Furthering Exploration - International Space Station Experience

AIAA 2006-5978 SpaceOps 2006 Conference Furthering Exploration – International Space Station Experience Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 Gary Kitmacher* and Theresa G. Maxwell† NASA Headquarters, Washington, DC, 20546, USA The International Space Station (ISS) is instrumental to the exploration of space. As we expand human presence from low Earth orbit, in the next decade to the Moon, and later, to Mars and beyond, we will face challenges in management; integration; remote, longduration, assembly and maintenance operations; science and engineering; and international culture and relationships. The ISS Program is providing critical insight and amassing new knowledge in all of these areas. Use of the expertise gained in the ISS Program can reduce risks in the human exploration of space. This paper discusses the applicability of the ISS experience to the Vision for Space Exploration (VSE), specifically in the areas of crew operations, spacecraft systems operations, and crew-system interface operations. I. Introduction – the Vision for Space Exploration On January 14, 2004, President Bush announced the Vision for Space Exploration. It establishes a course that expands the human presence beyond the Earth - first, in near-Earth orbit on the ISS; then in the next decade, to the Moon; and later, to Mars and beyond. The National Aeronautics and Space Administration (NASA) has unveiled plans for the next generation spacecraft, the Crew Exploration Vehicle (CEV), which will take us there. Completing assembly of the ISS by the end of the decade, and fulfilling commitments to the International Partners, is a crucial first step in human exploration. NASA is refocusing ISS research to meet the VSE requirements. As humans venture further from Earth, and as program timetables and mission logistics increase in time, distance and complexity, it will be crucial to have crews and vehicles that can be sustained with greater reliability in the harsh rigors of space. The ISS mission can directly support these Agency needs in the following areas: 1. 2. 3. Develop, test and evaluate biomedical protocols to ensure human health and performance on longduration space missions Develop, test and evaluate systems to ensure readiness for long-duration space missions Develop, demonstrate and validate operational practices and procedures for long-duration space missions II. The International Space Station Experience The International Space Station is a technological undertaking of global scope. Elements of the ISS are provided and operated by an international partnership of governments and their contractors. The principals are the space agencies of the United States, Russia, Europe, Japan, and Canada. The ISS has been continuously crewed for more than five years and is about 50% complete with approximately 186 metric tons of mass on orbit. There are 15 elements in orbit today, 9 elements ready for launch at the Kennedy Space Center in Florida, and 7 elements in process at International Partner sites. When assembly is complete, the ISS will be comprised of 453 metric tons (nearly a million pounds) of hardware, orbited in about 40 separate launch packages over the course of more than a decade. To date, there have been over 50 flights to the ISS, including flights for assembly, crew turnaround, and logistical support. Figure 1 depicts the final configuration of the ISS when assembly is complete, and identifies those elements which are currently on orbit and those which are awaiting launch. * † International Space Station, Space Operations Mission Directorate, 300 E Street SW, Mail Code: 7P39 International Space Station, Space Operations Mission Directorate, 300 E Street SW, Mail Code: 7P39 1 American Institute of Aeronautics and Astronautics This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Docking Compartment (DC) 1 P1 Truss SO Truss Segment S1 Truss Segment Mobile Segment Servicing System Zarya Control Module Zvezda Service Module PMA 1 SM MMOD Shields Research Module (RM) *ESP-3 Port Photovoltaic Arrays S3/4 Truss Segment P6 Truss Segment Multipurpose Laboratory Module (MLM) and ERA MLM Outfitting *ELCs P5 Truss Segment S6 Truss Segment *ELC S5 Truss Segment Canadarm2 SPDM/”Dextre” Starboard Photovoltaic Arrays Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 P3/4 Truss Segment Mobile Remote Servicer Base System (MBS), Mobile Transporter (MT) JEM ELM-PS Z1 Truss Segment JEM RMS & Exposed Facility Airlock ESP-2 Elements Currently on Orbit Node 1 Node 3 PMA 3 Cupola U.S. Lab Columbus ESP-1 Node 2 JEM PM PMA 2 Elements Pending US Shuttle Launch Elements Pending Russian Launch * For Reference Only Figure 1. International Space Station Configuration at Assembly Complete NASA will use the Space Shuttle, prior to its retirement in 2010, to complete the ISS assembly. Assembly priorities are to: Complete the truss segments. Establish the life support, thermal control and power systems that can sustain the assembly-complete station. Attach the International Partner elements, including the Japanese Experiment Module (JEM), the European Columbus Module and the Canadian Dextre robotic manipulator. Provide the logistics to sustain the ISS. Russia will launch its remaining assembly elements to the ISS, including the Multipurpose Laboratory Module and the Research Module. The final ISS configuration will support growth to six crewmembers in 2009 with the delivery of additional crew quarters, galley, waste management system, and new oxygen generation system. During this period, the Russian Progress vehicle will be used to augment Space Shuttle logistics capacity, and the Russian Soyuz vehicle will be used for some crew rotations. Once operational, the European Automated Transfer Vehicle (ATV) will also be used to supply logistics. Once the Shuttle is retired in late 2010, NASA and its International Partners will use a combination of their collective assets to support and maintain the ISS in orbit. The Russian Soyuz can be used to carry crew while the Russian Progress, European ATV, and Japanese H-II Transport Vehicle (HTV) can be used to share the burden of logistics support. NASA is also seeking a commercial provider to supply logistics and crew transport to the ISS. Within two to four years after the Space Shuttle’s last flight, the new NASA Crew Exploration Vehicle should be ready to support flights to and from the ISS. The International Space Station Program has endured now for 22 years – a full generation of technical expertise that has designed, developed, operated and managed the Program. ISS personnel have successfully adapted to changing circumstances, whether driven by technical or operational difficulties, transportation shortfalls, budgetary considerations, or political redirections. This has included several major redesigns of the ISS vehicle, as well as major changes in ISS operations. For example, reductions in launch and return capability after the Columbia accident have taught ISS engineers and scientists how to deal with logistics shortfalls and to adapt ISS research to new operational realities. 2 American Institute of Aeronautics and Astronautics The Exploration Program will not likely be completed within the careers of the personnel now establishing it. The personnel who will implement the Mars landing may not yet have begun their careers. Through the ISS Program, personnel are developing the experience, knowledge and skills to overcome the inevitable contingencies that will arise in the Exploration Program. These valuable lessons should be factored into the Exploration Program from the beginning, so that they do not have to be re-learned by the next generation of engineers and scientists who will take humans further into the solar system. As we expand human presence beyond the Earth, first in orbit, in the next decade to the Moon, and later, to Mars and beyond, the International Space Station experience can help to guide our success in Exploration. Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 III. Areas of Applicability to the Exploration Program Through the ISS Program, NASA and its Partners have acquired experience in building and operating complex space vehicles. The ISS has been a tremendous challenge of integrating the hardware, computer software, command and control interfaces, crew procedures, logistics, ground support teams, and research, with the added dimension of dealing with different languages and cultural paradigms - in the largest, most complex spacecraft ever devised. This technical challenge is certainly one of the most difficult any international partnership has ever faced. Perhaps as significant as the technological sophistication is the complexity of the multinational and multiorganizational elements involved. The ISS has been the most politically complex space exploration program ever undertaken. It involves multiple aerospace corporations and nearly every international space agency working as program partners. Further, it integrates international flight crews, multiple launch vehicles, globally distributed launch/operations/training/engineering and development facilities, communications networks, and the international scientific research community. Elements launched from different countries and continents have never been mated together until they reach orbit, and some elements launched later in the assembly sequence had not been built when the first elements were placed in orbit. The ISS Program’s greatest accomplishment is as much a human achievement as it is a technological one – how best to plan, coordinate, and monitor the varied activities of the Program’s many organizations. Getting all of the personnel elements to effectively work together has been a continuing challenge for the program