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Nuclear thermal propulsion – Progress and potential
Abstract
This paper describes the current research and development effort s currently underway within the United States on Nuclear Thermal Propulsion (NTP), with a particular focus on the Demonstration Rocket for Agile Cislunar Operations (DRACO) project, a joint effort of the United States Defense Advanced Projects Agency and the National Aeronautics and Space Administration. However, to put the DRACO project into context, the prior United States’ prior effort s on NTR are described and the foundation those efforts pro- vided to enable DRACO. The impact of NTP propulsion on both human and scientific exploration of the Solar System will also be discussed. And finally, the topic of advanced NTP propulsion will be addressed, including liquid fuel NTP engines.
... There are two main types: nuclear thermal propulsion (NTP), which heats liquid hydrogen for thrust, and nuclear electric propulsion (NEP), which generates electricity to power thrusters [3]. NTP is often preferred for human and scientific missions due to its higher thrust-toweight ratio [4]. ...
As interest in cislunar exploration grows, nuclear thermal propulsion (NTP) is emerging as a key technology for future missions. While NTP offers efficiency and high thrust-to-weight ratios, its safety risks remain a concern , particularly in the event of an in-space fragmentation. This paper investigates potential breakup events of an NTP-powered rocket near the Earth-Moon L 2 La-grange point, analyzing debris dispersion and impact sites on the lunar surface. Additionally, we assess the observ-ability of these fragments using space situational awareness (SSA) strategies. Furthermore, the potential radiation dose rate from nuclear-contaminated debris impact-ing the lunar surface is shown in the different scenarios. Our findings highlight the need for enhanced monitoring and mitigation measures to ensure the safe deployment of nuclear technology in cislunar space.
... In high temperature gas-cooled reactors, uranium carbides enhance fuel performance in particle fuels when fabricated as an uranium oxycarbide composite (UO 2 + UC x ) [5][6][7][8]. Additionally, uranium carbides such as UC 2 have previously been studied in early nuclear fueled rocketry [9,10], and advances in uranium carbide fuels are currently being explored for nuclear thermal propulsion as a solid solution with zirconium carbide (U,Zr)C [11][12][13]. In both applications, the formation of fission products will be of concern for either fuel performance or nuclear waste management. ...
As advances are being made regarding the performance of nuclear fuels, uranium carbides, and composites, such as (U,Zr)C and UO2 + UCx, have recently gained significant interest for deployment in nuclear space propulsion and high temperature gas-cooled reactors, respectively. However, the phase equilibria of several fission products in carbide systems remain unknown and may impact the overall fuel performance, specifically for particle nuclear fuels that are designed for commercial nuclear energy. Furthermore, comprehensive thermodynamic data on Rare Earth (RE) carbides, such as the Nd-C and Ce-C binary systems, remain limited. Presented in this study are the synthesis methods and characterizations of several Nd-C and Ce-C compositions. The findings from this research provide insights on the stability of RE-C binaries that form in irradiated nuclear fuels and address a critical knowledge gap in the current state of thermodynamics for two key RE-C systems.
This chapter explores humanity’s complex challenges and opportunities as we seek to explore and colonize the stars. The space colonization process emphasizes the importance of technological adaptation while also considering biological adaptation and the input of sociocultural forces, thus demonstrating the need for interdisciplinary collaboration to find solutions to challenges such as ineffective propulsion systems, harmful health effects of long-term exposure to space, inefficient propulsion systems, and governance problems. This conversation highlights breakthroughs like nuclear thermal propulsion, in-situ resource utilization and CRISPR-based therapies to allow for long-term human habitats in space. Ethics, planetary protection and equitable resource distribution are themes that emerge alongside calls for mechanisms for international cooperation similar to those inspired by the Antarctic Treaty System. Grounded in theory and practice, it calls for a shift in focus towards an approach that prioritizes technological rigor, human adaptability, and ethical stewardship to ensure extraterrestrial benefits and terrestrial sustainability.
