·How did we get here? ·Are we alone? The Space Science Strategic Plan outlines the long-term goals,near term objectives and proposed strategies that address these challenges.NASA must consolidate the results and recommendations from many external organizations including the National Research Council,The Planetary Society,universities,Congress,the international science community and others in order to proceed with specific missions targeted at achieving the science goals. As the missions become more challenging NASA must also develop the enabling technologies that make them possible.For planetary class exploration,Nuclear Electric Propulsion and Power(NEPP)systems offer capabilities that can make missions possible that are not possible today and can significantly enhance the scientific return of all other planetary missions.Increased power allows for new levels of science by providing higher levels of power for instruments and high bandwidth communications,allowing sufficient time to conduct experiments,providing access to areas previously not possible, enabling mobility at destinations and providing a resiliency for sustained operations.The use of nuclear electric propulsion can also decrease the time it takes for spacecraft to travel to the outer planets in addition to enabling multiple destinations,orbital change maneuvers and dynamic mission planning.Although the potential space applications for NEPP are vast,a pragmatic progressive approach that begins with planetary exploration, before moving to human missions,offers significant scientific returns for the investment and can potentially be leveraged for numerous future space applications. 12
• How did we get here? • Are we alone? The Space Science Strategic Plan outlines the long-term goals, near term objectives and proposed strategies that address these challenges. NASA must consolidate the results and recommendations from many external organizations including the National Research Council, The Planetary Society, universities, Congress, the international science community and others in order to proceed with specific missions targeted at achieving the science goals. As the missions become more challenging NASA must also develop the enabling technologies that make them possible. For planetary class exploration, Nuclear Electric Propulsion and Power (NEPP) systems offer capabilities that can make missions possible that are not possible today and can significantly enhance the scientific return of all other planetary missions. Increased power allows for new levels of science by providing higher levels of power for instruments and high bandwidth communications, allowing sufficient time to conduct experiments, providing access to areas previously not possible, enabling mobility at destinations and providing a resiliency for sustained operations. The use of nuclear electric propulsion can also decrease the time it takes for spacecraft to travel to the outer planets in addition to enabling multiple destinations, orbital change maneuvers and dynamic mission planning. Although the potential space applications for NEPP are vast, a pragmatic progressive approach that begins with planetary exploration, before moving to human missions, offers significant scientific returns for the investment and can potentially be leveraged for numerous future space applications. 12
1.2 Definition and Purpose This document seeks to establish a promising set of NEPP candidate architectures for future detailed concept definition and technology investment efforts.Although the United States has only flown one nuclear reactor in space,a significant amount of work has been completed on nuclear technologies and space power systems since the 1950s although,as a matter of national policy,very limited efforts have occurred over the last decade.Previous space nuclear efforts and planning activities have spanned a broad range of technologies and missions including Nuclear Thermal Rockets (NTR),multi- megawatt systems,multi-use platforms and interplanetary human missions.Over time an appreciable amount of concept designs,component testing and subsystem development for NEPP and other non-nuclear related space power and propulsion systems has been amassed.This activity will build upon previous efforts but will focus solely on planetary exploration class missions in the power range of 75 to 250 kW that can be achieved within ten to twelve years.This power range is based on previous and current NASA studies for planetary science missions.This requires a balanced approach to meeting mission requirements,assessing current capabilities and developing useful methods of concept selection.Additionally,the candidate architectural set must provide a viable pathway to a sustainable NEPP capability for NASA without either succumbing to near term flight gratification for political gains,which may compromise long-term objectives, Although the Bush Administration's 1992 National Space Policy Directive(NSPD-6),Titled,Space Exploration Initiative,stated,"NASA,DOD,and DOE shall continue technology development for space nuclear power and propulsion..."Congress did not support the proposed initiative and insufficient funds were available in existing budgets for reactor development.Further,under the 1996 Clinton Administration,Presidential Decision Directive/National Science and Technology Council (PDD/NSTC- 8)doctrine,Titled,National Space Policy,stated,"The Department of Energy will maintain the necessary capability to support space missions which may require the use of space nuclear power systems..." however,the policy set by OMB and the Administration focused funding on RTG efforts. 13
