Introduction

Microfabrication technologies (MST) allow the realisation of highly sophisticated products integrated with mechanical, optical and electronic functions completely. Such products are already exploiting very big markets in automotive and biomedical applications. Other sectors including aerospace are still observing the developments and are becoming aware of the potential applications of MEMS (Microelectromechanical Systems), MOEMS (Micro-opto-electromechanical Systems) and MST. The products will be accepted if the specifications are met. Companies, universities and research groups are more open to experimentation and many research projects in this field are financed from Space Agencies in the advanced countries (Europe, USA, Japan, Korea, etc.) (Liu 2003).

The field of nanotechnology is now explored worldwide by academia, research centers and industries. Applications and products are appearing on the market progressively. Nanotechnology is often merged with microtechnology to enable particular functionalities. It is applied on macroscopic products to enhance functionality. Many disciplines such as physics, chemistry, engineering, biology, and informatics are now converging into multi-disciplinary department in order to develop micro- and nanotechnology based products. One can note that the application of micro and nanotechnology to the aerospace field requires a very interdisciplinary engineering approach with a very wide field of view. In fact, from the consideration of material property at molecular level which affects the manufacturing at nanoscale (10 nm = 10E-9 m) to the comprehension of space mission requirements related to positioning of the satellite in orbit the engineer in charge of the mission study must have a general knowledge on a field spread over more than 15 orders of magnitude (Manzoni 2003) and a very specific knowledge on the 6 to 9 orders of magnitude (Fig. 9.1) covering the nanometer to millimeter size. Devices for aerospace application very often require environmental ranges, which are more extended than similar terrestrial devices. This complicates the design or the selection of materials and requires sometimes a dedicated manufacturing process. The system designers should consider the following aspects prior to the conception of component and architecture.

• The availability of reliable technologies

• The active and passive components of MEMS device

• Manufacturability

• Precision and standard

• Dimension and tolerances

• Performance

Fig. 9.1. Order of magnitudes

The aerospace field is characterised from the fundamental need of vehicle mass reduction in comparison with the payload mass and the reduction of the payload size and mass in relation to the mission requirements. Therefore, miniaturisation of the payload of the transporting vehicle is of primary interest. Space applications require also a higher level of intelligence of the systems since in the majority of the cases the human intervention is impossible or not fast enough. For such reasons, the miniaturisation and autonomy of the system are among the first targets of aerospace engineering and research. Such targets can be achieved by means of integrated systems involving microtechnology methods and using materials with enhanced performances. Several architectures are possible for the different systems, but in any case subsystems usually composed by hundreds of assembled elements, which are realised on a silicon wafer. A satellite is a very expensive product, composed by non-standard components, qualified with very high reliability and with a mission, from the initial phase, through realisation, launch and operation. It is also characterised by a very complex and expensive management. The concept is not any more fully justified for the enormous costs involved and therefore different possibilities can be explored. The Fig. 9.2 represents the mass of several satellites as a function of application domains. We observe how the mass can range from less than 1 Kg as in the PicoSat of DARPA to some tons like for the Hubble Space Telescope. Further classification follows.

• Mini satellite: above 100 kg

• Pico satellite: below 1 kg

Fig. 9.2. Satellite trends

Considering the launch cost and the structure of the satellite we clearly recognise the benefit of miniaturisation as the payload is directly related to the mass. Different principle and technology can separately influence the volume and mass of the spacecraft. Since the first concept of nanosatellite, represented in Fig. 9.3, fully manufactured on silicon wafers, has been proposed by "The Aerospace corp." in 1995 (Janson 1995), many research groups are on the way to produce and demonstrate such promising idea.

The reasons to build a nanosatellite are obviously related to the cost reduction in terms of launch and to the possibility to realise totally new class of missions. In practice the space community is still waiting the boom of the nanosatellite constellations, mainly because the enabling technologies are still not fully available or maybe because a different approach is necessary. The economical reasons to accomplish a mission only with nanosatellites are not fully demonstrated, especially due the high investments necessary to first demonstrate the reliability of such concept. The following four types of space based systems will be commercially developed in the next 10 to 15 years.

• Global positioning and navigation services

• Global communication services

• Information transfer services

• Global reconnaissance services

Fig. 9.3. The nanosatellite of the Aerospace Corporation

Each of these systems will be part of the infosphere, the volume where most of the information will transit. Other exciting concepts, like the use of many nanospacecrafts for planetary exploration and solar sail steering (http://solar-thruster-sailor.info/sts/sts.htm) are under study at Intelligent Space Systems Laboratory of the University of Tokyo (Furoshiki Satelllite). In that case the nanospacecrafts can have the function to provide attitude control to the sail and to realise the necessary tension by spinning the payload for the exploration. When such kind of collaborating sailing spacecraft fleet will reach the target, the sail can be removed and the spacecrafts can form a constellation around the planet, some of them can land on it and perform the exploration.

Each such nanospacecraft will perform one specific function and therefore will be equipped with one single instrument integrated with sensors and COTS (Component Off The Shelf). The whole concept of microspacecraft makes sense if it is designed based on a high modularity and standardisation. Several possible payloads can be easily connected to the spacecraft bus. Given the size of available sensing devices like imaging CCD or CMOS, accelerometers and gyros and magnetic sensors. It is reasonable to design a fully performing nanosatellite of the size of 10 cm. On this class of spacecrafts (Fig. 9.4) there is a big research activity worldwide, especially at university level, like the CubeSat XI developed at the University of Tokyo (Nakasuka et al. 2003), which was successfully launched on the 30th of June 2003. While CubeSat does not have all the functionality of conventional spacecrafts, it is expected that in near future it will be fully competitive. As can be seen in the Fig. 9.5 there is no much difference from a well packed PC or other electronic devices and a nanosatellite at the present status of development. Employing advanced technology and integrating miniaturised sensory systems the nanosatellite design would make the difference.

Fig. 9.4. CubeSat "XI" of the University of Tokyo

Fig. 9.5. CubeSat XI structure

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