mission areas. Specifically, low-power EP systems are currently used for small robotic interplanetary missions (e.g., Hayabusa and Dawn), for post-launch circularization of the orbits of large geosynchronous communications satellites (e.g., Advanced Extremely High Frequency satellite), and stationkeeping for a wide range of spacecraft (e.g., GOES-R and commercial communications satellites). Development of high-power EP systems (30 kW to 600 kW) will enable larger scale missions with heavy payloads, including development of a more efficient in-space transportation system in Earth-space, sample returns from near-Earth objects (NEOs), the martian moons, other deep space destinations (including extensions of the JUNO mission to Jupiter), precursor demonstrations of in situ resource utilization (ISRU) facilities, and pre-placement of cargo for human exploration missions. In addition to these specific propulsion and power system technologies, demonstration of large scale EP vehicles is required to ensure adequate control during autonomous rendezvous and docking operations necessary for either cargo or small body proximity operations.

2. Cryogenic Storage and Transfer. Enable long-term storage and transfer of cryogens in space and reliable cryogenic engine operation after long dormant periods in space.

Deep space exploration missions will require high-performance propulsion for all mission phases, including Earth departure, destination arrival, destination departure, and Earth return, occurring over the entire mission duration. Both high-thrust propulsion options, LOX/H2 chemical propulsion and LH2 nuclear thermal rocket (NTR), will require storage of cryogens for well over a year to support all mission phases. Chemical and NTR engines must also operate reliably after being dormant for the same period. While LOX can currently be stored for extended periods, LH2 boil-off rates using state-of-the-art technology are far too high for deep-space missions, allowing only a few days of storage. Additionally, cryogenic fluid transfer technology would enable other exploration architectures, including propellant aggregation and the use of propellants produced using ISRU facilities. This technical challenge is enabling for the most plausible transportation architectures for human exploration beyond the Moon.

3. Microsatellites: Develop high-performance propulsion technologies for high-mobility microsatellites (<100 kg).

The broader impact of small satellites is hindered by the lack of propulsion systems with performance levels similar to those utilized in larger satellites (high ΔV, high Isp, low mass fractions, etc.). Most existing propulsion systems are not amenable for miniaturization and work is needed to develop concepts that scale and perform favorably. In addition to small satellites, high-performing miniature propulsion would also provide functionality in different applications, for example in distributed propulsion for controlling large, flexible structures and address missions requiring fine thrust for precise station keeping, formation flight, accurate pointing and cancellation of orbital perturbations. A moderate investment in many of these technologies (including chemical, electric, and advanced propulsion concepts, such as tethers and solar sails) could validate their applicability to small satellites.

4. Rapid Crew Transit: Establish propulsion capability for rapid crew transit to/from Mars.

Trip times for crewed missions to NEOs, Phobos, and the surface of Mars should be minimized to limit impacts to crew health from radiation (galactic and solar), exposure to reduced gravity, and other effects of long-duration deep space travel. Developing high-performance, high-thrust propulsion systems to reduce transit times for crewed missions would mitigate these concerns. Two realistic high-thrust options exist that could be available for missions in the next 20 years: LOX/H2 and NTR. Engines used for rapid crew transport must be capable of multiple restarts following prolonged periods of inactivity, and they must demonstrate extremely high reliability. There are no engines of either type currently available that meet the requirements of performance, reliability, and restart capability. The two LOX/H2 engines that come closest are the J2X, with about ~250,000 pounds of thrust and the RL-10, with about ~25,000 pounds of thrust. Both are high-performance engines and both have some restart capability, but neither has demonstrated the ability to accomplish multiple restarts following prolonged dormancy. Also, NTRs have never been tested in space, and the last ground test was conducted more than 40 years ago. There is also considerable uncertainty regarding the effort it would take to reconstitute the state of the art as it existed 40 years ago or to define test and operational requirements, and the environmental issues are substantial.

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