Space technology demands

In the following, some substantial requirements for future space travel technologies and systems are summarized, which were defined by the European Space Agency (see ESTEC 1999, ESA 2001) and to which nanotechnology could contribute significant solutions.

4.1.1 Cost reduction Space Transportation

The main starting point for the cost reduction in space travel are savings in space transportation by reduction of mass and volume of spacecrafts and payload. At present, the costs amount to approx. 10.000 to 20.000 €/kg for transport into the earth's orbit (Janovsky 2001). Therefore a high incentive results for the miniaturization of spacecraft, which is possible in principle both on the level of components and modules as well as who- Miniaturization of satei le spacecrafts. Regarding the miniaturization of complete space systems ites at present, so called „Nano"-satellites (m< 10 kg)20 and even „Pico"-satellite (m< 1 kg) were examined, which possess as independent satelli- „Nanosatellites" tes among other things their own propulsion and control systems. The development of such satellites makes a progressive miniaturization of all subsystems and the supply of efficient and lightweight power supply systems necessary. The miniaturization of satellites however only makes sense if the payload can be miniaturizised without capability losses. This Um^l-mr^ °f mm^l-un-is for example not the case, if large antennas for observation or commu-nicaton are necassary or large solar cell panels are needed for the power supply. As further critical factors concerning the miniaturization of the payload, a sufficient signal response of instruments, devices for the cryogenic cooling of detectors or the equipment for the on board data handling should be mentioned.21 Nanosatellites are thus generally only suitable for special applications and missions. In the RAMES study (DARA 1995), as promising reference missions for nanosatellites, the detection of space debris or the measurement of the earth's magnetic field vector were zation

20 The definition of nanosatellites with a mass range of 1 to 10 kg is not unequivocal, some sources refer to other mass ranges e. g. up to 20 kg, see Caceres 2001)

21 see „Space Instrument Size Drivers" NASA Instrument and Sensing Technology

proposed among other things. Up to the year 2000, more than 20 nanosa-tellites were launched worldwide primarily for universitary or military research purposes. Meanwhile, also first beginnings of a commercial use of nanosatellites appear, operating for example from carrier platforms in space (Caceres 2001). On a long-term time horizon the application of nanosatellite swarms and constellations, which form huge highperformance sensor networks as „virtual satellites", seems to be very promising. The following tasks and application fields for nanosatelllite

Swarms of nanosatel- swarms can be mentioned:22

lites for visionary space applications • Simultaneous wide-area measurements for earth observation or plane tary exploration

• Sensor networks distributed over large orbit segments, which form a huge "virtual" sensor (for example for 3-D photographs)

• Swarms of co-operating small satellites, which form giant optical or microwave based devices for observation and communication missions

The potential of micro-/nanosatellite swarms is the subject of current investigations in the space community and will gain importance in the future. The ESA at present is in the phase of preliminary studies and prepares calls for proposals on this topic.

While the miniaturization of complete satellites is suitable only for special tasks, the reduction of weight and volume as well as energy-savings can generally be regarded as a priority objective for cost reduction in space. At present, however, a contrary development at least within the range of the telecommunications satellites occurs, where a trend to ever

Trend to ever hea larger and heavier satellites can be observed. But in this context, more vier satellites is limited by technologi- and more technical and financial borders arise from rarely manageable cal and economical huge solar cell panels, problems with the heat dissipation of electronics barriers as well as capacity bottlenecks of the carrier systems for space transportation. Therefore the need for lightweight, smaller and energy-saving

High demand for mass space systems will grow in the future. Nanotechnology could contribute and energy savings in solutions in this context in many areas, e.g.:23


• Data processing and system control (highly integrated avionics, wireless data communication, sensors etc.)

• Energy generation and storage (e.g. solar cell and fuel cell technology)

• Structure and thermal control elements (lightweight materials, miniaturized cooling loops and heat exchangers)

• Propulsion (electric propulsion technologies, MEMS-propulsion technologies)

22 personal communication Dr. Schlitt OHB-System AG, Bremen, March 2002

23 see Creasey et al. 2001

Data processing and control systems are the main energy consumers in spacecrafts. The development of miniaturized energy saving electronics could therefore lead to mass-savings through secondary effects for other subsystems (e.g. energy production, structure, thermal control elements). Further mass savings are expected by wireless data communication and highly integrated electronics. Within the range of systems for energy production, storage and distribution, which constitute up to 30 % of the dry weight in today's satellites, a further mass reduction can be obtained not only by energy-saving electronics but also by an increased efficiency. Reduction of mass can also be achieved within the structure range. New construction techniques e.g. grid construction instead of conventional sandwich constructions could lead to mass savings up to 60 % according to estimations of the ESA (Creasey et al. 2001). As explained in more detail in chapter 5, nanotechnology in a mid to long term time scale could also contribute significantly to mass savings in spacecrafts by lightweight construction materials, high-efficient energy production and storage technologies as well as energy saving highly efficient electronic components.