Recent advances in chemical propulsion have brought a revolution in the ability to economically access near-Earth orbit. As the world turns its eye on returning to the Moon and then on to Mars, understanding the factors to overcome from a technical perspective of space propulsion is important. First, readers are provided with motivation for why sustained missions to Mars is not as simple as sending a rocket to the planet (but still a momentous step for humanity). Next, near-term technologies and their capabilities are discussed along merits and limitations. Finally, we are in one of those exciting inflection points of human exploration and some suggestions are provided on how we must continue enabling exploration through the lens of space propulsion technology maturation.
The Centrifugal Nuclear Thermal Rocket (CNTR) is a Nuclear Thermal Propulsion (NTP) concept designed to heat propellant directly by the reactor fuel. The primary difference between the CNTR concept and traditional NTP systems is that rather than using traditional solid fuel elements, the CNTR uses liquid fuel with the liquid contained in rotating cylinders by centrifugal force. If the concept proves feasible, CNTR could provide specific impulse (Isp) as high as 1800s at high thrust, which may enable (i) viable near-term human Mars exploration by reducing round- trip times to 420 days and (ii) direct injection orbits for scientific missions to the outer planets of the solar system and Kuiper Belt objects. The CNTR could also use storable propellants such as ammonia, methane, propane, or hydrazine at an Isp of 900 s, enabling long-term in-space storage of a dormant system. Research is presently underway to determine resolutions for the significant engineering challenges that the CNTR concept presents. Papers were presented at the 2021 and 2022 International Astronautical Congresses which described these challenges, the study plan to address them, and progress to date. In particular, the 2022 paper highlighted the challenge of neutronics driving the heat generation gradient in the liquid uranium annulus, which results in the greatest heat generation on the outer wall of the annulus, where the maximum temperature of the containment wall is constrained to maintain structural integrity. This constraint was resulting in performance projections well below the theoretical projection. This paper provides a follow-on update which summarizes progress of the overall research effort, including strategies and key results to date on leveling the heat generation gradient in the liquid uranium annulus. The paper will also summarize the 3D modeling of the gaseous hydrogen bubbles in the liquid uranium and experimental results on gaseous and liquid analogs to validate the analytical models. Finally, estimates of engine key performance parameters including specific impulse, thrust and thrust to weight ratio will be given, and mission analyses of scientific missions to various Solar Systems destinations including Kuiper belt objects.
Highly efficient nuclear thermal propulsion (NTP) can enable a new class of planetary science missions for deep space exploration. This paper presents Jupiter and Saturn rendezvous missions using an NTP system that will focus on end-to-end trajectory analysis. The complexities of Earth escape and planetary orbital insertion using finite burn maneuvers are highlighted. With respect to the mission design problem, NTP-powered missions need to integrate the requirements and constraints of mission objectives, spacecraft design, NTP system design, and launch vehicle limits into a self-consistent model. Using a single high-performance-class commercial launch vehicle with a lift capability to low Earth orbit of 22 metric tons, an NTP-powered mission can deliver NASA’s large strategic science missions in a direct transfer trajectory to Jupiter in 2.1 years and to Saturn in 4.7 years.
The international planetary science community met in London in January 2020, united in the goal of realizing the first dedicated robotic mission to the distant ice giants, Uranus and Neptune, as the only major class of solar system planet yet to be comprehensively explored. Ice-giant-sized worlds appear to be a common outcome of the planet formation process, and pose unique and extreme tests to our understanding of exotic water-rich planetary interiors, dynamic and frigid atmospheres, complex magnetospheric configurations, geologically-rich icy satellites (both natural and captured), and delicate planetary rings. This article introduces a special issue on ice giant system exploration at the start of the 2020s. We review the scientific potential and existing mission design concepts for an ambitious international partnership for exploring Uranus and/or Neptune in the coming decades.
This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.
