Assuming relatively modest investment in the coming decade for technology maturation and risk reduction demonstration, the human race could have the ability to effectively defend itself against the threat of asteroid impact starting as soon as the 2030’s. In this paper we summarize from an engineering perspective, some of the most important attributes and trades involved of such a Planetary Defense (PD) system-of-systems. We consider operational scenarios and present new results comparing and contrasting simulated realizations of the efficacy of available mitigation (asteroid deflection) technologies, especially the trade between impulsive techniques (Kinetic Impact) and low-thrust concepts such as Ion Beam Deflection. New results from the successful DART mission indicate the momentum enhancement factor "beta" due to KI impact ejecta could be as high as 4 or 5; while this makes the deflection momentum transfer of KI more effective, it also increases the chance of disruption proportionally. If the delivered KI energy is limited to avoid the risk of disruption due to high values of beta, then the cases where KI is safe to use severely limit the effectivity of this method, whereas the Ion Beam Deflection (IBD) method does not suffer this limitation. Effective KI deflection also likely requires a precursor rendezvous mission to measure the target asteroid’s mass, whereas an IBD strategy inherently includes a rendezvous, simplifying the overall defensive operation. For these reasons and others, we suggest that IBD should be strongly considered for further analysis and potential technology demonstration risk reduction. We conclude by illustrating one such concept of a cost-effective IBD demonstration mission.

Perovskite solar cells (PSCs) are studied in low-intensity low-temperature (LILT) conditions before and after low energy proton irradiation to characterize device performance at deep space mission-relevant light intensities and temperatures.

Electric propulsion is now in both widespread use and active flight-implementation across a broad spectrum of commercial and government applications ranging from cubesats, LEO constellations, GEO comsats, deep space science missions, and even the human-tended Lunar Gateway. It has been my good fortune to witness, and even participate in, to some small extent, the transition of electric propulsion technology from laboratory development to its current wide-ranging acceptance. This paper summarizes my recollection on how this happened from the perspective of working on electric propulsion for 44 years, most of it at NASA’s Jet Propulsion Laboratory. Space limitations dictate that this summary cannot include all (or maybe even most) of the important details and developments of a story that spans more than 40 years. The objective was to identify the key factors that drove electric propulsion’s successes.

Dwarf planet Ceres is a compelling target as an evolved ocean world with, at least, regional brine reservoirs and potentially ongoing geological activity. Situated in the Main Belt of asteroids, Ceres is the most water-rich body in the inner solar system (in relative abundance) and is accessible enough for a sample return with the resources of a typical medium-class (New Frontiers) NASA mission. NASA's Dawn mission at Ceres revealed the presence of liquid, brine-driven activity, organic matter, and a rich salt chemistry. With this evidence, the overarching goals of the mission concept presented herein are to quantify Ceres' current habitability potential and origin. A sample return from young evaporite deposits in Occator crater offers greater science return than an in situ exploration mission by enabling high-resolution analysis of (1) organic matter expected from terrestrial and chondritic analogs that are trapped in salt minerals and (2) isotopes of refractory elements for a similar cost and less science risk. The sample return concept would be executed with a single flight system due to Ceres' relative proximity to Earth and low gravity. Solar electric propulsion was identified as the most cost-effective approach for getting to Ceres and back. De-orbiting, landing, and takeoff are performed with a throttleable monopropellant hydrazine system. The solar arrays are stowed prior to landing and takeoff. Sample acquisition builds on the pneumatic system designed by Honeybee Robotics. The sample return mission concept relies on the availability of key technologies: an enhanced landing vision system leveraging investments from Mars 2020; retractable/redeployable solar arrays, which have been demonstrated on the International Space Station but not at Ceres' gravity; and an emerging design from upcoming missions for sample transfer from the pneumatic sampling system to the sample return capsule. Return of a sample of mass $\sim 100\mathrm{g}$ from Ceres, maintained at $\leq-20^{\circ}\mathbf{C}$ , is without precedent for any currently advocated Ocean World mission, enabling a vast range of experimental techniques back on Earth with sensitivities and accuracies far beyond those feasible with in situ instruments. A sample of this size also enables analyses to benefit from techniques that will become available in the future.