A further starting point for reduction of costs in space transportation is the development of re-usable space transport systems, which require among other things an advancement of re-entry technologies (e.g. reusable, high temperature-resistant components such as heat control surfaces and shields).

On a mid to long term scale significant mass savings are expected by the use of nanotechno-logical components and materials On-Board Autonomy

By increasing the on-board autonomy of spacecrafts, (e.g. autonomous attitude and orbit control, payload data processing, health monitoring of astronauts etc.) the operating costs for routine operations and fault corrections could also be lowered. This could be achieved by nanotechnolo-gically improved information and communication technologies and sensor technology.

Cost reduction by increased on-board autonomy COTS-Technologies

Further cost savings can be realized by using of COTS (Commercial off the Shelf) technologies. Cost-intensive technology developments, e.g. within the ranges of micro- or nanotechnology, are usually not feasible for the space sector due to budget restrictions. In these cases the space sector acts no longer as „technology pusher" but rather as a „technology follower" which examines market-ready technologies regarding their suitability for space applications and adjusts them for the specific space conditions. For this, application-specific modifications as well as space qualification of the terrestrial components have to be performed, to guarantee the required reliability and durability under the extreme space

Space sector in the range of nanotechnology rather a „technology follower" than a „technology pusher"

conditions (radiation, vacuum, mechanical impacts and vibrations, extreme temperature gradients etc.).

4.1.2 Increased capabilities

Improved capabilities of future space systems are a further substantial objective both for scientific and commercial applications. In context with possible applications of micro-/nanotechnologies, innovation task forces were established by the ESA dealing with the following topics:

• Improved communication performance

• Instruments and sensors breakthroughs

• Innovative components and materials

• Intelligent space systems operation

The objectives of these innovation task forces will be described in the following. Improved communication performance

Optical data transmission for future satellite communication systems

Optical Intersatellite links demonstrated by ESA in the frame of the ARTEMIS-Mission

Within the range of satellite telecommunications the aim is a drastic increase of transmission capacity and efficiency, in order to supply broadband communication services especially for mobile users and to manage the increasing data flood within the range of scientific space missions. Main starting point for this is the use of higher frequency ranges not only in the EHF range of conventionally used radio-/microwaves, but also in the optical frequency range in particular in the near infrared (NIR). The transition of radiowaves with working frequencies of about 40 GHz to optical satellite communication with frequencies of approx. 193 THz in the NIR range would increase the transmission capacity by several orders of magnitude. Also regarding size-, weight- and energy-savings, optical data communication offers clear advantages. To realize the potential of optical data communication, optical intra- and inter-satellite links as well as intersystem connections to ground stations have to be established.

Optical intersatellite links in space have already been successfully demonstrated by the SILEX terminal in the context of the ARTEMIS mission of the ESA (ESA 2001). The technological advancement of optical telecommunication systems is promoted by the DLR in the context of the LCT/MEDIS programme. One objective of the MEDIS mission is to demonstrate an optical inter-satellite link with a high data transmission rate between the European IS S module Columbus and a MEO satellite (Smutny et al. 2002). While optical intersatellite links thus have already been demonstrated, the realization of an "all optical" satellite communication still is far away. To be mentioned here for example is the transmission of optical signals from space to terrestrial ground stations, which is possible only with a cloudless sky due to light absorption in the atmosphere. This problem may be solved by a high redundancy of ground stations, sited on mountain summits in different regions, to increase the probability of a functioning data-link (Bland Hawthorn et al. 2002). In addition the availability of space-qualified micro-optoelectronic components such as lasers, amplifiers and modulators has to be improved significantly. Instruments and sensors breakthroughs

One focal point of the scientific and increasingly also commercial space applications is the earth observation. In this field improved instruments and sensors shall allow new applications in the future. Technological objectives to be mentioned in this context are (see Roederer 2001):

• a significant reduction of mass, ernergy consumption and costs of the instruments,

• improved detection methods in particular from geostationary orbit in the optical and microwave frequency range

• improved data communication and on-board data handling

Concrete technology developments are pursued e.g. for improved LIDAR systems (laser, new active components etc.), innovative optical sensor systems (micro-optical systems, camera-on-a-chip etc.) as well as for microwave sounding technologies from the GEO (antennas, front-end etc.). In this context the application of micro system technology will play a central role. Innovative components and materials