Robotic space exploration to the outer solar system is difficult and expensive and the space science community works inventively and collaboratively to maximize the scientific return of missions. A mission to either of our solar system Ice Giants, Uranus and Neptune, will provide numerous opportunities to address high-level science objectives relevant to multiple disciplines and deliberate cross-disciplinary mission planning should ideally be woven in from the start. In this review, we recount past successes as well as (NASA-focused) challenges in performing cross-disciplinary science from robotic space exploration missions and detail the opportunities for broad-reaching science objectives from potential future missions to the Ice Giants.
This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.
This paper presents a literature review of technical papers and reports on mission studies conducted for robotic missions using Nuclear Thermal Propulsion (NTP) system for interplanetary missions. The paper outlines the basic principle and design of a nuclear thermal engine including performance assessment of small and large class NTP engines. Robotic mission concepts using NTP to various destinations such as Europa, Pluto and Ice giant planets are reviewed. The paper highlights the knowledge gaps in mission concept studies and focuses on various areas requiring detailed analysis to fulfill those knowledge gaps.
The future of human exploration missions to Mars is dependent on solutions to the technology challenges being worked on by the National Aeronautics and Space Administration (NASA) and industry. One of the key architecture technologies involves propulsion that can transport the human crew from Earth orbit to other planets and back to Earth with the lowest risk to crew and the mission. Nuclear thermal propulsion (NTP) is a proven technology that provides the performance required to enable benefits in greater payload mass, shorter transit time, wider launch windows, and rapid mission aborts due to its high specific impulse and high thrust.
Aerojet Rocketdyne (AR) has stayed engaged for several decades in working NTP engine systems and has worked with NASA recently to perform an extensive study on using low-enriched uranium NTP engine systems for a Mars campaign involving crewed missions from the 2030s through the 2050s. Aerojet Rocketdyne has used a consistent set of NASA ground rules and they are constantly updated as NASA adjusts its sights on obtaining a path to Mars, now via the Lunar Operations Platform-Gateway. Building on NASA’s work, AR has assessed NTP as the high-thrust propulsion option to transport the crew by looking at how it can provide more mission capability than chemical or other propulsion systems.
The impacts of the NTP engine system on the Mars transfer vehicle configuration have been assessed via several trade studies since 2016, including thrust size, number of engine systems, liquid hydrogen stage size, reaction control system sizing, propellant losses, NASA Space Launch System (SLS) payload fairing size impact, and aggregation orbit.
An AR study activity in 2018 included examining NTP stages derived from Mars crew mission elements to deliver extremely large cargo via multiple launches or directly off the NASA SLS. This paper provides an update on the results of the ongoing engine system and mission trade studies.
The fundamental capability of Nuclear Thermal Propulsion (NTP) is game changing for space exploration. A first generation Nuclear Cryogenic Propulsion Stage (NCPS) based on NTP could provide high thrust at a specific impulse above 900 s, roughly double that of state of the art chemical engines. The foundation provided by development and utilization of a NCPS could enable development of extremely high performance systems. The role of the NCPS in the development of advanced nuclear propulsion systems could be analogous to the role of the DC-3 in the development of advanced aviation. Progress made under the NCPS project could help enable both advanced NTP and advanced Nuclear Electric Propulsion (NEP).
The feasibility of rebuilding and testing a nuclear thermal rocket (NTR)
for the Mars mission was investigted. Calculations indicate that an NTR
would substantially reduce the Earth-orbit assemble mass compared to
LOX/LH2 systems. The mass savings were 36 and 65% for the cases of total
aerobraking and of total propulsive braking respectively. Consequently,
the cost savings for a single mission of using an NTR, if aerobraking is
feasible, are probably insufficient to warrant the NTR development. If
multiple missions are planned or if propulsive braking is desired at
Mars and/or at Earth, then the savings of about 4 to 5 billion. The concept of developing a full-power test stand at
Johnston Atoll in the Pacific appears very feasible. The added expense
of building facilities on the island should be less than $1.4 billion.