Dwarf planet Ceres is a compelling target for future exploration because it hosts at least regional brine reservoirs and potentially ongoing geological activity. As the most water-rich body in the inner solar system, it is a representative of a population of planetesimals that were likely a significant source of volatiles and organics to the inner solar system. Here we describe possible medium-class (around $1 billion) mission concepts that would determine both Ceres’ origin and its current habitability potential. Habitability is addressed through a combination of geological, geophysical, and compositional investigations by (i) searching for evidence from orbit of past and ongoing geological activity near landforms interpreted as brine-driven volcanic structures and (ii) probing the brine distribution below one of these regions with electromagnetic sounding (in situ). Two approaches were considered for compositional measurements, which address both habitability and origins: (1) in situ exploration at two sites and (2) sample return from a single site. Both concepts targeted material at Occator crater, which is one of the youngest features on Ceres (∼20 Ma) and a site rich in evaporites evolved from recently erupted brine sourced from a region >35 km deep. We conclude that a sample return architecture from these young evaporite deposits offers greater science return by enabling high-resolution analysis of organic matter (trapped in salt minerals) and isotopes of refractory elements for a similar cost and less science risk than in situ analyses. This manuscript describes the six science objectives and the two implementation concepts considered to achieve those objectives.

Deep understanding of planetary habitability requires identifying key factors that govern the surface environment over time. Venus is the ultimate control case for understanding how Earth developed and maintained conditions suited to life. Venus very likely had elements essential to habitability such as past surface water and a dynamo. Tectonism and volcanism, which create chemical disequilibrium, very likely persist today. What caused Earth and Venus to diverge down different evolutionary paths? VERITAS would create foundational, co-registered data sets of high-resolution topography, imaging, spectroscopy, and gravity, on par with those available for Mercury, Mars, and the Moon. VERITAS would answer outstanding fundamental questions about the evolution of Earth's twin. The VERITAS payload consists of the Venus Interferometric Synthetic Aperture Radar (VISAR) and the Venus Emissivity Mapper (VEM), plus a gravity science investigation. VISAR is an X-band radar that provides: 1) a global digital elevation model (DEM) with 250-m postings and 6-m height accuracy, 2) Synthetic aperture radar (SAR) imaging at 30-m horizontal resolution globally, 3) SAR imaging at 15-m resolution for $> \boldsymbol{25\%}$ of the surface, and 4) surface deformation from repeat pass interferometry (RPI) with 2-cm vertical precision for $> \boldsymbol{12} \boldsymbol{(\sim 200\ \mathrm{x}\ 200\ \text{km})}$ targeted areas. VEM covers $\boldsymbol{ > 70\%}$ of the surface in six near-infrared (NIR) bands sensitive to iron composition located within five atmospheric windows, plus eight atmospheric bands for calibration and water vapor measurements. VEM would provide near-global maps of mafic to felsic rock type and will search for active and recent volcanism. VERITAS would use two-way Ka-band uplink and downlink from a low circular orbit $\boldsymbol{(< 250\ \text{km})}$ to create a global gravity field with 3-mGal accuracy of 155-km resolution (degree and order 123). An onboard technology demonstration, the Deep Space Atomic Clock (DSAC-2), may support radio science and navigation with one-way tracking. VERITAS data would enable estimation of elastic thickness (a proxy for thermal gradient) and density differences due to subsurface structures, as well as constraining interior structure, including core size and state. Lockheed Martin builds the spacecraft. VISAR is built by JPL, with the Italian Space Agency (ASI) providing the low power electronics. ASI also provides transponders and a high gain antenna for the telecom system. CNES provides the Ka-band traveling wave tube amplifiers (TWTA). The German Space Agency (DLR) provides VEM and contributes algorithms for VISAR ground and onboard data processing.

