Gordon I. Johnston*
Jet Propulsion Laboratory
Washington, D.C.
Wayne R. Hudson**
NASA Headquarters
Washington, D.C.
*Program Manager, Space Research and Technology Base; Member AIAA
**Assistant Director for Space; Member AIAA
Included in the strategic planning of the NASA OAST Space Directorate is the Global Change Technology Initiative, focused on the Mission to Planet Earth, technology for Earth system science, and the challenge of understanding global climate change.[1] As stated by Dr. Ride in her report to the NASA Administrator, "Mission to Planet Earth requires advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life." [2] The Global Change Technology Initiative will address these needs through research and development in observation technologies, information technologies, operations technologies, and spacecraft technologies.
The goal of this Global Change Technology Initiative is to provide the technology needed to enable and enhance the long term observation, documentation, and scientific understanding of the Earth as a system. This goal will be accomplished through both basic and focused technology research and development. Specific objectives include: the development of space-based instrument and observation technologies to enable the accurate measurement of key parameters crucial to the understanding of global change; development of data and information technologies to enable the long term documentation of these parameters and the timely understanding of the data; and the development of operations, servicing, and spacecraft technologies to enable consistent long-term collection of these data through increased system reliability and improved operations effectiveness. This initiative is being developed for a fiscal year 1991 start.
We are all becoming more aware of concerns such as the ozone hole and ozone layer depletion, the build-up of greenhouse gasses and the potential for global warming, the damage to our lakes and forests from acid rain, and the continuing drought. These are not only of international scientific interest, but of great national public, media, and congressional concern as well, with large potential impacts to our economy and way of life. Yet our current understanding of how our global environment behaves is incomplete, and does not allow us to predict with confidence the consequences or long term significance of these events.
Humanitarian concerns motivate us to observe the Earth as well. Natural disasters cause considerable damage world wide. Within the United States, the Federal Emergency Management Agency provided $133M in FY 1988, $149M in FY 1987, and $448M in FY 1986 in assistance for Presidentially declared disasters. In 1985, a tropical cyclone in Bangladesh killed approximately 10,000 persons, the Mexico City earthquake killed 10,000 persons, and the eruption of the Nevada del Ruiz volcano in Columbia killed 22,000 persons.[3] In December of 1987 the United Nations General Assembly designated the 1990s as the International Decade for Natural Disaster Reduction. Space based observations can help to mitigate the effects of natural hazards such as severe storms through improved understanding and prediction.
Space-based remote sensing observations of the Earth are required to document long term trends in global climate, and to form the basis for the detection and diagnosis of these trends. These observations will provide the input to computer programs that couple sub-models of the atmosphere, land, oceans, etc. into global models of the Earth system. Based on these models, predictions will be made and tested against actual observation to improve the quality of the models and the understanding they are based on. Research in the specific disciplines and processes will continue to improve and enhance the sub-models used.
A recent report of the Earth System Sciences Committee of the NASA Advisory Council listed seventy variables that require measurements for global analyses of the Earth, including fifty-six global variables that require sustained, long-term observation and measurement.[4] Of these, fourteen were rated essential for understanding global change, ten of which lack global coverage or are currently of poor or questionable accuracy.
Table 1. Essential Earth Variables ---------------------------------------------------------------------- atmospheric pressure rainfall/precipitation vegetation cover/color index soil moisture, biome parameters (extent, productivity, and nutrient cycling) sea surface temperature ocean wind stress ocean circulation ocean color/chlorophyll small scale plate deformations ---------------------------------------------------------------------- Long-term, sustained measurement are required for understanding global change. Currently lack global coverage or are of poor or questionable accuracy. ----------------------------------------------------------------------
In another report, a Committee on Space Research (COSPAR) ad hoc group identified what new technology should be developed to measure those parameters which have so far remained unobservable from space, but are crucial for the study of global change.[5] Included among these parameters are the rate, intensity, and distribution of global rainfall, the changing chemistry of the troposphere, and the fluxes of energy and gasses between the biosphere and the atmosphere. The ad hoc group recommends that substantial support be given to the development of instruments to fill the identified gaps, and high priority should be given to studies of techniques for the measurements of precipitation, soil moisture, and tropospheric chemistry and aerosols, both from space and from the surface with conventional techniques.