management, regardless of whether they were from the United States or other nations, the various NASA centers, or civil service and industry. The various communities often have differing priorities and are competing for the same resources. The Program has succeeded by developing management processes which address the needs and constraints of the various organizational elements. Roles, responsibilities, authority and interfaces were negotiated and documented. Control boards, reviews, documentation, procedures, and information systems have been designed to facilitate program management and coordination. These ISS operations management processes and tools have continually evolved to accommodate changing needs, to address problem areas, and to take advantage of potential efficiencies. Examples are given throughout the paper. The ISS Program provides valuable lessons for current and future engineers and managers. ISS provides real world examples of what works and what does not work in space, as well as lessons in the management of space programs here on Earth. Specific operational areas in which the ISS experience can be applied to the VSE include: Crew Operations, Spacecraft Systems Operations, and Crew-System Interface Operations. A. Crew Operations High performing crews are critical to successful long duration missions. Mission failures can result from degradation of human performance, either physiologically or psychologically, after long duration exposure to the space environment and to the stress of isolation. Specialized skills and training of international crewmembers, as well as advanced protocols, procedures and tools were developed for the ISS and can be used to reduce the risks to future exploration missions. The interaction of the international crew with multiple mission control centers is also a significant element that can make a space mission highly successful or bring work to a standstill. The ISS provides an environment to improve the interaction between crew and ground and make missions safer and more effective. Working for months with crewmembers from other countries and cultures is an important aspect of the ISS program. Developing methods to work with our partners on the ground and in space is critical to providing innovative solutions to operations challenges. 3 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 1. Long Duration Crew Operations By necessity, the ISS Program adopted crew operations philosophies and support tools that are conducive to long duration operations. Unlike rigidly scheduled short duration missions, long duration crew schedules must be carefully balanced to provide dedicated work time in addition to crewmember time for exercise, hygiene, rest and sleep, and personal time. The number of tasks required to be performed according to a predefined schedule has been minimized; the crew has the flexibility to execute other routine, non-critical, and non-hazardous tasks from a pre-defined “task list”. The increased scheduling flexibility permits the crewmembers to better manage their own activities and time. One crewmember commented recently that ‘the difference between the work week and his weekend is that on weekends he gets to choose the work he wants to do.’ This makes work on the Space Station more Earth-like, providing the crew more autonomy. With greater autonomy the crewmember realizes a heightened sense of professionalism, and greater enjoyment and an enhanced feeling of accomplishment. It has also frequently been beneficial in terms of the quantity of work performed. Some have suggested that for future missions, the ideal of crew autonomy be taken to new levels. Use the professional education, expertise and initiative of the highly trained and motivated crewmembers as the basis for planning the program of research to be conducted rather than using the crewmember as a simple equipment operator. A new freedom in communications has been realized on the ISS. E-mail enables easy, frequent, and routine communications with professionals, colleagues, friends and family. Use of the Internet Protocol (IP) telephone enables verbal communications as easily as if the crewmember is in the office. Routine communications between the crew in orbit and managers or researchers on the ground have expedited the exchange of thoughts and information. Planned daily conferences at the start of each workday permit flight and ground crews to identify and prioritize tasks requiring attention, and at the end of each workday, to identify the tasks that have been completed. This allows the ground to track mission accomplishments and issues while keeping unnecessary communications to a minimum. Because current hardcopy versions of crew procedures and flight plans cannot be maintained onboard, the Program implemented software systems to electronically view and manage this information. Any needed updates to the procedures are made on the ground then uploaded to the ISS for immediate access by the crew. The onboard crew also has an electronic version of the ISS flight plan. Capabilities are provided for the crew to make annotations on their planned activities and to perform some limited plan editing. Updates to the crew flight plan are uploaded on a daily basis. Psychological support of the crewmember has gained a new level of attention on ISS with proactive review of operations processes and requirements by psychologists and managers on the ground and with routine ‘care packages’ provided from the crewmember’s home and family to orbit. In addition to regular communications sessions between crewmembers and the families, special communications sessions have been arranged between crewmembers and recognized world experts in science, technology, philosophy, music and entertainment. The extent of crew-controlled task scheduling, the degree of crew autonomy, and the importance of psychological support will become more critical as missions become longer, as missions take place at greater distances, and as the potential for any interruption in communications grows. The Exploration crews can build on this ISS experience. The ISS has been a cornerstone in advancing knowledge about how to live and work in space for long, continuous periods of time and the knowledge gained will remain critical to our future exploratory journeys. 2. Crew Training The ISS crew must be able to handle both nominal and off-nominal operations. This requires general training on the onboard hardware and systems as well as specific training on the procedures to be performed for specific operations. Effective training is essential since the crews may be required to control or to restore systems in the event of automated systems failures, loss of communications with the ground controllers, or other malfunctions and emergencies. Some ISS systems and crew procedures are quite complex, which can make it difficult for the crew to deal with contingencies if not adequately trained. The international nature of the ISS led to the development of a training program that is geographically distributed. Each Partner is responsible for training the crew on the operations of their respective elements and systems. There are, therefore, training facilities in the United States, Russia, Europe, Japan, and Canada. Scientific equipment training further widens the geographically distributed nature of the training requirements. This adds overhead and logistical complexity to the training schedule, which, if not effectively managed, can result in crew fatigue prior to launch. A two year or longer training regimen has been required by most long duration crews. New processes for managing the crewmembers’ time before and after missions have been developed. 4 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 The ISS experience has shown that U.S. and Russian training methods for flight crew and ground personnel differ considerably. U.S. training focuses more on specific task and procedural training while Russian training focuses on overall understanding of design functionality and system operations. Advantages can be seen in both approaches. Generic training on system design and functionality provides a knowledge base which the crew can use when dealing with unforeseen events, while specific task training is beneficial for very complex or hazardous operations. For long duration missions such as the ISS, “refresher” training and systems and hardware reference data is especially important in preparing for complex, intricate, critical, or hazardous operations, since the crew’s initial training on the operation may have been months or even years earlier, and on the ground. The Crew On-Orbit Support System, developed initially for use on Mir, and expanded upon greatly for the ISS, provides onboard computer based training (CBT) capability for the flight crew. A library of software provides lessons and reference data covering many systems and critical operations and is available for crew use and review. New and impromptu operational procedures have been developed and uplinked for use in space. The ability to effectively train onboard will be key to future exploration missions when Earth-based training last occurred months or years earlier. Significant investments were made in ISS training resources, processes and facilities due to the complexity of the spacecraft systems and mission requirements. These investments can now be applied to Exploration. 