Power beaming involves the wireless transfer of power, and could provide a revolutionary new way to power spacecraft and vehicles operating in difficult to access regions. Power beaming has the potential to represent an alternative solution to power spacecraft and landers where sunlight is unavailable. It could provide a source of power to robotic systems in permanently shadowed regions or power landers and rovers from orbiting spacecraft (e.g.,Moon, Mars, Europa, Enceladeus, Miranda).

Electric propulsion (EP) is an important technology for NASA. It has played a major role on three missions, namely Deep Space 1, Dawn and Space Technology 7, and it is planned for use on many more. The ion propulsion system for the ongoing Dawn mission has achieved several notable accomplishments, including providing a total velocity change (delta-V) of over 11 km/s to the spacecraft. As a result of these successes, solar electric propulsion (SEP) is now broadly recognized as an essential technology for both robotic and human exploration. NASA is currently conducting many projects focused on research and development of EP for a variety of applications. All three of NASA's mission directorates that deal directly with space exploration are actively engaged in supporting work in this area. This paper describes these projects in more detail, including the specific engineering activities being conducted at NASA's main centers for EP technology development, namely Glenn Research Center (GRC) and the Jet Propulsion Laboratory (JPL).

NASA is currently developing the Asteroid Redirect Robotic Mission (ARRM) that would have the capability to retrieve a boulder from a Near Earth Asteroid and place it in Lunar orbit where astronauts in an Orion spacecraft would rendezvous with the ARRM vehicle and explore the boulder. This mission is conceived as a capabilities demonstration mission that would path-find high power Solar Electric Propulsion (SEP) and Earth-independent human spaceflight operations for Mars missions in the 2030s, as part of NASA's Journey to Mars initiative. The high-power, light-weight solar arrays and high-power, magnetically-shielded hall thrusters being developed for ARRM will dramatically increase NASA's in-space transportation capability. These two technologies could propel affordable human missions to asteroids, Martian moons, and asteroids. In addition, these technologies could be used to enhance human access to cis-lunar space and the Lunar surface. This paper will provide an overview of ARRM and discuss how ARRM-developed technologies would feed forward to human deep space exploration missions.

Electric propulsion may play a crucial role in the implementation of the gravity tractor planetary defense technique. Gravity tractors were devised to take advantage of the mutual gravitational force between a spacecraft flying in formation with the target celestial body to slowly alter the celestial body's trajectory. No physical contact is necessary, which bypasses issues associated with surface contact such as landing, anchoring, or spin compensation. The gravity tractor maneuver can take several forms, from the originally proposed constant thrust in-line hover to the offset halo orbit. Both can be enhanced with the collection of mass at the asteroid. The form of the gravity tractor ultimately impacts the required thrust magnitude to maintain the formation, as well as constraints on the vectoring of the thrust direction. Solar electric propulsion systems provide an efficient mechanism for tugging the spacecraft-asteroid system due to their high specific impulse. Electric propulsion systems can generate thrust continuously at high efficiency, which is an ideal property for gravity tractors that may require years of operation to achieve the desired deflection because of the very low coupling force provided by the gravitational attraction. The performance and feasibility of the deflection are predicated on having the propulsion capability to maintain the gravity tractor. This paper describes the impacts of constraining the solar electric propulsion thrust magnitude and thrust vectoring capability. It is shown that uncertainty in asteroid density and size, when combined with the enforcement of the electric propulsion constraints, can preclude the feasibility of certain gravity tractor configurations. Additionally, odd thruster configurations are shown to drive the gimbal performance and to have major impacts on eroding incident spacecraft surfaces due to plume interaction. Center of gravity movement further exacerbates issues with gimbaling and plume interaction. A tighter plume divergence angle is therefore always desired, but this paper shows that there is an optimal momentum balance between plume interaction and asteroid-plume avoidance. Several gravity tractor techniques are compared based on metrics of time efficacy, as measured by the induced asteroid delta-V per unit time, and mass efficiency, as measured by the induced asteroid delta-V per unit mass of fuel. Given the propulsion constraints, halo orbits can be infeasible for smaller asteroids unless the mass of the spacecraft is augmented with collected material through a technique called the Enhanced Gravity Tractor. Another proposed method is to alter the halo period by canting the thrusters. In-line hover gravity tractors can always be moved along the net thrust direction to conform to the given propulsion system at the expense of performance, except in the case of smaller asteroids with propulsion systems that are limited in lower throttle range or maximum gimbal angle. Alternative strategies, such as on-off pulsing the thrusters to lower the effective thrust are considered. An example is described for deflecting asteroid 2008 EV5 (341843), which currently serves as the reference asteroid for the proposed Asteroid Redirect Robotic Mission.