Within the United States, space-based remote sensing observations of the Earth are used extensively by private industry as well as the Federal Government.[6] NASA is responsible for advanced research and development activities to preserve U.S. preeminence in the exploration and exploitation of space. These responsibilities include conducting research and development in advanced space technology and systems; conducting research and experimentation to expand through space technology our understanding of the Earth and its environment; establishing an active, long-term Earth science program at NASA in coordination with other Federal programs; and providing for archiving and access to data acquired through NASA-supported research in remote sensing and Earth sciences. NOAA is responsible for environmental science applications and service programs to monitor the oceans, atmosphere, and land. NOAA also is responsible for developing and administering ocean and coastal resources management and regulatory programs. Other Federal agencies that use remote sensing data of the Earth to pursue their missions include the Department of the Interior (including the U.S. Geological Survey), the Department of Agriculture, the Department of Defense, the U.S. Army Corps of Engineers, the National Science Foundation, the Agency for International Development, the Department of Transportation, the Department of Energy, the Department of State (Bureau of Oceans and International Environmental and Scientific Affairs), and the Environmental Protection Agency. Satellites to observe the Earth are operated by NASA, NOAA, the DoD, and private industry. The NASA OSSA strategic plan includes the continued development of Earth observation capabilities and missions.[7]
Understanding the global environment is an international problem that requires an international approach. To verify satellite observations, ground truth measurements must be made in the regions being observed, requiring cooperation from the governments of these areas. Currently there are extensive international as well as national programs that are cooperatively working to improve the understanding of the Earth. Programs under such organizations as the World Meteorological Organization, the United Nations Environment Program, and the International Council of Scientific Unions (ICSU), seek to bring together and scientists from around the world to confront these problems. One important effort under the ICSU, focused on global change on time-scales of decades to centuries, is the International Geosphere-Biosphere Programme. ESA, Japan, the USSR, India, China, and France all have operating satellites to observe the Earth.
To further pursue this international effort to understand the Earth, a concept has been developed called "Mission to Planet Earth." This effort, described in both the Ride and Donahue reports, would consist of two to six polar platforms, Five geostationary platforms, and a series of special missions such as Space Shuttle payloads, Space Station Freedom attached payloads, and Explorer class Earth Probe missions. [2],[8] International partners such as ESA and Japan could provide two each of the polar and geostationary platforms. A complementary in situ observing system is included in the Donahue report, using automated and manned ground stations, balloons, aircraft, ships, ocean floats, and sounding rockets.
The key product of these efforts will be information and understanding concerning the Earth system. In the coming decades, national and international decision makers will be faced with critical and difficult policy issues, many of major economic impact. The restriction of chlorofluorocarbon production and use, the installation of scrubbing equipment on the smoke-stacks of mid-west power plants, and the future of low lying areas threatened by sea level rise, are just a few examples of the issues that future policy makers will face. In comparison with the potential economic impact of these decisions, the investment in the Mission to Planet Earth is small.
Current OAST programs, especially the Civil Space Technology Initiative, support the near term missions of the Office of Space Science and Applications with technology for sub-millimeter and laser sensors, coolers, on-board data processing and storage, automation and robotics, and dynamic control of large flexible structures. What is needed is a technology effort that takes the next step, beyond the five year scope of CSTI. As Pathfinder does for planetary exploration, this technology initiative will support and complement the programs of the Office of Space Science and Applications, looking to missions that are ten to twenty years in the future.
After a decade of reduced funding, the OAST Space R&T program has been seen continuing growth in recent years, motivated by a national recognition that investment in advanced technology is key to the health and future of the national space effort, which in turn is key to national leadership, prestige, and economic competitiveness. In addition to the level of effort Space R&T Base program that conducts generic and fundamental research in space technology, OAST has recently initiated the Civil Space Technology Initiative focused on near term and low Earth orbit missions, and Pathfinder, which is now focused on the key technical issues to enable future decisions on robotic and human exploration of the solar system. OAST has developed the In-Space Technology Experiment Program (In-STEP) for Fiscal Year 1990 to conduct technology experiments in the space environment, in those cases when the experiment cannot be conducted in Earth-based laboratories. To address the technology needs of the Mission to Planet Earth (the fourth Bold New Initiative option in the Ride report, and the only option not addressed by Pathfinder) OAST is developing a Global Change Technology Initiative for initiation in Fiscal Year 1991.