3. Extravehicular Activity (EVA) Operations To date, there have been 28 Space Shuttle-based and 36 Space Station-based EVAs at the Space Station, totaling over 385 hours. More EVAs are being planned as the assembly continues. The majority of these EVAs have been for assembly tasks, but several were for maintenance, repairs, and science. These tasks were conducted from three different airlocks - the Shuttle airlock, ISS Joint Airlock, and the Russian Pirs; using two different space suit designs - the U.S. Extravehicular Mobility Unit (EMU) and the Russian Orlan. In some instances, collaboration between the U.S. and Russian EVA Flight Control Teams has been particularly close. Control Moment Gyroscopes (CMG) are used to maneuver the ISS, using naturally replenished electrical power to operate the motion control system, instead of attitude control thrusters which use fuel that must be resupplied from Earth. The system includes 4 CMGs, even though only 3 are required for full operation and two CMGs can provide adequate although degraded control. When CMG #1 failed, the cause of the failure of its rotational bearing could not be resolved through telemetry transmitted to the ground. The CMG would have to be replaced and the failed unit returned on the next available Shuttle so the failure could be analyzed. The remainder of the CMG system could continue to maintain vehicle attitude control. But when a second Control Moment Gyro (CMG #2) lost power because of the failure of its Remote Power Controller Module (RPCM), planning for an EVA to change out the RPCM started immediately. The RPCM change-out EVA was a great example of multinational cooperation, as this was the first EVA to be performed in Russian suits on the U.S. Segment of ISS. The Russian Flight Control Team was in control of the EVA while the crew was on the Russian portion of the Station. The Russian and U.S. Flight Control Teams worked together flawlessly to assist the crew in translating from the Russian to U.S. Segment and back, and in monitoring crew health and Orlan EVA suit status. During the EVA, the Canadian Space Agency (CSA)-developed Space Station Remote Manipulator System was used to monitor the status as the astronauts worked outside the spacecraft. One major lesson the ISS Program learned is the importance of designing and certifying EVA equipment for longer lifetimes, with the capability to perform maintenance in space and with an understanding of the on-orbit certification criteria prior to continued use. The EMUs are normally planned to be returned to Earth on the Shuttle for servicing. During the Shuttle down time after the Columbia accident, two of the three EMU suits on orbit, as well as the U.S. Joint Airlock, experienced technical issues that prevented their use in support of spacewalks. Root cause of the loss was contaminants in the suit and airlock coolant water that blocked filters and disrupted magnetic coupling of the suit pump rotor. Water pump rotors also had de-bonded over time. The Russian Airlock and Orlan suits were relied upon to conduct ISS EVAs during this period. But through the ingenuity of the engineers on the ground, and the skills of the crew in space, the EMUs were repaired and made serviceable. Although these EMUs were not used for an EVA, this was a break-through in the normal maintenance philosophy for the EMUs, as all critical maintenance had previously been performed on the ground. The ingenuity of the ground team and the crewmembers was demonstrated by developing the procedures to troubleshoot and repair the EMU cooling pump impellers on orbit with no training and limited tool selection. U.S. EVA capability on the ISS was not fully restored until the July 2005 Shuttle flight, which replaced the two EMUs and delivered a filter/iodinization kit that was successfully used to complete airlock restoration, and will continue to be used in the future to assure EVA readiness. The lesson for the Exploration Program is to design its EVA equipment for in-situ servicing and repair and to develop specific criteria for continuing certification. 5 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 The ISS experience with EVA training is applicable to other long-duration missions, such as a journey to Mars. The Space Shuttle EVA training philosophy has been to train crewmembers on the specific tasks to be accomplished, in the specific order they would be performed. A Space Shuttle-based EVA is a well-practiced and carefully orchestrated ballet, in which everyone knows his or her part by rote. ISS crews, on the other hand, may be faced with both planned and unplanned, or contingency, EVA tasks. Time and resources in which to prepare the ISS expedition crews for EVAs is limited. In order to most efficiently use the available preflight crew time and training resources a different philosophy has evolved based on crewmember recommendations. This new philosophy is to train the crewmembers on a skill set that is applicable to most EVA tasks they will encounter. If there is an especially complex task required of a crew, some specific task-based training may still be required. This skills-based philosophy prepares expedition crewmembers to be able to react to nearly any EVA contingency or repair task that might arise while they are on orbit. This philosophy has repeatedly shown its value during several unplanned EVA tasks that were required to replace failed external hardware on the ISS. Preflight training on both the U.S. and Russian EVA systems is augmented with on-orbit training. Each EVA is preceded by an on-orbit training session, in which the EVA crewmembers review their procedures and practice the EVA, including donning/doffing of the suits. These sessions can be used to train the crew on-the-fly for EVA tasks that were not planned preflight. Another aspect of EVA operations that should be considered when designing exploration missions is the extent of EVA preparation and support that is required. Each EVA requires a significant amount of crew time in addition to the actual EVA. Besides the preflight and on-orbit training requirements, numerous operations must occur immediately before and after an EVA, including preparing the airlock, inspecting the suits, pre-breathe protocol procedures, servicing the suit after an EVA, and closing out the airlock. This additional overhead should be considered when defining EVA requirements and strategies for the Exploration Program. During nominally planned ISS EVA operations, the EVA crew is supported by one or more crewmembers inside the vehicle. When three crewmembers were available on the ISS, a crewmember inside the ISS supported the two EVA crewmembers outside. When the ISS crew was reduced from 3 to 2 crewmembers in the wake of the Columbia accident, EVAs became two-person operations. With no one remaining inside the vehicle, systems monitoring and spacecraft operations are turned over to mission control. The ground also assumes the role of the third crewmember in helping to coordinate the EVA. This kind of operation is not new to either the United States or Russia. During the Apollo moon landings the crew worked on the Moon’s surface while ground controllers monitored the spacecraft systems. During Salyut and Mir, Russian cosmonauts routinely left the spacecraft untended during spacewalks. This mode of operation is possible as long as the ground has the ability to monitor and control the vehicle. The operational lessons of the ISS in the areas of EVA suit maintainability, training and EVA support may prove critical for long duration crewed missions which venture even further from the Earth. B. Spacecraft Systems Operations Efficient, reliable spacecraft systems are critical to reducing crew and mission risks. Optimizing systems performance and characterizing system performance in space will reduce mission risks and advance capabilities for planetary distances and autonomous vehicle and systems management. Confidence in life support systems for water and waste recovery, oxygen generation, and environmental monitoring technologies becomes more critical as the distance and time away from Earth increase. The ISS is the first space vehicle in the U.S. space program in which reliance upon recycled water and oxygen has been critical to continuation of the mission. For the first time, NASA engineers have developed the closed loop recycling systems and tested them in space. The ISS is NASA’s closed loop life support test bed for demonstrating these advanced capabilities. The systems will serve as the basis of the expertise required to send crewed spacecraft to the planets. Maintaining crew health is key for long duration flights. The ISS must provide exercise and environmental monitoring systems that are in use continuously over many years of operation. ISS also proved an important lesson in defining the criticality of these systems. Much has been learned about developing exercise equipment and its effectiveness for maintaining crew fitness in microgravity. More long-duration experience with these systems is needed before extended missions on the Moon or to Mars are attempted. Operations protocols and support tools which minimize the ground support infrastructure needed to monitor and control spacecraft systems are also essential for long duration missions. The ISS operations concepts and ground facilities continue to evolve due to ongoing efforts to increase effectiveness and minimize operations costs. 1. System Design for Long Term Operations The United States and Russia evolved different approaches to system design and operations. The ISS experience has shown that, for long term operations, there are advantages and disadvantages to both approaches. 