This paper provides a summary of the 2007 Mars Design Reference Architecture 5.0 (DRA 5.0), which is the latest in a series of NASA Mars reference missions. It provides a vision of one potential approach to human Mars exploration, including how Constellation systems could be used. The strategy and example implementation concepts that are described here should not be viewed as constituting a formal plan for the human exploration of Mars, but rather provide a common framework for future planning of systems concepts, technology development, and operational testing as well as potential Mars robotic missions, research that is conducted on the International Space Station, and future potential lunar exploration missions. This summary of the Mars DRA 5.0 provides an overview of the overall mission approach, surface strategy and exploration goals, as well as the key systems and challenges for the first three concepts for human missions to Mars.
This paper describes the design for the Particle Bed Reactor (PBR) that we considered for the Space Nuclear Thermal Propulsion (SNTP) Program. The methods of analysis and their validation are outlined first. Monte Carlo methods were used for the physics analysis, several new algorithms were developed for the fluid dynamics, heat transfer and transient analysis; and commercial codes were used for the stress analysis. We carried out a critical experiment, prototypic of the PBR to validate the reactor physics; blowdown experiments with beds of prototypic dimensions were undertaken to validate the power-extraction capabilities from particle beds. In addition, materials and mechanical design concepts for the fuel elements were experimentally validated.Four PBR rocket reactor designs were investigated parametrically. They varied in power from 400 MW to 2000 MW, depending on the mission's goals. These designs all were characterized by a negative prompt coefficient, due to Doppler feedback, and a moderator feedback coefficient which varied from slightly positive to slightly negative. In all practical designs, we found that the coolant worth was positive, and the thrust/weight ratio was greater than 20.
View Video Presentation: https://doi.org/10.2514/6.2021-3604.vid The Centrifugal Nuclear Thermal Rocket (CNTR) is a Nuclear Thermal Propulsion (NTP) concept designed to heat propellant directly by the reactor fuel. The primary difference between the CNTR concept and traditional NTP systems is that rather than using traditional solid fuel elements, the CNTR uses liquid fuel with the liquid contained in rotating cylinders by centrifugal force. If the concept can be successfully realized, the CNTR would have a high specific impulse (~1800 s) at high thrust, which may enable viable near-term human Mars exploration by reducing round-trip times to ~420 days. The CNTR could also use storable propellants such as ammonia, methane, or hydrazine at an Isp of ~900 s, enabling long-term in-space storage of a dormant system. Significant engineering challenges must be addressed to establish the technical viability of the CNTR, and the plan for addressing these engineering challenges is the subject of this paper.
The development and qualification of nuclear thermal propulsion (NTP) fuel element technologies would be aided by an in-depth model of material response and failure modes at operating conditions. Integrated computational materials engineering techniques have the potential to provide such a model, as demonstrated here through three case studies focused on a tungsten–uranium mononitride (UN) cermet fuel. The first case focuses on the erosion of tungsten (also named wolfram), a nominal coating/cladding and fuel element matrix material, in hot hydrogen. Ab initio techniques are used to calculate erosion rates and thermal expansion at NTP operating conditions. The second focuses on the stability of UN fuels at high temperature and in the presence of hydrogen. Phase diagram techniques augmented with ab initio thermodynamic data reveal potential instabilities and decomposition pathways at high hydrogen concentrations. The third focuses on using microstructure information to predict high-temperature mechanical response and failure of tungsten. Combined finite element and discrete dislocation dynamics techniques provide mechanical properties in agreement with experimental methods. The integration of these techniques for an all-encompassing material model is discussed.