This initiative builds upon the experience and expertise within NASA to pursue the technologies needed to observe global change. Studies are being conducted to further define and develop the content of this initiative. Basic research, in areas such as spacecraft lifetime and reliability, that are generic in nature but are crucial to enabling long term missions to observe the Earth, will be encouraged under the Space R&T Base program. A focused technology program will be pursued beginning in Fiscal Year 1991 to develop and demonstrate key technologies for future generations of Earth observing missions in both low and geosynchronous Earth orbit.
The NASA Office of Aeronautics and Space Technology (OAST) conducts systems analysis studies under the Spacecraft Systems Analysis program to identify key technology needs and opportunities, and to develop integrated technology plans and objectives.[9],[10] The results of these studies support the development and advocacy of technology thrusts and focused technology initiatives within the OAST Space Research and Technology program. The recent successful advocacy of the Civil Space Technology Initiative (CSTI) and Project Pathfinder drew heavily upon the results of this program.
In Fiscal Year 1989 and 1990, studies are being conducted to identify and define the proper program content and emphasis for the Global Change Technology Initiative. These studies will identify technology needs and opportunities, develop integrated technology objectives and "road-maps", and refine funding requirement estimates and identify potential milestones and deliverables. Joint studies and workshops (with the NASA Office of Space Science and Applications, OSSA) will ensure that the scientific and operational requirements are addressed, and will facilitate the transfer to future users of the technologies to be developed under the focused technology initiative.
The Space Research and Technology program recognizes both the requirements of future missions and the opportunities for new technology to enable new missions. Since it can take as long as twenty years for a technology to transition from conceptualization to mission application, a projection of future mission needs is essential to aid in planning the program. The missions are described below.
The Mission to Planet Earth is an initiative to understand our home planet, how forces shape and affect its environment, how that environment is changing, and how those changes will affect us. The goal of this initiative is to obtain a comprehensive scientific understanding of the entire Earth System, by describing how its various components function, how they interact, and how they may be expected to evolve on all time scales. The challenge is to develop a fundamental understanding of the Earth as a system, and of the consequences of changes to that system, in order to eventually develop the capability to predict changes that might occur, either naturally or as a result of human activity.
The guiding principle behind the Mission to Planet Earth is the adoption of an integrated approach to observing Earth. The observations from various sensors on platforms and satellites will be coordinated to perform global surveys and to perform detailed observations of specific phenomena. Mission to Planet Earth requires advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life. Sophisticated sensors and information systems must be designed and developed, and advances must be made in automation and robotics (whether platform servicing is performed by astronauts or robotic systems).[2]
As stated by the Earth System Sciences Committee of the NASA Advisory Council, the goal of Earth system science is to obtain a scientific understanding of the entire Earth system on a global scale by describing how its component parts and their interactions have evolved, how they function, and how they may be expected to continue to evolve on all time-scales. The challenge to Earth system science is to develop the capability to predict those changes that will occur in the next decade to century, both naturally and in response to human activity. To meet this goal and challenge, the committee recommends: sustained, long-term measurements of the global variables to record the vital signs of the Earth system; fundamental description of the Earth and its history to deepen our basic understanding of the planet on which we live; research foci and process studies to bring research efforts to bear on key Earth-science problems; development of Earth system models to integrate data sets, guide research programs, and simulate future global trends; an information system for Earth system science to facilitate data reduction, data analysis, and quantitative modeling; coordination of federal agencies to ensure effectiveness and efficiency in program implementation; and international cooperation to further U.S. partnership in a worldwide research effort. Such a program may logically be implemented during two distinct eras of research opportunities: a near-term era, before 1999, that includes the flight of currently planned space missions and the conduct of essential process studies; and a new era, beginning around 1999, marked by the establishment of a comprehensive observing program for Earth system science employing a new generation of space technology and an integrated suite of ground-based instruments. [4] It is in support of this new era of Earth observations and study that this technology initiative is being developed.