6 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 The Russian modules and systems of ISS are essentially identical to those used in the Russian Mir Station and were developed beginning with the Salyut designs of the early 1970s. Russian design philosophy embraces simplicity and robustness. Many of the systems, however, require frequent crew interaction for maintenance and operation. The systems are usually reliable and easy to operate and, when maintenance is required, permit crew access and interaction. Emphasis is placed on operability and functionality, but the minimal telemetry means that systems may unexpectedly malfunction before corrective measures can be planned. The on-orbit crew is expected to operate with a level of independence from the ground that requires the crew to take on the responsibility to ensure the systems remain operational. Russian system reliability is based on periodic maintenance and component replacement. Most of the U.S. modules and systems now part of ISS have little heritage from prior spaceflight programs. The U.S. systems tend to be more complicated than their Russian counterparts. The U.S. systems provide considerable data to flight controllers via telemetry. This allows the crew to rely on the flight control team to monitor the performance of the systems. Frequently ground controllers have more data than the onboard crew and they may have more control than the onboard crew. Most of the U.S. systems are computer-controlled. This permits a high degree of automation and ground monitoring and control, but this also means the systems may not operate at all unless computers and software are operating nominally. ISS has shown that a system driven by computers and software must have sufficient redundancy; the design must accommodate the microgravity and radiation environments; and a priority factor to consider in the software design from the standpoint of operations is the ease of crew interaction and control. The ISS experience has shown that crew input into the design of onboard computer displays is essential to ensure that the displays are intuitive, easy to navigate, and contain the information needed for effective crew operations, especially in a time-critical situation. The U.S. Command and Data Handling (C&DH) architecture is a tiered approach with the triple redundant Command and Control (C&C) computers providing the top level of control. Prior to the installation of the U.S. Laboratory the U.S. Node was controlled with two of its own computers and software. Once the Lab was integrated to the ISS and activated the C&C computers in the Lab took over the top level of control of the U.S. C&DH architecture. The Node computers continued to control the Node functions and communicated with the C&Cs. The Node computers also retained a safety net function referred to as Mighty Mouse. In the event that all three C&C computers should fail, the Mighty Mouse software would activate and the Node computers would take over a limited portion of the C&C's role and attempt to restore the C&C computers. Approximately four months after the Lab activation all three C&C computers did fail within hours of each other. The Mighty Mouse function performed as designed and kept the critical U.S. systems running. Failure analysis revealed that the cause of the C&C failure was related to mechanical issues with the hard drives on those computers. The hard drives were replaced with Solid State Memory Units (SSMU) and there have not been any problems since. The potential failure of the hard drives had been identified some time before, and development of the SSMUs had begun nearly a year before the on-orbit failure, allowing replacement less than 9 months from the time of the failure. Laptop computers are used both as the crew interface to the C&DH and to perform less critical functions such as display of procedures, email and IP Phone. Laptop hardware has been upgraded twice since First Element Launch, providing a higher performance platform than would have been possible with more traditional avionics equipment. The maintenance of avionics software on ISS has been another success story. The software upgrade process was originally launch-driven, with most initial software loads resident on the computers launched as part of each ISS element. Software updates (both patches and new releases) were provided via uplink or through the use of CD ROMs flown on the Shuttle, Soyuz, and Progress. After the Columbia accident, the update process was continued, and virtually all of the Space Station’s U.S. and Russian software has been upgraded at least once since. Continuing the software update process in the absence of Shuttle flights has allowed the implementation of software fixes and improvements with a corresponding reduction in operational workarounds and increase in operational efficiencies. In addition, this approach has allowed the software team to live within decreasing budget allocations planned before the Columbia accident. 2. Habitation and Life Support The ISS is demonstrating the importance of habitability in sustaining crews and spacecraft operations over the long time periods that will be critical for lunar and planetary habitats and Mars transit vehicles. Habitability issues are important for maintaining crew health and feelings of well-being. Inadequate attention to habitability presents serious mission and safety risk. Noise levels were a concern from the outset of the ISS program, beginning with requirements definition. Inadequate attention in the design and development stages and, in some cases, use of decades-old technologies, combined with inadequate noise standards, led to a noisy environment in which personal hearing protection for the 7 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 crew has become the norm. In some circumstances, the noisy environment makes it difficult for crews to communicate with one another or with the ground. As systems are replaced, this noise problem is being reduced, but on long distance missions upgrades are not an option. Reliable operation of the life support systems in human spacecraft is critical and will become much more significant as crews and spacecraft venture further from their logistics source on Earth. Dissimilar redundancy in key life support systems has proven critical to the ISS. The United States and Russia used different hardware design reliability philosophies. The Russian systems are made of modular, stand-alone hardware. Though these components endure periodic failures and anomalies that reduce performance, frequent, simple maintenance can keep the systems operating, and when there are more significant problems, replacement components or assemblies can be launched on Progress logistics missions. The U.S. systems were designed independently from the Russian systems, are more complex, experienced different operational failure modes, and required varied maintenance and repair solutions. The Russian “Elektron” system, for example, has been the primary generator of oxygen onboard ISS. Its major component, the “Liquid Unit” generates breathable oxygen by electrolysis of water recovered from the cabin air and separation into oxygen and hydrogen. A series of failures of the fluid micro-pumps, caused by air bubbles and contaminants in the fluid lines, occurred during the Shuttle down period. The failures necessitated the change-out of three Liquid Units in succession, and then considerable hands-on maintenance by the crewmembers in order to maintain partial operability. Replacing these Liquid Units creates manifesting challenges on Progress resupply missions. Backup systems, like the solid-fuel oxygen generators (SFOGs) in the Service Module, have been pressed into service. U.S. and Russian-supplied stored oxygen provide a third leg of redundancy. In the near future, a new electrolysis-based U.S. oxygen generation system will be launched to the ISS to provide additional oxygen generation capability needed to support a six-person crew. The Carbon Dioxide Removal Assembly (CDRA), in the U.S. Segment, processes the cabin air to remove carbon dioxide, as does the “Vozdukh” system in the Russian Segment. Failure of the desiccant containment, valve contamination and corrosion resulted in some partial failures of the CDRA. However, by using new and innovative cleaning processes and by pre-positioning key spare components the system was maintained throughout the Shuttle down period. The advantage of having two totally different designs, one U.S. and one Russian, for carbon dioxide removal was evidenced. In the wake of the Columbia accident, as logistics constrained the number of environmental samples being returned from ISS, the ISS Program re-assessed minimum requirements for sampling. Environmental monitoring systems, such as the Major Constituents Analyzer (MCA) have been used less frequently and for only the most critical measurements. When the Volatile Organics Analyzer (VOA) failed, the United States and Russia shared returned air samples for analysis and monitoring of the cabin atmosphere. In order to reduce the number of environmental samples being returned, the crew performed previously unplanned microbiological measurements insitu to verify water quality. The important lesson for other long duration missions is to critically assess and limit the number of environmental samples that must be returned, and to give the onboard crew the means to monitor and maintain the onboard environment. When either a U.S. or Russian component has failed, the other country’s system has always been relied upon for support. Despite the systems failures, multiple independent systems have proven complementary and have ensured maintenance of a safe, breathable atmosphere and a potable water supply. Dissimilar redundancy should be a strong consideration for Exploration systems. Other systems also demonstrate the philosophical differences in design. The U.S.