Nuclear thermal propulsion (NTP) is an in-space propulsion technology capable of both high specific impulse (850–1000 s) and thrust (44–1112 kN), which can help reduce trip times for crewed missions beyond low Earth orbit. NTP technology has been demonstrated during historic programs. Over 20 ground test reactor experiments were performed, which demonstrated the prototypic reactor operations, during the Nuclear Engine for Rocket Vehicle Application (NERVA)/Rover program (1955–1972). Although historical programs have shown that NTP is a viable in-space propulsion technology, developing NTP in modern programs is contingent on the development and qualification of ultrahigh-temperature nuclear fuel technologies that can withstand engine operating conditions. In historical NTP development programs such as NERVA/Rover, prototypic reactor/engine schemes were ground tested to assess the overall system feasibility and to qualify the reactor fuel forms for eventual flight systems. Although this approach is effective to verify fuel performance under prototypic conditions, relying solely on full-scale NTP reactor tests as the pathway for verifying or qualifying fuel is inefficient and cost prohibitive today. Additionally, modern nuclear licensing requirements state that before test reactor approval, reactor components and fuel elements should be qualified via non-nuclear (out-of-pile) and nuclear (in-pile) testing under representative operating conditions. Using this methodology, fuel matures as production scale fabrication methods are established, and as produced fuel performance is demonstrated. This paper provides an overview of historical approaches to NTP fuel performance maturation, including fuel screening and qualification needs, and provides insight for establishing an efficient testing paradigm that can be implemented to rapidly and affordably develop NTP fuel forms for eventual qualification.
This paper is concerned with the possibility of implementing Low-Enriched Uranium (LEU) fuels in NERVA (Nuclear Engine for Rocket Vehicle Application) type Nuclear Thermal Rockets (NTR). It presents a detailed study of the requirements for implementing LEU in a NERVA type NTR revolving around the softening of the neutron spectrum. This work has been done to demonstrate the possibility to enhance the proliferation resistance of these nuclear space systems in the hope of fostering the groundwork for their eventual commercial use. The necessity of thermalizing the neutron spectrum is discussed along with a detailed study of various methods by which this can be achieved. The effect of minimizing non fission neutron loss through the selection of appropriate structural materials is then presented along with a cursory study of various options. Finally a reference conceptual reactor configuration is proposed and compared with previous NTR designs where its relative shortcomings and obvious advantages are presented and discussed. It is concluded that an LEU-NTR is not only a definite possibility, but also offers distinct advantages over existing Highly-Enriched Uranium (HEU) NTR designs, particularly the increased proliferation resistance of the system and the reduction of the fuel cost and development schedule. Finally, some suggestions are made for future work that would enable the progression of the current conceptual design to a more finalized reactor design.
This paper is the second paper on the study regarding the feasibility of low-enriched uranium (LEU) fuel in nuclear thermal rocket (NTR) applications. While the first paper dealt primarily with the issue of achieving and demonstrating criticality of the LEU-loaded NTR, this companion paper addresses other important reactor parameters which need to be verified in order to demonstrate the practicability of the LEU-NTR. Following the conclusions of the first paper, this second paper seeks to characterize various neutronic and performance parameters in order to definitely demonstrate the technical feasibility of the LEU-NTR. The parameters characterized and presented in this study are the reactor burn-up behavior including effects due to the build-up of Xe-135, the temperature coefficient of reactivity, the reactivity capability of the control drum system, and a rocket performance comparison with previous NTR systems. Impacts of the transient Xe reactivity change during operation have been characterized though various reactor depletion analyses using the MCNP5.1 and MCNPX codes in combination with the ENDF/B-VII.0 nuclear data library. The necessary level of excess reactivity of the LEU-loaded reactor has been quantified and the reactivity control drum, system has been assessed in term of the reactor shutdown capability. In this work, the reference core design presented in the first paper has been modified in order to improve the operational performance of the LEU-NTR to operate with a 900 s specific impulse and a 110 kN thrust.
This paper compares material issues for cermet and graphite fuel elements. In particular, two issues in NTP fuel element performance are considered here: ductile to brittle transition in relation to crack propagation, and orificing individual coolant channels in fuel elements. Their relevance to fuel element performance is supported by considering material properties, experimental data, and results from multidisciplinary fluid/thermal/structural simulations. Ductile to brittle transition results in a fuel element region prone to brittle fracture under stress, while outside this region, stresses lead to deformation and resilience under stress. Poor coolant distribution between fuel element channels can increase stresses in certain channels. NERVA fuel element experimental results are consistent with this interpretation. An understanding of these mechanisms will help interpret fuel element testing results.