The three mission classes that form the space segments for the Mission to Planet Earth are the low Earth polar orbit Earth Observing System (Eos) platforms, the Earth System Explorer Missions, and the advanced Geostationary Earth Science Platforms.
The first space segment, Eos, consists of two U.S. and one ESA polar platform. The two U.S. platforms are currently being developed by NASA for launch in 1996 and 1998, respectively. Current plans include servicing missions to provide for growth in the number and quality of remote sensing capabilities on these platforms through the year 2000 and beyond, and they are expected to operate at full capacity through 2010. Many of the crucial measurements would be obtained for a period of at 15 years. Plans for the two U.S. Eos polar platforms are well under way. The platforms are being developed as part of the Space Station Freedom program, and the Office of Space Science and Applications will announce the selection of the scientific investigations in February of 1989. The Eos will establish the research capability of advanced instrumentation, such as high-resolution spectrometers, multi-channel radars, and space-based lidars. Such instruments will yield important new and improved Earth measurements, including mineral composition, land-surface vegetation, cloud properties, and the deformation of continental plates, as well as the measurement of atmospheric winds, aerosols, boundary-layer properties, and certain trace constituents. The extensive set of individual instruments in the Eos payload is required because of the different measurement capabilities needed to observe the various regions of the electromagnetic spectrum by passive and active remote sensing techniques.
The second space segment is a series of Explorer-class missions and the use of well established instruments mounted on long term platforms such as the Space Station. In spite of the powerful synergisms within Eos, there are some observing needs that require other low Earth orbit configurations or dedicated spacecraft. Notable needs in this category include measurements of the the Earth's gravitational field from an orbit sufficiently low to yield adequate spatial resolution, measurements of the precipitation throughout the diurnal cycle with active microwave techniques, observations of the Earth's magnetic field using sensors isolated from electrical interference, and investigation of the properties of the thermosphere for which in situ sampling is necessary.
The third space segment of a total system for global Earth observation will be provided by advanced platforms in geosynchronous orbit. These offer several fundamental advantages over other platforms. First, high temporal resolution, limited only by instrument design and cost, can be brought to bear on the study of rapidly changing, global atmospheric phenomena. Another major advance would be passive-microwave sensing of regions of precipitation. The capability of microwave sounding is not now available because of the large antenna required for adequate spatial resolution at geosynchronous orbit altitudes. Geosynchronous orbit furthermore provides a fixed reference geometry for a given Earth location, facilitation data analysis and interpretation. Operational geosynchronous satellites have been in service since 1974 and carry imager/sounder instruments providing high resolution visible and infrared images of the Earth. The infrared channels of the sounding instruments provide temperature and moisture profiles over large areas of the Earth with high frequency. NOAA now operates two GOES geostationary satellites and will continue to support the operational need for weather monitoring and prediction.
As stated by Dr. Ride in her report to the NASA Administrator, Mission to Planet Earth requires advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life. Sophisticated sensors and information systems must be designed and developed, and advances must be made in automation and robotics (whether platform servicing is performed by astronauts or robotic systems). In addition, Mission to Planet Earth will require large Earth observatory platform structures in both low and geosynchronous orbits. Since the envisioned geostationary platforms are quite large, they will either need to be lifted to low-Earth orbit, assembled at the Space Station, and then lifted to geosynchronous orbit with a space transfer vehicle; or they will require the direct deployment in geosynchronous orbit of complex precision structures on a scale never before achieved. If assembled at the space station, these platforms will require well-developed orbital facilities, the Space Station will have to be able to support on-orbit assembly, and a space transfer vehicle must exist.
Earlier reports have discussed the technology requirements for Eos, and this information has been used to incorporate the needs of the Eos into the current Space R&T program.[9],[11] The Global Change Technology Initiative is not needed for the implementation of Eos. However, current Eos plans include servicing missions that will allow the addition of improved mission and instrument capabilities and subsystems. The Global Change Technology Initiative will include research and technology development for instruments and subsystems that could be added to the Eos via servicing, as well as capabilities for future generations of low Earth orbit missions beyond the current Eos missions. A key area for continued development will be long life, tunable solid state lasers for future lidar applications.