The topic „innovative components and materials" deals in particular with:

• innovative methods for three-dimensional integration of electronic components in compact modules (3 D Stacking),

• wide-band-gap semiconductor components (e.g. SiC, GaN)

• evaluation of MEMS for space application24

In particular within the range of WBG semiconductors, nanotechno-logical processes for the production of electronic components such as transistors, diodes and lasers will be essential. Components from WBG materials possess better characteristics compared with conventional semiconductors (e.g. GaAs) like an increased breakdown voltage, a better thermal conductivity, a higher temperature working area and radiation hardness. Thus on the one hand smaller and more efficient electronic components for applications under harsh conditions, e.g. for electronics in the proximity of rocket engines, and on the other hand also improved opto-electronic components within the UV range will be possible.

WBG semiconductors for radiation hard electronics

24 see Boetti et al. 2001 Intelligent space systems operation

In the frame of the Innovation Task Force „intelligent space system operations" the following objectives are pursued:

• increased „system-intelligence" (on-board autonomy, intelligent fault recognition and correction, increased fault tolerance and autonomy of the spacecraft crew etc.)

• remote controlled/ tele-present operations (user-interfaces for intelligent information and visualization systems, improved tele-manipulation systems, data capture and compression technologies etc.)

• suitable „End-to-end"-system architectures (for the autonomous operation of space systems e.g. for formation flights of satellite constellations, the monitoring of space transportation and reentry systems, the payload operations etc.)

• innovative space systems (miniaturized inspection probes for satellites or the ISS, innovative robotic systems for the exploration of space

The objectives aim at a long-term time horizon for future European space missions. Connecting factors to nanotechnology exist particularly in the development of high-performance and energy-saving data processors, storage and transmission as well as nanotechnological sensors.

One of the most important aspects for increased capabilities of space systems is an improved power supply, which is needed in particular within the range of telecommuncation satellites. For this on the one hand, high-efficient, lightweight and durable energy generation and storage systems (solar arrays, fuel cells, batteries and supercapacitors) must be made available and on the other hand the energy consumption of the space systems must be reduced by means of miniaturization. Since components of the energy generation, storage and distribution constitute at present up to 30 % of the mass of satellites, this would also contribute substantially to the cost savings by reduction of the launch mass (see chapter 4.1.1).

4.1.3 Lowering of mission risks

The costs of payload development for space missions and of the space transportation are usually very high, so that a reduced mission risk is gi-Nanomaterials and nano- ven a high priority. An important objective is therefore an increased reli-

technologically improved ability and durability of space components and systems. This might be sensors for more safety , • , r i u • j r ^ ■.,.■ j achieved for example by improved fault recognition and correction mein future space systems . .

thods as well as an increased fault tolerance. Here nanomaterials with improved mechanical and possibly intrinsic fault recognition and self-healing properties as well as nanotechnologically improved sensors could supply a substantial contribution. A further possibility of lowering the risk of space missions is to increase the redundancy of space components and systems. For example, if the mission task would be distributed among a multiplicity of small satellites, the loss of one satellite would be far less serious than if only one satellite would be used, whereby its loss would usually cause the entire mission to fail. Beyond that the capability of the whole system could also be increased by a network of small cooperating satellites (see chapter 4.1.1). In this context miniaturized spacecrafts (nano-, pico-satellites, inspection probes, etc.) will play an important role in the future.

4.1.4 Innovative system concepts

A further objective within the range of the space technologies is the realization of new system conceptions for different targeted applications. For example, the following space systems are under discussion, which partly possess visionary character:

• Constellations and swarms of miniaturized satellites and probes („na-no"-, „pico"-satellites, „flying chips" etc.)

• Stratospheric platforms (aerostats and gliders) for altitudes up to 45 km to complement satellites in some specific applications

• Gossamer Spacecrafts (very large light and self-unfoldable space Visionary space systems systems with integrated subsystems e.g. thinfilm solar cells or pha- based on nanotechnology sed-array-antennas) with applications in telescopes, mirrors, antenn- applications nas, starcovering-structures for the detection of planets outside the solar system, solar sails, solar power plants in space (e.g. European Sail Tower or NASA Sun Tower)

• Inspection probes, controlled either by the ground station or the spacecraft crew, for maintenance and monitoring of the spacecraft (satellite, space stations etc.) and/or the exploration of space objects (planets, meteorites etc.)

• Space elevator (visionary conception, consisting of a cable, which has its center of gravity in geosynchronous orbit and is manufactured from ultra strength materials with extremely high strength-to-weight ratio, like for example carbon nanotubes exhibit on molecular level, (see chapter

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