1.2 Definition and Purpose This document seeks to establish a promising set of NEPP candidate architectures for future detailed concept definition and technology investment efforts. Although the United States has only flown one nuclear reactor in space, a significant amount of work has been completed on nuclear technologies and space power systems since the 1950s although, as a matter of national policyi , very limited efforts have occurred over the last decade. Previous space nuclear efforts and planning activities have spanned a broad range of technologies and missions including Nuclear Thermal Rockets (NTR), multimegawatt systems, multi-use platforms and interplanetary human missions. Over time an appreciable amount of concept designs, component testing and subsystem development for NEPP and other non-nuclear related space power and propulsion systems has been amassed. This activity will build upon previous efforts but will focus solely on planetary exploration class missions in the power range of 75 to 250 kW that can be achieved within ten to twelve years. This power range is based on previous and current NASA studies for planetary science missions. This requires a balanced approach to meeting mission requirements, assessing current capabilities and developing useful methods of concept selection. Additionally, the candidate architectural set must provide a viable pathway to a sustainable NEPP capability for NASA without either succumbing to near term flight gratification for political gains, which may compromise long-term objectives, i Although the Bush Administration’s 1992 National Space Policy Directive (NSPD-6), Titled, Space Exploration Initiative, stated, “NASA, DOD, and DOE shall continue technology development for space nuclear power and propulsion…” Congress did not support the proposed initiative and insufficient funds were available in existing budgets for reactor development. Further, under the 1996 Clinton Administration, Presidential Decision Directive/ National Science and Technology Council (PDD/NSTC- 8) doctrine, Titled, National Space Policy, stated, “The Department of Energy will maintain the necessary capability to support space missions which may require the use of space nuclear power systems…” however, the policy set by OMB and the Administration focused funding on RTG efforts. 13
or being so focused on future growth that the immensity of the challenge and diffused mission objectives cause the program to fail under its own design.Further,primary system goals and objectives must be focused on performance from a science and customer perspective rather than solely on a series of technical specifications constructed on the basis of creating a high performance NEPP system alone. 2.0 Problem Description and Background 2.1 Planetary Exploration Challenges 2.1.1 Available Power Power availability challenges are inherent to space exploration.Table 1 illustrates planetary distances in Astronomical Units (AU)and the corresponding solar intensity in terms of the solar constant and incident energy in mW/cm2.The fractional amount of total solar flux available makes solar power systems,such as photovoltaic or solar dynamic,impractical for outer planet missions.Additionally,performing missions in protracted shadowed environments or polar missions of planets nearer to the Earth also make such systems impractical due to energy generation and storage limitations. Planetary exploration scenarios also must consider environments that are clouded and contain high natural radiation environments that further preclude the use of solar power systems.Radiation damage in solar cell devices occur when neutrons or charged particles (electrons,protons,ions)collide with the atomic nuclei and electrons in the device material.The collisions cause ionization,where electrons are removed,and atomic displacement,where atoms are displaced from their lattice structure,which collectively degrade both the voltage and current characteristics of the cell. 14
or being so focused on future growth that the immensity of the challenge and diffused mission objectives cause the program to fail under its own design. Further, primary system goals and objectives must be focused on performance from a science and customer perspective rather than solely on a series of technical specifications constructed on the basis of creating a high performance NEPP system alone. 2.0 Problem Description and Background 2.1 Planetary Exploration Challenges 2.1.1 Available Power Power availability challenges are inherent to space exploration. Table 1 illustrates planetary distances in Astronomical Units (AU) and the corresponding solar intensity in terms of the solar constant and incident energy in mW/cm2 . The fractional amount of total solar flux available makes solar power systems, such as photovoltaic or solar dynamic, impractical for outer planet missions. Additionally, performing missions in protracted shadowed environments or polar missions of planets nearer to the Earth also make such systems impractical due to energy generation and storage limitations. Planetary exploration scenarios also must consider environments that are clouded and contain high natural radiation environments that further preclude the use of solar power systems. Radiation damage in solar cell devices occur when neutrons or charged particles (electrons, protons, ions) collide with the atomic nuclei and electrons in the device material. The collisions cause ionization, where electrons are removed, and atomic displacement, where atoms are displaced from their lattice structure, which collectively degrade both the voltage and current characteristics of the cell. 14
Table 1:Planetary Distances and Solar Intensities? Planet Astronomical Units Solar Constant Incident Energy (mW/cm2) Mercury 0.39 6.6735 902.900 Venus 0.72 1.9113 258.600 Earth 1.00 1.0000 135.300 Mars 1.52 0.4300 58.280 Jupiter 5.20 0.0369 4.999 Saturn 9.54 0.0109 1.487 Uranus 19.18 0.0027 0.368 Neptune 30.07 0.0011 0.149 Pluto 39.44 0.0006 0.087 Power is necessary for advanced scientific investigations and to date has been limited to tens to hundreds of Watts.Allowing scientific payloads to move from hundreds to thousands of Watts provides for active experimentation in addition to enhanced passive observation.This includes new suites of radar experiments,advanced spectrometry,multi-spectral imaging,increased temporal resolution and the ability to provide high data rate communications.NEPP systems offer significantly higher power levels for science,provide the ability to operate in a variety of hostile planetary environments and generate power independent of solar distance. 2.1.2 Propulsion Requirements Issues relating to propulsion include the ability of delivering increased payloads to greater distances,reducing the time required to deliver the payload,flexibility in launching independent of planetary alignments,performing orbital maneuvers at the destination and enabling multiple destinations.It should be noted that NEPP systems are still dependent on chemical stages to achieve Earth orbit from which they depart. Presently,total chemical systems only have enough propulsive energy to achieve a flyby 15
