Robotic exploration of our Solar System is made possible by the ability to propel and deliver a spacecraft to its destination (and sometimes back to Earth), and to provide the power required to operate the instruments and systems that acquire scientific data and transmit them back to Earth. Challenging deep-space missions frequently require large spacecraft velocity changes (delta-v) from advanced propulsion systems to reach their target and maneuver to obtain data and samples, and the missions often need significant power in extreme environments. Advanced propulsion and power systems are thus critical elements in spacecraft design and play a role in determining overall mission capabilities and performance.
The future of deep-space exploration depends on developing technologies in five key areas of advanced propulsion and power:
- Electric propulsion: Increased capabilities and higher-efficiency thrusters are being developed to reduce cost and risk, and to enable credible mission proposals.
- Chemical propulsion: Future large mission classes depend on increased capabilities in feed systems -- such as pressurization systems, low-mass tanks, and cryogenic storage components. In addition, advances in propulsion-system modeling are being developed to increase chemical thruster capabilities.
- Precision propulsion: Advances are being made in micro- and milli-newton thruster development to provide extended life and reliability for precision formation flying and orbit control in next-generation Earth-observation and other science missions.
- Power systems: Higher-efficiency and higher-specific-power solar arrays and radioisotope power systems are desired to provide increased power and mission design flexibility for deep-space missions.
- Energy storage: Improved primary batteries, rechargeable batteries, and fuel cells with high specific energy and long-life capability are needed for the extreme environments that will be faced by future missions. Advances in these technologies would make more challenging missions possible and could reduce the system cost sufficiently to enable new Flagship, New Frontiers, Discovery, and space physics missions.
Selected Research Projects/Areas of Research
Advanced Electric-Propulsion Technologies
Advanced electric-propulsion technologies consist of electric-propulsion systems based on ion and Hall thrusters. These capabilities were successfully demonstrated on the Deep Space 1 and Dawn missions. Because electric-propulsion systems can deliver more mass for deep-space missions and can accommodate flexible launch dates and trajectories, they could enable many future missions. Development of an electric-propulsion stage using advanced thruster technologies and accompanying components, including solar electric power sources, is critically needed for future flagship missions and would be directly applicable to other missions as the technology matures and costs decrease. Solar electric propulsion is presently flying on Dawn, which uses 2.3 kW NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) engines. Other thruster technologies are emerging with higher power, thrust, and specific impulse (Isp) capabilities.
Advanced Chemical-Propulsion Technologies
Advanced chemical-propulsion technologies include milli-newton thrusters, monopropellant thrusters, ultra-lightweight tanks, and 100 to 200 lb–class bipropellant thrusters. Advances are being made to improve thruster performance and reduce risk and costs for attitude control system, and entry, descent, and landing (EDL) systems. Specific improvements include the development of electronic regulation of pressurization systems for propellant tanks, lower-mass tanks, pump-fed thruster development, and variable-thrust bipropellant engine modeling, as well as deep-space-propulsion improvements in cryogenic propellant storage systems and components.
Precision Micro/Nano Propulsion
Advanced thrusters are required for precision motion control/repositioning and high Isp for low-mass, multiyear missions. Solar pressure and aerodynamic drag compensation and repositioning requirements dictate Isp and thrust level, while precision control of attitude and interspacecraft distance drive minimum impulse. These thrusters produce micronewton thrust levels for solar-wind compensation and precision-attitude control. Precision noncontaminating propulsion is needed, especially for science missions with cryogenic optics and close-proximity spacecraft operations, to keep payload optical/infrared surfaces and guidance-navigation-control sensors pristine. Additional requirements are for high-efficiency thrusters that enable 5- to 10-year mission lifetimes that include significant maneuvering requirements. Performance targets for micro/nano propulsion include a miniature xenon thruster throttleable in the 0–3 mN range and with a 10-year life. Continued development and flight qualification of this thruster is required for some potential future missions.
Power Sources for Deep-Space Missions
Power source options for deep-space missions include solar cell arrays and radioisotope power systems (RPS). Solar arrays with specific power in the range of 40–80 W/kg are currently used in Earth-orbital missions and deep-space missions at distances up to about 4 AU. Future orbital and deep space missions may require advanced solar arrays with higher efficiency ( > 35%), and high specific power ( > 200 W/kg). Some deep space and planetary-surface missions may require advanced solar arrays capable of operating in extreme environments (radiation, low temperatures, high temperatures, dust). Using advanced materials and novel synthesis techniques, such high-efficiency solar cells and arrays are under development for use in future spacecraft applications. These advanced cells would increase power availability and reduce solar array size for a given power, and may also have applications for terrestrial energy production applications as well, if fabrication costs can be driven to sufficiently low levels.
Radioisotope power systems (RPS) with specific power of ~3 W/kg are currently used in most deep-space missions beyond ~4 AU, or for planetary surface missions where there is limited sunlight. JPL has long used RPS for deep space missions, including Voyager, Galileo, and Cassini, and will be using RPS for the Mars Science Lander (MSL), the next Mars rover. Future deep-space missions may require advanced RPS with long-life capability ( > 20 years), higher conversion efficiency ( > 10%), and higher specific power ( > 6 W/kg). Some deep-space mission concepts require the ability to operate in high radiation environments. Advanced thermoelectric radioisotope generators are under development by NASA for future space missions. The capabilities of smaller RPS are being explored for future exploration missions. The development of small RPS can enable smaller landers at extreme latitudes or regions of low solar illumination, subsurface probes, and deep-space microsatellites.
Energy Storage for Deep-Space Missions
The energy storage systems presently being used in space science missions include both primary and rechargeable batteries. Fuel cells are also being used in some human space missions. Primary batteries with specific energy of ~250 Wh/kg are currently used in missions such as planetary probes, landers, rovers, and sample-return capsules where one-time usage is sufficient. Advanced primary batteries with high specific energy ( > 500 Wh/kg) and long storage-life capability ( > 15 years) may be required for future missions. Some planetary surface missions would require primary batteries that can operate in extreme environments (high temperatures, low temperatures, and high radiation). JPL, in partnership with industry, is presently developing high-temperature ( > 400 ºC) and high-specific-energy primary batteries (lithium–cobalt sulfide, LiCoS2 ) for Venus surface missions and low-temperature ( < ?80 ºC) primary batteries (lithium–carbon monofluoride, LiCFX) for Mars and outer-planet surface missions. Rechargeable batteries with specific energies of ~100 Wh/ kg are currently used in robotic and human space missions (orbiters, landers, and rovers) as electrical energy storage devices. Advanced rechargeable batteries with high specific energy ( > 200 Wh/kg) and long-life capability ( > 15 years) may be required for future space missions. Some missions could require operational capability in extreme environments (low temperature, high temperature, and high radiation). JPL, in partnership with other NASA centers, is presently developing high-energy-density Li ion batteries ( > 200 Wh/kg) that can operate at low temperatures (~ ?60 °C) for future space missions.
Fuel cells, such as those used on the Space Shuttle, can be particularly attractive for human space science missions. These fuel cells have specific power in the range of 70–100 W/kg and a life of ~2500 h. Advanced fuel cells with high specific power (200 W/kg), higher efficiency ( > 75%), long-life capability ( > 15,000 h), and higher specific power may be needed for future human space missions. JPL is working on the development of such advanced fuel cells.