Many of the technology requirements for the Mission to Planet Earth, especially the Eos, are being addressed by current OAST programs, including the Civil Space Technology Initiative (CSTI) and Project Pathfinder. CSTI, which is a near-term (five year) program focused on low Earth orbit applications, includes program elements in automation and robotics; sensors; on-board data processing and storage; large structures and control; and aerobraking (for servicing vehicle return). Project Pathfinder will develop technology options for potential Agency bold new initiatives with the exception of the Planet Earth initiative, but does include program elements in optical communications; autonomous rendezvous and docking; in-space assembly and construction; and fault-tolerant systems which are of importance to the Mission to Planet Earth.
The Global Change Technology Initiative will complement and build upon the existing OAST programs, addressing the specific needs of Earth observation applications. For example, the sensors work in CSTI is directed towards low background astrophysics applications, such as the Large Deployable Reflector (LDR) infrared telescope. The LDR would not be able to tolerate the heat load it would experience if it were used to observed the warm Earth rather than the cold background of space. In this case the Global Change Technology Initiative will complement the ongoing CSTI program by developing technologies for high background observation applications.
The three themes identified by Dr. Ride, "advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life," form the basis for the structure of the Global Change Technology Initiative (Figure 1). The Observation Technologies thrust will advance the technology to enhance observations, the Information Technologies thrust will advance the technology to handle and deliver the enormous quantities of data, and the Operations Technologies and Spacecraft Technologies thrusts will advance the technology to ensure a long operating life.
The Observation Technologies thrust will focus on the scientific requirements for sustained, long-term measurements of global variables through the development of spacecraft and space-based instrument technologies. This thrust will include precision pointing and vibration control, optical systems, cryogenic systems, laser and radar sources for active sensing, sensors and detector arrays, and the study of approaches to enhance instrument stability and calibration with decreased degradation from contamination and space environmental effects. Of special concern are the technology developments to enable large (e.g. 40-80 meter diameter) precision antennas in geostationary orbit for precipitation monitoring, atmospheric temperature and pressure sounding, and soil moisture measurements.
A major advantage of geostationary orbits for Earth observing platforms is the ability to acquire continuous observations with high temporal resolution. As shown in figure 2, Earth systems processes which require high temporal resolution observations also require high spatial resolution. Despite the substantially greater altitude of geostationary orbits over the orbits of the Eos and the Explorer class missions, the phenomena observed require the highest possible spatial resolution. This in turn places greater requirements for precision pointing and platform control.
Technology developments in precision pointing and vibration control for remote sensing instruments are important for the accurate pointing of multiple instruments and for large (40-80 meter diameter) radiometric antennas. This will allow simultaneous and continuous observation of the Earth by multiple instruments with minimal interference. This effort will work synergistically with the platform structural concepts effort described under the Spacecraft thrust below, and will develop technologies for precision alignment and/or compensation for deformations, momentum compensation for scanning instruments, and inter-instrument isolation. The pointing requirements of the precision instruments on the platform require that the large radiometric antennas not disturb the dynamics of the entire platform. The need to decouple the motions of these structures may require novel solutions such as tethered platforms or a network of free flying spacecraft.
To allow quality, long-term, and continuous observation of Earth processes on local to synoptic scale from geosynchronous orbit, optical systems technology research will be conducted, including diffraction gratings, ultraviolet thin films, electro-optic crystals, acousto-optic filters, hologram optical elements, and optical system performance modeling. This will develop an optics technology base supporting ultra-violet, visible, and infrared high resolution (spatial, spectral, and temporal) observation of the Earth.
Technology research and development is needed for cryogenic systems that can provide the necessary long-term instrument cooling while handling the thermal input of continuous observation of the warm Earth. These coolers and systems will require integration with platform thermal systems. Unique technology problems are associated with servicing cryogenically cooled instruments.
Active sensing using radar or lidar is not currently planned for advanced geostationary platforms due to the high output power required to obtain adequate return signals. Precise geodesy measurements using laser sources on the geostationary platform and Earth based retro-reflectors may be conducted. Future advances in spacecraft power and radar and laser source efficiency may allow consideration of active sensing from geostationary orbit. Developments in long life, tunable solid state laser sources for active sensing to measure winds and troposphere trace gasses will be required for future Eos servicing and follow-on missions.
