A space Debris Monitoring System for the ISS Based on Optical fibers

The purpose of the paper is to study the realization of panels with embedded optical fibers to show the feasibility of: i) an integrated health monitoring system for aerospace structural components, ii) an integrated modal parameteridentification system for aerospace structural components, iii) a space debris monitoring system. The last point has been the object of an informal presentation at the workshop meeting on the scientific exploitation of the International Space Station (ISS). The idea is that one of exposing a plate with embedded optical fiber sensors, either made of isotropic (aluminum alloy) or anisotropic (composite laminate) materials to the space environment around the space station. Space debris monitoring is one of the major concerns for the aerospace community because the debris population is the ever increasing and might endanger all future space activities. Once exceeded a certain threshold a chain reaction of impacts that generates more and more debris of smaller dimensions could inhibit any safe space mission. For this reason many studies are in progress to estimate the debris population and its evolution. To this end computer codes have been developed. These propagators assume an initial debris distribution which is indeed not well known for particles smaller than about 5cm. A plate with an embedded sensing system exposed to the space environment could give information on impact frequency, discriminate between single impacts or impacts from a group of particles, direction, intensity, and impact location. This last aspect even if hard to accomplish on the basis of the time delay signals caused by the interaction of ultrasonic elastic waves with the sensors could be tackled and could be usefully employed to determine the correspondence between the impact information described above and the chemical composition of the debris, this last one obtained after recovery of the plate. Chemical analysis can in fact be useful to distinguish between artificial debris and natural meteoroids. For this particular application it is not necessarily required a metallic plate but this one seems to offer many advantages over the polymeric composite one: i) the know-how acquired in the embedding process could be used also for the structural parts of space habitat, space structures, some of the shields used in the space debris bumper assembly; ii) easier modeling of elastic wave propagation in isotropic material, iii) metallic materials are prone to permanent deformation due to impact that can be measured at any time after the event. Let us now turn to the reasons why optical fibers are being considered in this study. Fiber optic and fiber optic sensors can be used as a very attractive sensing system because of the many advantages they offer with respect to conventional sensors. Most optical fibers, including the very common silica based optical fibers are very thin (125 - 155 microns in diameter), flexible (curvature radius lower than 0.01m), immune to electromagnetic and radio-frequency interference and non-conductive. This last aspect is essential when one thinks about embedding the optical fiber into a metallic material. As a further advantage one can think of the fiber (or chosen sections of it if you use the so-called intrinsic sensors) as both the sensing element and the transmission cable. By measuring variations of amplitude, wavelength, phase or polarization of the light due to environmental effects one can obtain information of almost any kind of physical property such as strain, stress, temperature, pressure. Due also to their small dimensions, their relatively high melting temperature and their very low transmission loss, optical fibers are very good candidate for being embedded into metallic materials with not very high melting temperature. Aluminum alloys for aerospace applications are for instance a very good choice. The possibility of embedding can suggest to use the optical fiber system not only during operation but also to monitor manufacturing parameters during forming, important for instance during the curing cycle of polymeric composites but also for metallic casting and subsequent thermal treatment. An embedded fiber optic system could be the sensing part of a more complex smart structure that is capable of reacting to the sensed environment. But it can work independently from the acting system with a negligible amount of energy and without complex control algorithms and hardware. Considering that optical fiber strain gauges have usually much greater frequency response than their electrical counterpart they can measure both static and dynamic structural response. A structure with an embedded fiber optic network can give therefore actual information of its dynamic properties and its evolution in time due to environmental effects. Consequently fiber optic sensors can also be used to identify the dynamic properties of a space structure or of space habitat under its actual operating conditions, that are very difficult to simulate with ground tests where air and gravity can play a not negligible role.