Table 1: Planetary Distances and Solar Intensities2 Planet Astronomical Units Solar Constant Incident Energy (mW/cm2 ) Mercury 0.39 6.6735 902.900 Venus 0.72 1.9113 258.600 Earth 1.00 1.0000 135.300 Mars 1.52 0.4300 58.280 Jupiter 5.20 0.0369 4.999 Saturn 9.54 0.0109 1.487 Uranus 19.18 0.0027 0.368 Neptune 30.07 0.0011 0.149 Pluto 39.44 0.0006 0.087 Power is necessary for advanced scientific investigations and to date has been limited to tens to hundreds of Watts. Allowing scientific payloads to move from hundreds to thousands of Watts provides for active experimentation in addition to enhanced passive observation. This includes new suites of radar experiments, advanced spectrometry, multi-spectral imaging, increased temporal resolution and the ability to provide high data rate communications. NEPP systems offer significantly higher power levels for science, provide the ability to operate in a variety of hostile planetary environments and generate power independent of solar distance. 2.1.2 Propulsion Requirements Issues relating to propulsion include the ability of delivering increased payloads to greater distances, reducing the time required to deliver the payload, flexibility in launching independent of planetary alignments, performing orbital maneuvers at the destination and enabling multiple destinations. It should be noted that NEPP systems are still dependent on chemical stages to achieve Earth orbit from which they depart. Presently, total chemical systems only have enough propulsive energy to achieve a flyby 15
or "snapshot"on outer planetary missions rather than an orbital opportunity in which detailed studies could be undertaken over a longer period of time. Reaching the outer planets and beyond takes tremendous amounts of propulsive energy and requires planetary launch assists to accomplish chemical only missions.For example,the Galileo mission used gravity assists from Venus and Earth to gain enough momentum to travel to Jupiter.As a result,Galileo spent the first three years of its journey making flybys of Venus and Earth before it was ready to swing outward toward Jupiter.Cassini is currently on a similar tour of the solar system,on its way to Saturn, and is using a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist)trajectory. Planetary assists are essentially auxiliary propulsion.They take time,are directly coupled to the ability to perform the mission and consequently can become a significant launch constraint when planning outer planetary missions.Although the use of planetary gravity assists is not necessary with NEPP they could be used,if desired,to augment NEPP mission trajectory designs. Increasing the efficiency of the propulsion system is directly related to increased payloads.Developing NEPP systems with low weight to power ratios,or specific mass, will result in increased payloads over that of current chemical systems for planetary missions.NEPP systems also provide acceleration over a large part of the mission trajectory that results in higher velocities.As mission distance increases,the trip time may decrease relative to chemical missions due to the increased velocities achieved. One of the most demanding requirements is for orbital maneuvers at the destination or having capability to move from one destination to another.Orbital mission flexibility at the destination allows for plane,altitude and eccentricity changes that enable 16
or “snapshot” on outer planetary missions rather than an orbital opportunity in which detailed studies could be undertaken over a longer period of time. Reaching the outer planets and beyond takes tremendous amounts of propulsive energy and requires planetary launch assists to accomplish chemical only missions. For example, the Galileo mission used gravity assists from Venus and Earth to gain enough momentum to travel to Jupiter. As a result, Galileo spent the first three years of its journey making flybys of Venus and Earth before it was ready to swing outward toward Jupiter. Cassini is currently on a similar tour of the solar system, on its way to Saturn, and is using a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist) trajectory. Planetary assists are essentially auxiliary propulsion. They take time, are directly coupled to the ability to perform the mission and consequently can become a significant launch constraint when planning outer planetary missions. Although the use of planetary gravity assists is not necessary with NEPP they could be used, if desired, to augment NEPP mission trajectory designs. Increasing the efficiency of the propulsion system is directly related to increased payloads. Developing NEPP systems with low weight to power ratios, or specific mass, will result in increased payloads over that of current chemical systems for planetary missions. NEPP systems also provide acceleration over a large part of the mission trajectory that results in higher velocities. As mission distance increases, the trip time may decrease relative to chemical missions due to the increased velocities achieved. One of the most demanding requirements is for orbital maneuvers at the destination or having capability to move from one destination to another. Orbital mission flexibility at the destination allows for plane, altitude and eccentricity changes that enable 16