Advanced sensors and detector arrays are required for remote sensing across the electromagnetic spectrum from the microwave to the ultraviolet. Two regions of the spectrum, the sub-millimeter/far infrared (30-2000 GHz) range and the thermal infrared (5-17 micron) range, are especially critical for understanding global climate change. The sub-millimeter range is used for the precipitation monitoring as described below, and will require developments in sensors as well as supporting systems such as mixers and oscillators. The thermal infrared range corresponds to the peak of thermal emission from the surface of the Earth, and is the region of the spectrum in which the "green-house" gases play a crucial role.[12] The materials for these sensors must be highly reliable and stable, with minimum impact from contamination, radiation, and the space environment, in order to provide the long term and sensitive measurements necessary to detect long term changes in global climactic parameters that are buried beneath the day-to-day variations in weather.
Improvements must be made in the ability to calibrate and compare data from diverse instruments and spacecraft, for both relative and absolute measurement values, in order to allow the synthesis of information obtained over time and across the spectrum. Techniques to improve instrument performance and reduce instrument noise, such as chip-level integration of data preprocessing (e.g. including the pre-amp and analog-to-digital conversion electronics on the sensor chip) could maximize the performance from both existing and new sensor technologies.
Large precision antennas are required in order to obtain microwave sounding with adequate spatial resolution from geosynchronous orbit. Since it is estimated that about half of the Earth's rainfall occurs in short lived, small scale storms, resolution corresponding to the size of these storms (10 kilometers) is needed to provide complete rainfall monitoring data. As shown in figure 3, observations at 36 GHz with an Earth footprint of 10 kilometers require an antenna diameter of 40 meters. These large antennas will require precision shape correction and steering to allow coverage of the globe, either mechanically or through receiver array adjustments. Measurements above about 36 GHz require solid surface reflectors while lower frequency measurements can use large mesh reflectors. Unfilled aperture or interferometric techniques could provide an alternative approach for these large (greater than 40 meters) antennas required for frequencies less than 36 GHz. Special microwave transparent structural materials may be required to achieve the instrument performance requirements.
In the Information Technologies thrust, human factors research in scientific visualization, and research in both on-board and ground data/information technologies such as software engineering, advanced computing, data storage, and data/information networks, will support the technology needs for the Mission to Planet
Earth. This will include development of integrated computer models for Earth system science and an information system capable of enabling the analysis and understanding of integrated data sets from both space-based and in situ instruments. Optical communications developments will enable the very high rate collection and relay of data and information between spacecraft in low inclination, polar, and geosynchronous orbits.
By the mid-1990s, space-based remote sensing observations of the Earth will be acquired at the rate of about 10 trillion bits per day, and this trend will continue with the addition of the Earth Science Geostationary Platforms. Converting this stream of numbers into information that can be understood and used by the scientific community will be a high pay-off area for technology development. Human factors research to enhance scientific visualization will seek ways to improve the man/machine interface for the interchange of scientific information, and to best use the unique pattern recognition and cognitive capabilities of human beings to review and assimilate the massive amounts of data that will be received.
Mission to Planet Earth platforms will require on-board processors capable of handling the high data rates and large data volumes generated by the multiple scientific and operational instruments. Data system approaches with an "open" architecture, employing local area network management to support additional instrument and system upgrades, will be required to enable the servicing option. Research will be conducted for the development of space qualifiable component technologies such as on-board, high performance integrated circuit fiber optic transceivers and low power, high speed, radiation hard GaAs processors. Tools and techniques to design, simulate, produce, and test application specific integrated circuits will be developed. Research will be conducted into high performance parallel computing architectures appropriate for use in on-board data processing.
Software engineering research will develop the tools and techniques to enhance software reliability, and facilitate the use of new parallel architectures for high performance parallel processor computers, the processing architecture needed for global ocean-atmosphere-biosphere system simulations. Advances in software and computing technologies will facilitate the development of a system for Global Computing.