-provided health maintenance and exercise hardware is technically sophisticated, with vibration isolation and exercise performance monitoring systems, and provides excellent human and hardware performance data to the ground physicians and engineers. ISS has demonstrated their importance of resistive exercise systems for maintaining bone density and muscle tone. The Russian provided equipment is simpler and has limited monitoring or downlink capability, but it was based on systems that had been used for years on the Mir station and were specifically designed for simplicity, robustness, and on-orbit repair. The sophisticated U.S. exercise hardware was not designed for on-orbit maintenance. The Resistive Exercise Device (RED) and the Treadmill were deemed non-critical early in the ISS program and therefore were not tested to the same extent as systems identified as critical. But the U.S. exercise system hardware experienced failures soon after the first crew took up residence onboard. When requirements for the systems were investigated it was determined that exercise is a critical need for long duration missions and that the lack of adequate crew exercise can threaten the mission. At the outset, the U.S. systems were designed for periodic return to Earth and replacement with new systems launched on the Space Shuttle. However, once on-orbit maintenance was deemed critical and with the challenges the Program has faced in logistics, attention turned towards on-orbit maintenance by the crew. 8 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 In many instances it was found that the failed components of the exercise hardware are small. The crew has successfully been called upon to replace much smaller components than had ever been previously planned for repair in orbit. The maintenance operations necessitated some special zero-g considerations. For instance, the large gyroscope and flywheel of the Treadmill Vibration Isolation System (TVIS) had to be disassembled from the treadmill assembly. The sophisticated Vibration Isolation and Stabilization (VIS) system isolates the TVIS from the ISS structure, enabling crewmembers to run without transferring vibrations to the station or to sensitive experiments. On the ground this maintenance procedure is done on a workbench in a tightly controlled environment and with components resting on specially cleaned workbenches and with specially built restraints. In orbit, magnetic forces caused components to repel and fly away from one another. The crewmembers had to physically restrain the components and use considerable force to overcome magnetic forces during disassembly and reassembly. The increased on-orbit maintenance requirements and the sometimes unanticipated maintenance difficulties have given great insight into the certification and testing requirements for hardware and into the kinds of operations astronauts can be relied upon to perform during long duration exploration missions. 3. Spacecraft Operations and Ground Support One of the major challenges for long duration missions is the design of the ground support infrastructure needed to monitor and control the spacecraft systems. Because the ISS is an international program, it faces unusual complexities in the area of real-time flight operations. Operations functions for ISS have been decentralized, with each Partner taking on significant roles relating primarily to the hardware/systems they have developed. Over time, as the ISS moved into its operational phase, interdependencies have increased and they will do so to an even greater extent in the future. Figure 2. ISS Operations Centers Real-time operations and control of the ISS is geographically distributed across countries and International Partners, as depicted in Fig. 2. Each Partner will eventually have an operations control center participating in flight operations, in addition to a launch control center for its transportation elements. Currently there are three control centers operating 24 hours per day, seven day per week supporting the ISS: the Johnson Space Center (JSC) Mission Control Center (MCC) in Houston, Texas, MCC-Moscow, and the Payload Operations Center (POC) located at Marshall Space Flight Center (MSFC) in Huntsville, Alabama. The Mobile Servicing System (MSS) Operations Complex in Saint-Hubert, Quebec, supports operations of the Canadian robotics systems. The Columbus Control Center in Oberpfaffenhofen, Germany, and the JEM Control Center in Tsukuba, Japan, will come on line when the European and Japanese elements are launched. The control centers are interconnected, and each has its own unique functions and responsibilities. MCC-Houston and MCC-Moscow are responsible for the U.S. and Russian segments of the ISS, respectively. The POC is responsible for NASA payload operations, and generally falls under the authority of MCC-Houston. In addition to these prime control centers, there are also a variety of smaller operations centers supporting the research community. The prime control centers are not fully redundant, but have enough 9 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 common functionality that they can provide backup capabilities in special circumstances. For example, when the MCC-Houston at JSC was evacuated due to approaching hurricanes, ISS control responsibilities were transferred to MCC-Moscow for the duration of the evacuation. Long duration, round-the-clock operations require a different approach than is used for short duration missions such as the Space Shuttle. A prime consideration is the need to minimize overall costs for the ground support facilities and flight control teams. The ISS Program has attempted to reduce mission operations costs wherever possible, without sacrificing mission safety or operational effectiveness. Efficiencies have been realized in both the ground support facilities and the flight control teams. Another prime consideration for flight control team staffing on a long-duration mission is the human factors aspect. Long periods with shift or weekend work can disrupt family life, causing personnel burnout and high turnover. A variety of strategies have been employed to minimize these impacts to the flight control teams, such as reducing support on the weekends and off-shifts. Some flight teams cycle personnel on and off console. During the periods when they are not performing shift work, these controllers cycle back into planning or other operations support activities. The ISS prime shift hours were even driven by the very real constraint of the Russian flight controllers to utilize the Moscow mass transportation system, which does not operate from late evening to early morning. The Exploration Program will face some of the same challenges. Reductions in the number of flight control personnel have been achieved by adopting different operational paradigms. At MCC-Houston on most weekends where no critical activities are planned onboard ISS, only a single Flight Director and a few Flight Controllers may be working. The reduction in the number of support personnel means that Flight Controllers must be trained and proficient in more than one system in order to reduce manpower requirements and workload induced burnout. The flight control team can also be reduced through increased automation for routine monitoring of spacecraft systems. Even more streamlining may be done in the future. Another strategy is to reduce or simplify the “pre-mission” operations preparations activities that must be performed. A good example is the mission planning approach that has been adopted for the ISS. In contrast to the Space Shuttle Program, where detailed flight plans are developed long in advance of the flight, the ISS Program produces long term plans at a much less detailed level. These plans allocate flight activities to days, but do not assign specific times. The very detailed flight plans are not generated until a week or two before they are to be executed. The template for generating the long term plans has also been simplified. Initial concepts were to have three iterations of the plan. Over time, the ISS Program has reduced the number of iterations, thus reducing the overall time and manpower required. Reductions in both the level of detail and the planning template have helped to minimize the manpower requirements for this activity. The NASA ground support facilities continue to pursue reductions in sustaining costs, while increasing capabilities for the flight control teams. Both the MCC-Houston and the MSFC POC have been migrating legacy hardware/software to readily available and cheaper desktop systems, and have created internet versions of many basic flight control tools (e.g., voice distribution systems, information systems for flight support). This not only reduces facility sustaining costs, but allows more and more operations to be performed away from the control centers. MSFC has created a suite of low cost tools, including the Telescience Resource Kit (TReK) and the Internet Voice Distribution System (IVODS), which enable U.S. science users to remotely monitor and command their payloads from their home sites. Because of the progress made in these remote operations support tools, MCCHouston personnel were able to continue monitoring of ISS operations even after evacuating the MCC-Houston during recent hurricanes. Flight operations concepts have accommodated the additional interfaces, complexities, and coordination that are introduced with multiple flight operations centers both within the United States and internationally. New tools have been developed to facilitate distributed planning and operations information distribution. Cooperative software development and sharing of software tools across NASA centers and Partners, where feasible, has been used to reduce overall ground development costs. Through these experiences, the ISS Program has learned many valuable lessons in the areas of long term flight operations, ground facility/software development and distributed operations that will have applicability to the very complex and long term Exploration missions. C. Crew-System Interface Operations ISS has advanced robotic operations in space and demonstrated and validated the resulting human-machine robotic interactions and interfaces. The mixed crew and robotic operations have enabled the extensive in-space assembly and orbital maintenance and repair operations. These same capabilities may enable and enhance lunar missions in the near term but will be a prerequisite for assembling and maintaining the large space vehicles that will be required to take people to the planets. 