generally variable. Therefore simultaneous multifrequency observations are strictly necessary in order to understand the actual behaviour of cosmic sources. Space experiments have opened practically all the electromagnetic windows on the Universe. A discussion of the most important results coming from multifrequency photonic astrophysics experiments will provide new inputs for the advance of the knowledge of the physics, very often in its more extreme conditions. A multitude of high quality data across practically the whole electromagnetic spectrum came at the scientific community's disposal a few years after the beginning of the Space Era. With these data we are attempting to explain the physics governing the Universe and, moreover, its origin, which has been and still is a matter of the greatest curiosity for humanity. In this paper we will try to describe the last steps of the investigation born with the advent of space experiments, to note upon the most important results and open problems still existing, and to comment upon the perspectives we can reasonably expect. Once the idea of this paper was well accepted by ourselves, we had the problem of how to plan the exposition. Indeed, the exposition of the results can be made in different ways, following several points of view, according to: - a division in diffuse and discrete sources; - different classes of cosmic sources; - different spectral ranges, which implies in turn a sub-classification in accordance with different techniques of observations; - different physical emission mechanisms of electromagnetic radiation; - different vehicles used for launching the experiments (aircraft, balloons, rockets, satellites, observatories). In order to exhaustively present The Impact of Space Experiments on our Knowledge of the Physics of the Universe it would then have been necessary to write a kind of Encyclopaedia of the Astronomical Space Research, which is not our desire. On the contrary, since our goal is to provide an useful tool for the reader who has not specialized in space astrophysics and for the students, we decided to write this paper in the form of a review, the length of which can be still considered reasonable, taking into account the complexity of the arguments discussed.