The nature of geostationary orbits greatly simplifies the direct data communications requirements by remaining fixed relative to the Earth, allowing large dedicated ground stations that can provide the needed telemetry capability without interruption. However, these geostationary science platforms could be used as relay stations for the collection of data from other geostationary platforms not in view from the United States, from low Earth orbiting platforms including the Eos, and from aircraft, ocean floats, and other in situ sensors. These platforms could be used as well for the dissemination of scientific information to the national and international scientific community. For these applications, technology development leading to a possible flight demonstrations of optical communications will be pursued. Optical communications will increase the capacity and capability of information system data communications, as well as decreasing antenna size, which will result in improved platform stability and pointing. Technology research needed includes the development of high performance semiconductor lasers.
In the Operations Technologies thrust, automation technologies for on-board and ground-based systems will enable more flexible spacecraft and inter-spacecraft operations coordination. This could enable the direct control of scientific observations by the scientists from their home institutions (i.e. telescience), as well as increasing capabilities vs. costs by enabling more efficient ground operations for these long duration missions. Automation and robotics research will study the development of on-orbit robotic and telerobotic servicing to increase the mission lifetime, upgrade mission capabilities, and reduce the demand for astronaut EVA.
Operations automation technologies for on-board and ground-based systems will seek to automate the routine and/or repetitive house-keeping functions associated with the day-to-day operation of spacecraft, either on-board the spacecraft, utilizing the advanced data system capabilities discussed above, or in the ground mission operations computer system. System approaches that minimize the interaction of spacecraft subsystems will simplify the command generation process, and could allow telescience, the direct control of scientific observations by the scientists from their home institutions, as well as increasing capabilities vs. costs by enabling more efficient ground operations for these long duration missions.
Considerable experience will be gained concerning the tradeoffs between EVA, telerobotic, and robotic servicing as a result of the Space Station Freedom program. It is unclear whether and at what point servicing will become a viable option for geostationary platforms. The energy required to access geostationary orbits limits the ability to perform servicing missions and will increase the importance of long life, increased reliability, and increased efficiency in the use of expendables. Advanced geostationary platforms will be too massive to return to low Earth orbit and there are no piloted vehicles currently envisioned that could access geostationary orbit, so any servicing mission will require robotic or telerobotic capabilities. Even if initial geostationary missions are not designed to rely upon servicing, advanced and operational missions may depend upon servicing to repair, resupply, and upgrade the capabilities of the platform. Platforms designed for servicing will require adaptable power, data/control, and thermal management capabilities to provide the flexibility for these future upgrades, as well as instrument and subsystem designs incorporating serviceability. An on-board robot or telerobot designed for the specific environment of the platform could simplify servicing design requirements. Accurate ground models of the platform, instruments, and subsystems will be required to enhance development, integration, and to verify growth possibilities. Servicing missions could use aerobraking in the Earth's atmosphere for the return from geosynchronous orbit to reduce the mass and fuel requirements.
Under the Spacecraft Technologies thrust, the long term, sustained nature of the Mission to Planet Earth will be enhanced through basic technology research and development to increase spacecraft reliability and lifetime. This will include technologies in areas such as reliability and quality assurance, non-destructive inspection and evaluation, long life materials and structures, platform structural concepts, power and propulsion systems, thermal control systems, platform charging, and space environmental effects.
Studies in reliability and quality assurance will include investigations into testing techniques and tradeoffs such as thermal cycling vs. thermal dwell for testing and flight certifying electronic assemblies, development of improved transient dynamic load test techniques for spacecraft structures and spacecraft equipment, and improved vibration test techniques.
Non-destructive inspection and evaluation (NDE/NDI) techniques will be used to develop "smart structures." These will include the development of techniques to integrate fiber optics sensors within critical structural components to provide real-time information about strain, temperature, configuration, impact damage, and radiation degradation. These will be developed, along with NDE/NDI approaches using thermal and ultrasonic health monitoring and laser based optical monitoring systems, to provide reliable information about the state of the spacecraft during operations, and could enable on-board fault detection and correction.
Technology research in long life materials and structures will develop and characterize new structural materials for long-term operation in both low and geostationary Earth orbit environments. This will include scaling up and characterizing promising new material systems into structural sub-elements for large precision platforms and reflector support structures. Currently available materials may not provide the required specific stiffness for very large structures. These materials will need to be characterized for their long-term stability in the low and geostationary Earth orbit environments. Degradation mechanisms of composite materials in these environments are not fully understood, and the database for new materials is very small.