The nuclear thermal rocket (NTR) represents the next “evolutionary step” in high performance rocket propulsion. Unlike conventional chemical rockets that produce their energy through combustion, the NTR derives its energy from fission of Uranium-235 atoms contained within fuel elements that comprise the engine's reactor core. Using an “expander” cycle for turbopump drive power, hydrogen propellant is raised to a high pressure and pumped through coolant channels in the fuel elements where it is superheated then expanded out a supersonic nozzle to generate high thrust. By using hydrogen for both the reactor coolant and propellant, the NTR can achieve specific impulse (Isp) values of ~900 seconds (s) or more - twice that of today's best chemical rockets. From 1955-1972, twenty rocket reactors were designed, built and ground tested in the Rover and NERVA (Nuclear Engine for Rocket Vehicle Applications) programs. These programs demonstrated: (1) high temperature carbide-based nuclear fuels; (2) a wide range of thrust levels; (3) sustained engine operation; (4) accumulated lifetime at full power; and (5) restart capability - all the requirements needed for a human Mars mission. Ceramic metal “cermet” fuel was pursued as well, as a backup option. The NTR also has significant “evolution and growth” capability. Configured as a “bimodal” system, it can generate its own electrical power to support spacecraft operational needs. Adding an oxygen “afterburner” nozzle introduces a variable thrust and Isp capability and allows bipropellant operation. In NASA's recent Mars Design Reference Architecture (DRA) 5.0 study, the NTR was selected as the preferred propulsion option because of its proven technology, higher performance, lower launch mass, versatile vehicle design, simple assembly, and growth potential. In contrast to other advanced propulsion options, no l- rge technology scale-ups are required for NTP either. In fact, the smallest engine tested during the Rover program - the 25,000 lbf (25 klbf) “Pewee” engine is sufficient when used in a clustered engine arrangement. The “Copernicus” crewed spacecraft design developed in DRA 5.0 has significant capability and a human exploration strategy is outlined here that uses Copernicus and its key components for precursor near Earth object (NEO) and Mars orbital missions prior to a Mars landing mission. The paper also discusses NASA's current activities and future plans for NTP development that include system-level Technology Demonstrations - specifically ground testing a small, scalable NTR by 2020, with a flight test shortly thereafter.
The results of nuclear rocket development activities from the inception of the ROVER program in 1955 through the termination of activities on January 5, 1973 are summarized. This report discusses the nuclear reactor test configurations (non cold flow) along with the nuclear furnace demonstrated during this time frame. Included in the report are brief descriptions of the propulsion systems, test objectives, accomplishments, technical issues, and relevant test results for the various reactor tests. Additionally, this document is specifically aimed at reporting performance data and their relationship to fuel element development with little or no emphasis on other (important) items.
The particle bed reactor designed for 100 to 300 MW power output using hydrogen as a coolant is capable of specific impulses up to 1000 seconds as a nuclear rocket. A single space shuttle compatible vehicle can perform extensive missions from LEO to 3 times GEO and return with multi-ton payloads. The use of hydrogen to directly cool particulate reactor fuel results in a compact, lightweight rocket vehicle, whose duration of usefulness is dependent only upon hydrogen resupply availability. The LEO to GEO mission had a payload capability of 15.4 metric tons with 3.4 meters of shuttle bay. To increase the volume limitation of the shuttle bay, the use of ammonia in the initial boost phase from LEO is used to give greater payload volume with a small decrease in payload mass, 8.7 meters and 12.7 m-tons. 5 refs., 15 figs.