10 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 The Canadarm 2 robotic arm is used to assemble the large and massive ISS elements on orbit. Ground control of robotic activities enables more efficient use of valuable crew time. Development of displays and control for enabling astronaut control of these operations will be important for future spacecraft systems’ designs. Software tools, such as virtual reality, play a role in helping crews to practice EVA or robotic tasks before ever donning a spacesuit or powering up the robotic arm. The ISS provides a real world laboratory for logistics and maintenance concepts for future spacecraft. ISS crews have had to demonstrate repair capabilities as an indirect result of the Columbia accident and the reduced flow of logistics for the ISS. Crews and their ground maintenance counterparts have devised unique solutions that have kept the ISS functioning despite logistic shortfalls. 1. Systems Maintenance and Repair The ISS Program has demonstrated new capabilities to sustain spacecraft operations over long periods of time, which will be critical for lunar/planetary habitats and Mars transit vehicles. The lifespan of hardware is frequently limited by performance and materials constraints. Hardware may be designed to remain in space without maintenance or replacement, designed for periodic maintenance or replacement, or designed with a specific certification lifespan. The ISS Program uses a combination of analysis, testing, and simulations to define life limits. System performance is being tracked to understand the degradation of the vehicle and systems over time. As a result of the Shuttle loss and the resulting interruption of logistics support, all of these design features have been tested. Major challenges were posed by the limitations on size and mass of cargoes that could be launched to orbit, and by the inability to return failed hardware to the ground for failure analysis and refurbishment. As discussed in the section on Extravehicular Activity, Control Moment Gyroscopes (CMG) are used to maneuver the ISS. The system consists of 4 CMGs, although only three are currently required for full operation and two CMG’s can provide adequate control. When CMG #1 malfunctioned, the remainder of the CMG system could continue to maintain vehicle attitude control. The cause of the failure of the rotational bearing of CMG #1 could not be resolved through telemetry transmitted to the ground. During the Discovery Return to Flight mission, astronauts conducted an EVA to replace the failed CMG, getting the system back into fully operating condition. The failed CMG was returned to the ground for failure analysis. Significant crew time and stowage volume was required to maintain hardware that was not designed for on-orbit repair. An example of an external spare stowed inside a module due to environmental limitations is the Bearing Motor and Roll Ring Module for a solar array. It is over 18 cubic feet in volume. After a solar array experienced several stalls in its rotation mechanism, shortly after the array was installed, the spare was launched. It was determined that it would be too difficult to install without the Space Shuttle docked and the Shuttle has been grounded for three years. To date, the spare has been in storage for five years. For an Exploration mission there will be limited stowage available for spares and no opportunity to return hardware for failure analysis, so appropriate performance and diagnostic data must be available to support in-situ diagnosis and repair. Maintenance tools must be available to the crew. Maintenance trades must optimize between complexity, automation, reliability, repair, and replacement. Factors that must be considered in the trades include crew training, crew time, stowage, logistics, costs and vehicle functionality. Modular systems with commonality maximized across hardware and systems may be the best choice. The ISS is an ideal test bed for new maintenance methodologies and tools. 2. Logistics, Resupply and Stowage Resupply, logistics and onboard stowage have proven to be very important issues for the Space Station. Prior to the Columbia accident the nominal plan was to fly U.S.-provided consumable items as required on the Space Shuttle, and Russian-provided consumable items on the Progress cargo vehicle. This arrangement of frequent visiting vehicles provided a constant supply line that supported the crew and vehicle in orbit with less impact to onorbit stowage. The Russian cargo vehicles were already planned to carry a significant volume of replacement components as the Russian hardware was designed for frequent maintenance. Critical U.S. hardware had prepositioned spares, but all other hardware was to be flown on an as-needed basis. It was undesirable to preposition all hardware on the Space Station due to the limited stowage space available. Volumetric requirements for hardware and provisions stowage were addressed early in the ISS Program. But ISS configuration changes introduced unforeseen challenges in the provision of adequate stowage volume. Careful attention by crew and ground planners has been required to ensure that access to emergency provisions, fire ports, and the module pressure shell can be maintained in case of contingencies, even as stowage has occupied many interior surfaces. Stowage usually occupies the volume of modules that are used less frequently, such as the Joint 11 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 (U.S.) and Russian Airlocks and the Pressurized Mating Adapter to which the Shuttle docks. This incurs a penalty in terms of normal accessibility, difficulty in locating hardware and provisions, and increased crew time required to locate, unpack and repack stowage areas. In the closed and stowage-challenged spacecraft, inventory management gains critical significance. The items available onboard, their stowed locations, and their rate of use or lifespan must be tracked, forecast, and carefully planned. The computerized barcode inventory system used on the Space Station has been cumbersome in many instances. It is inefficient and maintaining the inventory demands considerable crew-time. The recent apparent loss of critical Orlan EVA components onboard the station meant that the Russian EVA capability could not be relied upon until the components were located. Radio-Frequency Identifier Devices (RFIDs) were explored early in the ISS program but decided against because of costs and technology availability. But the relatively low expense of these now commercially available systems may be a prime candidate for use on lunar and planetary spacecraft. In the wake of the Columbia accident, the resupply of the ISS depended on the limited capacity of Progress cargo vehicles. A food shortage could have necessitated abandonment of the ISS had a Progress not replenished the food supply. Out of necessity, the ISS Program carefully reevaluated the usage rates and requirements for critical consumables of air, water, food, and propellant. The reduction in the resupply requirements allowed continued occupancy and operation of the ISS. Good examples of resupply requirements reductions include: a nearly 85% reduction in crew clothing, down from 12 cubic feet to just over 2 cubic feet per crew member for their six-month stay on orbit; a 25% reduction in food overage volume; replacing packing materials with soft goods such as towels and clothes; replacing film with digital cameras; using electronic procedures instead of paper procedures, etc. More water is recycled by fully drying out clothes and towels prior to disposal, which has led to a reduced usage from 3 to 2 liters per day per person for consumption and hygiene needs. Propellant requirements were also reduced with careful planning. At 600 square meters, the US solar arrays on the ISS are the largest power arrays ever flown. Each is nearly the size of a football field’s end zone. These have the potential to create significant drag when oriented normal to the direction of flight. New array management techniques were devised to keep the minimum frontal area exposed while meeting required power demands. Such techniques include orienting the arrays’ edges into the direction of flight when in darkness and holding the maximum possible bias towards that orientation during the sunlit portion of the flight. This strategy requires that some reduction in the electrical power be accepted. Different orientation techniques, termed “Night Glider” and “Sun Slicer”, were developed for the Earth-oriented and solar inertial flight attitudes, respectively. A third flight orientation, termed “YVV”, essentially flies the ISS sideways and results in extremely low drag. Overall, these techniques have resulted in over 25% reduction in atmospheric drag, and this has translated into a reduction of over 600 kilograms of fuel for reboost over a 30 month period. With the conservation efforts of the crew and the close tracking of actual consumables usage, the Program was able to maintain two crewmembers on orbit using only Progress cargo vehicles. For Exploration, the potential resupply of some items forces operational, philosophical and hardware design trades. For example, should some food be grown to supplement the diet? How much trash and waste can be recycled? To what extent can you depend upon a closed-loop regenerative water or oxygen life support system? Can clothes and soft goods packing or food packaging be made more efficient? 