Platform structural concepts research will develop components, materials, and stable platform concepts to enable operation of large Earth science and operational platforms in geostationary orbit, including large (40-80 meter diameter) radiometers. Technology trade-off studies are required to determine if these large structures should be deployed or assembled in space. Development of long life/low vibration bearings and space devices (reaction wheels, magnetic torques, etc.) will be required to minimize interference with the observational systems.
Propulsion research and technology will develop long life, high specific impulse, low contamination propulsion systems for orbit transfer, orbit maneuvering, and station keeping, with minimum contamination and resupply requirement.
Power research and technology will develop high performance, autonomous, light weight, and reliable power generation, storage and distribution components for high density power systems to support platform operation, active sensing, and propulsion with minimum maintenance and ground interaction. These will allow advanced science, advanced propulsion, and improved communications, and the possible use of electric propulsion to minimize propellant requirements for geostationary platform low to geostationary orbit transfer and on-orbit station keeping.
Research and technology development in space environmental effects will be included. Integrated thermal control systems and designs will be developed for advanced Earth science platforms. Platform charging model capabilities will be developed to assess vulnerability to spacecraft charging of the Mission to Planet Earth platforms, and technical approaches for mitigation will be developed to allow long-term operation without interruptions or damage from geomagnetic substorms. Spacecraft charging is a concern for all geosynchronous spacecraft and the phenomena is increased by size and voltage levels. Similar efforts will be conducted in contamination, radiation damage, and debris damage.
Multi-instrument Earth science platforms planned for both low and geostationary Earth orbit will provide data from a suite of scientific instruments, and provide systematic and cross correlated data sets to address systematic and cross discipline scientific questions. These spacecraft will require highly complex systems that will allow a wide variety of simultaneous activities, some requiring coordination and some requiring independence, with high reliability and reduced ground operations cost. These missions will produce data at extremely high rates and in massive amounts.
Spacecraft will continue to grow more complex physically and functionally. They will require advanced detectors and optical systems across the full range of the electromagnetic spectrum, and will require improvements in precision pointing in order to optimize the output of the sensors. Substantial augmentation to spacecraft communications, on-board processing, and data storage capabilities will be needed. Many spacecraft will have to have utilities that function autonomously, and may be capable of on-board fault detection and correction.
Advanced geostationary and low Earth orbit science platforms will play crucial roles in addressing the challenge and uncertainty of global climate change, and will lay the foundations for an operational Earth science system that would benefit all of the inhabitants of our planet.
1. "1989 Long-Range Program Plan," National Aeronautics and Space Administration, Washington, D.C.
2. S. K. Ride, "Leadership and America's Future in Space," A report to the (NASA) Administrator, August 1987.
3. "Confronting National Disasters, an International Decade for Natural Hazard Reduction," Advisory Committee on the International Decade for Natural Hazard Reduction, Commission on Engineering and Technical Systems, National Research Council, U.S. National Academy of Sciences, U.S. National Academy of Engineering, National Academy Press, Washington, D.C., 1987.
4. Report of the Earth System Sciences Committee, NASA Advisory Council, "Earth System Science, A Closer View (A Program for Global Change)," January 1988.
5. "Potential of Remote Sensing for the Study of Global Change, COSPAR Report to the International Council of Scientific Unions (ICSU)," edited by S. I. Rasool, published for the Committee on Space Research (COSPAR) by Pergamon Press, Advances in Space Research, volume 7, number 1, 1987.
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9. Wayne R. Hudson, Eugene V. Pawlik, "Spacecraft Technology Requirements for Future NASA Missions," Paper No. 1160-cp, AIAA Space Systems Technology Conference, June 1986.
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12. Stephen H. Schneider, Randi Londer, "The Coevolution of Climate and Life," Sierra Club Books, San Francisco, pp. 195-198.
Conference paper presented at the AIAA 27th Aerospace Sciences Meeting (paper no. AIAA 89-0254), January 9-12, 1989. Converted to HTML November 25, 1994.