Research sponsored by the Atomic Energy Commission, the USAF, and NASA (later on) in the area of nuclear rocket propulsion is discussed. It was found that a graphite reactor, loaded with highly concentrated Uranium 235, can be used to heat high pressure liquid hydrogen to temperatures of about 4500 R, and to expand the hydrogen through a high expansion ratio rocket nozzle assembly. The results of 20 reactor tests conducted at the Nevada Test Site between July 1959 and June 1969 are analyzed. On the basis of these results, the feasibility of solid graphite reactor/nuclear rocket engines is revealed. It is maintained that this technology will support future space propulsion requirements, using liquid hydrogen as the propellant, for thrust requirements ranging from 25,000 lbs to 250,000 lbs, with vacuum specific impulses of at least 850 sec and with full engine throttle capability. 12 refs.
The SNTP Program was an advanced technology development effort aimed at providing the Nation a new, dramatically higher performing rocket engine that would more than double the performance of the best conventional chemical rocket engines. The program consisted of three phases. Phase I ran from November 1987 through September 1989. The objective of this phase was to verify the feasibility of the Particle Bed Reactor (PBR) as the propulsion energy source for the upper stage of a ground-based Boost Phase Intercept (BPI) vehicle. The BPl mission was of interest to the Strategic Defense Initiative Organization (SDIO) who sponsored the program. Phase II started under SDIO control and was transferred to the Air Force (AF) in October 1991. The BPI mission was de-emphasized, and engine requirements were revised to satisfy more general AF space missions. The goal of Phase II was to perform a ground demonstration of a prototypical PBR engine. (MM)
With the assumption that future attempts to explore our Solar System for life will be limited by economic constraints, we have formulated a series of principles to guide future searches: (1) the discovery of life that has originated independently of our own would have greater significance than evidence for panspermia; (2) an unambiguous identification of living beings (or the fully preserved, intact remains of such beings) is more desirable than the discovery of markers or fossils that would inform us of the presence of life but not its composition; (3) we should initially seek carbon-based life that employs a set of monomers and polymers substantially different than our own, which would effectively balance the need for ease of detection with that of establishing a separate origin; (4) a "follow-the-carbon" strategy appears optimal for locating such alternative carbon-based life. In following this agenda, we judge that an intensive investigation of a small number of bodies in our Solar System is more likely to succeed than a broad-based survey of a great number of worlds. Our priority for investigation is (1) Titan, (2) Mars, (3) Europa. Titan displays a rich organic chemistry and offers several alternative possibilities for the discovery of extant life or the early stages that lead to life. Mars has already been subjected to considerable study through landers and orbiters. Although only small amounts of methane testify to the inventory of reduced carbon on the planet, a number of other indicators suggest that the presence of microbial life is a possibility. Care will be needed, of course, to distinguish indigenous life from that which may have spread by panspermia. Europa appears to contain a subsurface ocean with the possibility of hydrothermal vents as an energy source. Its inventory of organic carbon is not yet known.
If we buy into the goals of the Space Exploration Initiative (SEI) and accept that they are worthy of the hefty investment of our tax dollars, then we must begin to evaluate the technologies which enable their attainment. The main driving technology is the propulsion systems; for interplanetary missions, the safest and most affordable is a Nuclear Thermal Propulsion (NTP) system. An overview is presented of the NTP systems which received detailed conceptual design and, for several, testing.
Pluto Orbiter/lander and sample return missions are not impossible using chemical propulsion, but are possible with nuclear thermal propulsion. Using the MITEE nuclear engine, a spacecraft could first orbit Pluto, mapping it, and then land at a selected site, 12 years after the departure form Earth. If surface water/ice is available, fresh H2 propellant could be manufactured by electrolysis of melt H2O using power from the bi-modal nuclear engine, enabling multiple hops to new sites for further data collection. A warm water probe could also be deployed to explore the interior of Pluto's ice sheets. After completing exploration, the spacecraft could return samples from Pluto to Earth with a 12-year trip time. Mission architectures and the design of the spacecraft, nuclear propulsion engine, propellant manufacturing unit and warm water probe are described herein.
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