3. In-Space Assembly Operations The size and complexity of the ISS presented a unique challenge to operations. The ISS at assembly complete will have a mass four times larger than any previous vehicle in orbit, and will be larger than a football field. The complexity, size and mass of the on-orbit vehicle prevented assembly of the ISS on the ground. Even using a heavy lift booster such as a Saturn V, many launch and assembly flights would be required, but with launch capacity restricted to Shuttle performance, a series of over forty assembly flights was needed. The elements that comprise the ISS are each between ten and twenty metric tons and with the exception of the Russian modules, each element is passive and must be carried to a rendezvous by the Shuttle and berthed using a combination of the Shuttle and ISS remote manipulator arms. While rendezvous between the ISS and Shuttle was typically planned to be in coplanar orbits, when the planes were out of phase, this introduced new problems for such massive space vehicles that had to be carefully factored into rendezvous maneuvers. Similarly, the berthing of such large ISS elements needed to be very precisely aligned in order for the complementary berthing rings to mate properly. In the early years of the Program this was implemented with the use of a remote vision system, but later in the Program, technology had improved to permit more precise alignments of elements using data from the Shuttle and ISS inertial measurement units in combination with the pointing data from the ISS and Shuttle manipulators. 12 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 The ISS international partnership introduced new challenges because elements and modules were designed and built by various International Partners using the unique techniques and components of their respective countries and cultures. Also, the ISS is being assembled over an extended period of time. Some components will not be in orbit until ten years after the launch of the first elements. Many of the components will have never interfaced with one another on the ground. The first time the elements, including those produced by International Partners, will be joined together and operated will be in orbit. The on-orbit construction of the ISS, starting from an initial single module, to the assembly complete configuration, will take over a decade; however, the ISS was required to be operational during all phases of construction. Not only did the major electrical power, thermal control, data management, environmental control, guidance, and propulsion systems need to be fully functional from the start, but the crew and research hardware was also functioning as the assembly program continued. And the ISS configuration is continually changing as additional elements are added and vehicles arrive, become part of the configuration, and then depart. The ISS is the first major human space system that was designed to be assembled, integrated and operated in space by people, and only in space. For the design of such a complex system, of paramount importance was to have complete understanding of the operations requirements throughout the life of the Program. Often, this phase is “short changed” because of schedule and resource pressure or lack of experience on the part of the designer. For the ISS Program, experienced engineers were allowed sufficient time and effort to analyze and understand the performance that would be required during the life of the Program. Design options were investigated and the merits of each debated at length and preliminary analysis performed, before a design was selected. The importance of this effort cannot be overly emphasized. The ISS design concept changed several times during the definition phase of the Program. Together with the extended period of assembly and the complexities and inevitable problems that could occur over the assembly period, it was recognized that the ISS would need to be able to accommodate unforeseen changes. The design required that control and operation of various systems and subsystems were distributed throughout the ISS. The system has several tiers of modularity, at the component level, at the rack level, and at the module or element level. The U.S. Segment of the ISS benefited from the establishment and adherence to this fundamental architectural principle. This most fundamental principle addressed hardware change-out and maintainability but required a system that was assemble-able. As configurations and launch sequence planning have changed over the years, the modularity of the ISS architecture in the U.S. elements has proven critical. Modular racks with standard interfaces to the modules have allowed flexibility in manifesting and on-orbit outfitting. Racks were offloaded from the U.S. Laboratory when the Program changed the ISS to a higher inclination in order to accommodate the Russian launches. The modular architecture designed nearly two decades earlier allowed these changes in configurations and launch parameters with no impact to the hardware design. The ISS is the first vehicle ever designed with rigid requirements for maintainability and re-configurability over a truly extended on-orbit lifetime. Each Apollo mission flew for only a matter of days and was used only once. Shuttles fly for a couple of weeks before they undergo major ground servicing and periodic major modifications. Skylab missions lasted for less than a year with no plan for continued use. Even Mir was designed for a five year life, although it lasted for somewhat longer (15 years) through the addition of new modules. The ISS was the first spacecraft ever to be physically assembled using extensive EVA and robotics in orbit. The Shuttle-based assembly operations and missions are complex; almost every assembly mission is different. Earth orbit has become a construction site where conditions alternate between freezing cold and searing heat. The construction workers are extravehicular astronauts; the cranes are a new generation of space robotics; and the tools must operate in a microgravity environment. Because of the complexity of ISS assembly, detailed assembly planning is crucial. Like an Earth-based construction site, certain activities must precede others, so the integrated assembly sequence must consider all such dependencies. The ISS Program plans and tracks the exact configuration of the ISS after each assembly stage, and ensures compatibility of the new elements into the existing on-orbit configuration. As new elements are brought on line, ISS documentation, software, procedures, operating plans, interfaces, and support tools are updated. The only analog for future long-duration human exploration missions with modular spacecraft assembled in space is the ISS. Onboard systems and hardware are highly representative in design, complexity, and reliability to what will be required for trips to the planets. Many of the operational constraints are similar to those that will be experienced in the assembly of a lunar base or for a Mars spacecraft. ISS is the only test bed available today to check out systems and operations for Exploration. 13 American Institute of Aeronautics and Astronautics Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 4. Robotics Efficiency, speed and precision of the ISS in-space assembly requires that much of the assembly work be done robotically. ISS robotic systems are operated at the large-scale (e.g., cranes), mid-scale (e.g., anthropomorphic robots), and small-scale (dexterous and/or micro manipulators). Other reliable, remotely operated, self-deploying and self-assembling systems were developed for use in Earth orbit, but are adaptable for use on the Moon, and beyond. Intelligent and robust docking mechanisms, as well as autonomous rendezvous and docking technologies and the test beds used to develop them are key Exploration mechanisms. ISS operations make use of an integrated suite of imaging sensors and manipulators for in-space assembly, inspection and operation. The capability to perform a wide variety of local inspection and control operations will be important to the long term, robust operation of diverse systems in deep space and on other worlds. Canada, which built the Space Shuttle remote manipulator in the 1970s, also developed the station’s primary mechanical arm. Called the Space Station Remote Manipulator System (SSRMS), the 55-foot-long arm has the capability to move around the station's exterior either like an inchworm, locking its free end on one of many special fixtures, called Power and Data Grapple Fixtures (PDGF), placed strategically around the station, and then detaching its other end and pivoting forward, or riding on a Mobile Servicing System (MSS) platform that will move on tracks along the length of the station's 350-foot truss, putting much of the station within grasp of the arm. Canada also is providing a new robotic hand for the SSRMS, the Special Purpose Dexterous Manipulator, also called Dextre. It consists of two small robotic arms that can be attached to the end of the main station arm to conduct more intricate maintenance tasks. Two other robotic arms will eventually be installed on the ISS. A European Robotic Arm (ERA), built by the European Space Agency, will be used for maintenance on the Russian Segment of the station and the Japanese laboratory module will include a Japanese robotic arm that will tend research equipment mounted externally on a "back porch" of the lab. These robotic systems introduce new techniques of human/machine interfaces. For example, training for the robotic operations is routinely performed with virtual trainers and actual on-orbit operation is performed remotely by the crew using computer and television