The presence in the space of micrometeoroids and orbital debris, particularly in the lower earth orbit, presents a continuous hazard to orbiting satellites, spacecrafts, and the international space station. Space debris includes all nonfunctional, man-made objects and fragments. As the population of debris continues to grow, the probability of collisions that could lead to potential damage will consequently increase. This work addresses a short review of the space debris "challenge" and reports on our recent results obtained on the application of self-healing composite materials on impacted composite structures used in space. Self healing materials were blends of microcapsules containing mainly various combinations of a 5-ethylidene-2-norbornene (5E2N) and dicyclopentadiene (DCPD) monomers, reacted with ruthenium Grubbs' catalyst. The self healing materials were then mixed with a resin epoxy and single-walled carbon nanotubes (SWNTs) using vacuum centrifuging technique. The obtained nanocomposites were infused into the layers of woven carbon fibers reinforced polymer (CFRP). The CFRP specimens were then subjected to hypervelocity impact conditions-prevailing in the space environment-using a home-made implosion-driven hypervelocity launcher. The different self-healing capabilities were determined and the SWNT contribution was discussed with respect to the experimental parameters.

Small space debris are a high risk for the walls of Composite Overwrapped Pressure Vessels (COPV), by making small holes and causing the fuel leak. Commonly the self-healing materials are used to keep the mechanical structure strength, here the hermeticity of the repaired portion is a stringent requirement, to prevent any potential fuel leak from the cryogenic tank in vacuum. The efficiency is compared for protective walls composed of a combination of various layers, using strong materials (Kevlar, Nextel) and self-healing commercial materials developed as bulletproof, e.g. the Ethylene-co-Meth Acrylic Acid (EMAA) and Reverlink TM .The small debris impact dynamic was detected and monitored with Fiber Bragg Gratings (FBG) sensors at very fast acquisition frequencies, up to 0.5 GHz (2 ns), measuring the variation of the total reflected signal by the FBG. The acquisition system is based on commercially available products. To measure the total wavelength spectrum, the fastest available spectrometer can go up to 2 MHz acquisition (Micron-Optics), which is insufficient to detect the hypervelocity impact. The impact pressure evolution of the FBG, placed in the middle layer, was compared with commonly used strain gauges placed a few layers further or on the back of the last layer. The measured impact time delay and relative intensity were compatible between the two sensing methods.Some samples were characterized in details using the X-ray Computed Tomography at ESTEC, they permitted us to confirm the results by observing the details of the healing and follow the impact trajectory visually.

It is surprising to see the wide range and versatile potential of applications of the VO2, due to its transition from a semiconductor phase at low temperature, to a metallic state at high temperature. Although this transition’s atomic mechanism is not yet well understood, the tuneability is very reproducible experimentally and can be monitored by various triggering schemes, not only by heating/cooling but also by applying a voltage, pressure, or high power single fast photonic pulse. Many of the recent applications use not only the low-temperature phase and the high-temperature phase, but also the transition slope to monitor a specific parameter. The paper starts with a summary of the VO2 thin film deposition methods and a table presenting its recent proposed applications, some of which our team had worked on. Then the development characterization and application of the VO2 as a smart thermal radiator is provided along with the recent progress. The experimental results of the emissivity were measured at low temperature and high temperature, as well as during the transition in vacuum based on the thermal power balance. These measurements were compared with those deduced from an average of Infrared Reflectance (2–30 µm) weighed with the blackbody reflection spectrum. The roadmap is to try alternatives of the multilayers in order to increase the emissivity tuneability, increase the device dimensions, have an easier application on space surfaces, while lowering cost.

SUMMARYThe Fiber Sensor Demonstrator (FSD), for ESA's Proba-2 satellite is the first demonstration of a full fiber-optic sensor network in the space environment on a satellite. MPB Communications (MPBC) has developed the FSD as a lightweight (<1.3 kg) fiber-optic sensing system. It was launched on Proba-2 in November 2009 and is still completely functional, almost seven years after its launching. FSD contains also the first fiber laser in space, it is used to sweep the spectral range and read the intensity at each wavelength between 1520 and 1560 nm. The optoelectronic components and data acquisition board are about 500 g, and the rest (<800 g) is the mass of the protective monolithic aluminium enclosure that has to resist to the launching mechanical shock and vibrations.The advantages of the FSD approach include a central interrogation module that is positioned remotely from the sensors and used to control a variety of different fiber-optic sensors, based on Fiber Bragg Gratings (FBG).The FSD contains six parallel fiber lines multiplexed within the interrogation module. Sixteen speciallypackaged FBG temperature sensors are distributed on four fiber lines. They monitor the temperature at different locations in the propulsion system and the spacecraft bus over the range of -60 °C and +120°C. The fifth fiber line is dedicated to a High-Temperature (HT) sensor to measure the transient temperature response of the SSTL thruster to beyond 350°C. The last fiber line contains an innovative Pressure/Temperature (P/T) sensor that provides both temperature and pressure measurements of the SSTL Xe propellant tank.The fiber sensors are monitored periodically over the seven years of flight. Their measurements fit well with the standard sensors used by the satellite. The evolution of the FSD response in space shows the fibers and Interrogation module parameters are within 10 % variations, including the laser diodes power and the FBG sensors intensity. The total gamma radiation received by the FSD is about 7 krad, during the 7 years.

MPB has developed a 10W Polarization Maintaining Optical Fiber amplifier (1550 nm) for space applications. The prototype is based on three stages of optical amplification with photodiodes at each stage, monitoring the output power. It includes the control electronics and software with feedback loops to dynamically control and monitor the amplifier. The design had to overcome many challenges to comply with the mechanical, thermal, radiation, and vacuum requirements for the LEO satellite space environment, while at the same time meeting the price targets for LEO constellations by maximizing the use of commercial off the shelf (COTS) components. The following were the main challenges: a) to effectively dissipate the heat generated (75-90 W); b) to select radiationtolerant electronics to drive the needed electrical current; c) to source and effectively implement components, such as the combiners and isolators, in the high power optical path compatible with vacuum at 10W output. The major challenge with regard to heat management was to find an optimal method to dissipate the heat from the third stage (high power) Erbium Ytterbium Doped Fiber. Commonly, this fiber is spooled on an Aluminium spool. The difference in the Constant Temperature Extension (CTE) between the fiber (low) and Aluminium (high) leads to a detachment of the fiber at low temperature with a high risk of breaking the fiber when passing from OFF to ON. At high temperatures, the Aluminium extends much more than the fiber, leading to an over tension on the fiber with a high risk of mechanical breakage. Different designs of the spool, supports inside the box, selection of materials, and process implementations were tried. An innovative, proprietary method was developed to satisfy this requirement. The unit successfully passed performance testing between -20°C and +40°C in vacuum with 10W output, with a wall plug efficiency of 11%. The lower temperature limitation was due to the specification of the high-power laser diodes. The higher temperature was limited by the local heating and risk of mechanical breaking of the third-stage COTS combiner and isolator. Vibration and mechanical shock are not foreseen to be an issue. The simulation demonstrated the prototype is complying with these requirements. Moreover, MPB has built similar instruments at lower power levels that have successfully passed these qualification tests. The components used were available as COTS products, including the radiation-tolerant electronics. All the components were qualified individually for > 30 krad, in vacuum, and for the temperature range -35°C to +65°C except for the highpower laser diodes which were limited to -25°C. MPBC is continuing the qualification, implementing minor design changes, in order to satisfy the complete temperature range (-35°C to +65°C).