screens and assisted by an automated vision system. A new mode of robotics operation was recently introduced on the ISS, when the MCC-Houston successfully performed a camera survey of portions of the ISS exterior, via ground control of the SSRMS. The onboard crew monitored the robotic operations, but did not actively assist in the event. The next day, the SSRMS was nominally moved via ground control to a clearance position to enable an upcoming Soyuz relocation. Future programs will certainly utilize robotics extensively for assembly and maintenance tasks. The ISS experience with multiple robotic devices, human-machine interfaces and crew robotics training should prove valuable. IV. ISS as an Operations Test Bed for Exploration The ISS affords a unique opportunity to serve as an operations test bed for the Exploration tasks. Because it is a large, complex spacecraft operating continuously in space and maintained by the onboard crew, the ISS is an ideal platform to test protocols and procedures that will enable greater crew autonomy and reduce dependence on the ground support team. Training tools, crew and robotic operations, time-delayed or intermittent ground communications, and on-orbit repair and maintenance can be demonstrated and validated in space. ISS can support demonstrations of new capabilities and tools required for sustaining spacecraft operations, including remote vehicle management, logistics management, in-space assembly and inspections, and flight demonstrations of new crew and cargo transportation vehicles. Similar to spacecraft that will support future missions beyond low-Earth orbit, ISS does not return to the ground for servicing, and provisioning of spares is severely constrained by transportation limits, especially after Shuttle retirement. The ISS mission increments can be used as temporal and operational analogs for Mars transit. The ISS is a viable, and the only, test bed available in the near term for increasing technology readiness levels and/or validating concepts and technologies for human space flight in the microgravity, thermal, radiation, and contamination environments of space. It is the only space-based operational laboratory available for testing critical Exploration spacecraft systems such as closed-loop life support, EVA suit components and assemblies, advanced batteries and energy storage, and automated rendezvous and docking. Table 1 describes some potential operations-related roles for the ISS as a test bed for operational experience and technology validation. 14 American Institute of Aeronautics and Astronautics Table 1. ISS as an Operational Test Bed for Exploration Mission Objective Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 Crew Operations and Training Extra Vehicular Activity (EVA) Capabilities needed Capabilities needed For Moon For Mars Crew Operations • Integrated International crews • Integrated International crews • Evolved operations tools and processes • Streamlined operations tools and processes • Skills based Intravehicular (IVA) and EVA training; evolved onboard training tools • Computer based IVA and EVA training • Improved EVA suit materials and on orbit maintainability • Highly reliable, maintainable suits; resilient to Mars dust • Enhanced suit mobility /flexibility; self don/doff • Reduced crew prep times for EVAs ISS Role • Develop and demonstrate protocols and procedures with international crews • Develop and demonstrate skills-based and onboard training tools • Prototype new EVA suit materials, components and subassemblies • Verify procedures for on-orbit repair and maintenance, self donning/doffing, and airlock management Spacecraft Systems Operations Advanced Habitation and Life Support Operations Communications Operations Protocols • Closed-loop life support • Long duration crew accommodations • Evolved medical care and countermeasures • Long distance crew provisioning and resupply • Evolve crew accommodations and planning systems for provisioning, food and clothing • Advanced environmental control and life support • Characterize operating conditions for next generation closed-loop life support • Long distance medical care and long duration countermeasures • Validate advanced health care and countermeasures • Remote systems management • Remote systems management • Systems monitoring tools for reduced ground support • Radiation-hardened hardware • Develop operations procedures for remote vehicle management and intermittent communications • Autonomous crew operations • Autonomous systems monitoring tools • Characterize operating conditions for radiation-hardened hardware and networks • Validate autonomous crew operations and reduce ground support 15 American Institute of Aeronautics and Astronautics Mission Objective ISS Role • Demonstrate test, repair and maintenance operations on orbit • Component commonality to support field repair without logistics resupply • Maximum component commonality to support on-orbit maintenance and repair • Reduced resupply requirements and trash generation • Reduced in route and on-site resupply requirements • Evolved logistics and inventory management • Autonomous logistics and inventory management tools Assembly Operations • Reliable in-space assembly operations • Autonomous in-space assembly operations • Demonstrate procedures for in-space assembly systems; selfdeploying systems; inspection and control Automation, Robotics and Human-Machine Interface • Combined crew and robotic operations • Autonomous crew and robotic operations with time delayed communications • Validate robotic designs, concepts, tools and operational scenarios for long distance assembly and maintenance tasks Systems Maintenance; Repair; Logistics Resupply and Sparing Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 Capabilities needed Capabilities needed For Moon For Mars Crew-System Interface Operations • Robotic exploration aids and EVA support • Ground controlled robotic operations • Combined airlock and robotic operations • Evolve logistics management, maintenance and sparing concepts NASA is using the ISS as a laboratory for research with direct applications to Exploration requirements in human health and countermeasures, as well as applied physical science for fire prevention, detection and suppression, multi-phase flow for propellant, life support, and thermal control applications. At the completion of assembly, the ISS will support research and technology development programs that meet the Agency’s needs for crew health and safety, technology advancement, and validated operational experience essential for long duration missions beyond low Earth orbit. With the transition to the Vision for Space Exploration, NASA’s plans for research and utilization of the ISS have undergone significant changes. The resulting research and utilization approach is still evolving to focus available resources on risk reduction associated with the NASA exploration architecture. However, NASA is well positioned to take maximum advantage of the window of opportunity provided by the ISS. V. Conclusions The operation of the International Space Station was dependent at its outset very directly on the knowledge that was gained during operation of the earlier Russian and U.S. space systems. The ISS, as we operate it in space today, is an evolution of space systems technologies that were developed by many countries with widely differing design philosophies. The significance of the Columbia accident and its impact on continuing operations during the Shuttle hiatus has been critical. Many of the operations, processes, functions and systems in use on the ISS today provide the capabilities that will be needed for future Exploration missions. Many of the hardware and software systems developed for ISS may be adapted for direct use on future systems. We are learning what is required to build and sustain a large space infrastructure over multiple generations. We have the test bed in place today to learn what does and does not work. Perhaps the most significant new operations knowledge gained from the ISS Program to date includes: 16 American Institute of Aeronautics and Astronautics New respect for the ability of the flight crew to independently perform system operations, maintenance, research and mission planning, when provided with the appropriate training and tools to ensure greater crew autonomy. The importance of the integral relationship between habitability, logistics and crew physical and psychological support for long duration missions. The importance of designing equipment for longer lifetimes, with the capability to perform maintenance in space, and with an understanding of the certification criteria for continued use. The complexity of the multinational and multi-organizational program management and operations, and the benefits as well as drawbacks these introduce. Perhaps most significantly, the ISS Program has educated a new generation of engineers and managers, providing first-hand experience in the design, development, integration and operation of advanced humanoperated spacecraft. Downloaded by 54.161.69.107 on June 18, 2020 | http://arc.aiaa.org | DOI: 10.2514/6.2006-5978 These valuable lessons should be factored into the Exploration Program from the beginning, so that they do not have to be re-learned by the next generation of engineers and scientists who will take humans back to the Moon and on to Mars. Acknowledgments This paper would not have been possible without the contributions of the men and women of the International Space Station program and their supporting contractors. Additionally, the following individuals provided important contributions and support: Mark Uhran, Steven Pittotti, Andrew Thomas, Michael Foale, James Voss, Merri Sanchez, and John Bacon. 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