Five potential methods of preventing an asteroid from colliding with Earth-Kinetic Impactor (KI), Ion Beam Deflection (IBD), Gravity Tractor (GT), Enhanced Gravity Tractor (EGT), and Laser Ablation (LA)-are compared, with the objective of helping to inform a NASA decision regarding which technology or technologies could be demonstrated in space on the proposed 2019 Asteroid Robotic Redirect Mission (ARRM). Three design blocks are considered, which differ in the power available to the deflection technology and the mass at low-Earth orbit. We plot the required warning time (up to 30 years between discovery of the hazard and potential collision with Earth) vs. asteroid diameter for each of four technologies (gravity tractor without enhancement is considered only as a trim/verification candidate) under each design block, and illustrate sensitivities of results to important parameters.

The Asteroid Redirect Robotic Mission (ARRM) concept seeks to rendezvous with, capture, and redirect to translunar space an entire small near-Earth asteroid with a mass of up to approximately 1000 metric tonnes. It would focus the capabilities of the science, technology, and the human exploration communities on a grand challenge creating a new synergy between robotic and human missions to advance human space exploration beyond low Earth orbit for the first time in 50 years. This paper addresses the key aspects of the ARRM concept and the options studied to assess its technical feasibility. Included are evaluations of the expected number of potential targets, their expected discovery rate, the necessity to adequately characterize candidate mission targets, the process to capture a non-cooperative asteroid in deep space, and the power and propulsion technologies required for transportation back to the Earth-Moon system. A class of distant retrograde lunar orbits that are stable for more than 250 years are identified as potential locations for storing the redirected asteroid. These orbits are reachable by the Asteroid Retrieval Vehicle transporting a 1000-t asteroid and are also reachable by crewed missions using the Space Launch System and Orion. The study concludes that the key aspects of finding, capturing and redirecting an entire small, near-Earth asteroid to the Earth-Moon system by the first half of the next decade are technically feasible. The study was conducted from January 2013 through July 2013 by the Jet Propulsion Laboratory (JPL) in collaboration with Glenn Research Center (GRC), Johnson Space Center (JSC), Langley Research Center (LaRC), and Marshall Space Flight Center (MSFC).

In-space transportation technology is the key to unlocking the material resources of near-Earth asteroids for the benefit of human spaceflight activities beyond low-Earth orbit. High-power solar electric propulsion, with power levels of around 50 kW represents the most capable, affordable, near-term propulsion technology available and is enabling for the capture and retrieval of entire small near-Earth asteroids. Future technology advances, stimulated by the successful retrieval of the first asteroid, will likely include scaling to higher power levels, operation at higher specific impulse levels, and ultimately the use of asteroid-derived materials as propellant.

This paper summarizes the results of a 2014 KISS workshop that identified a wide variety of ways that the technologies (and their near-term derivatives) developed for the proposed Asteroid Redirect Mission (ARM) would beneficially impact the Nation’s space interests including: human missions to Mars and its moons, planetary defense, orbital debris removal, robotic deep-space science missions, commercial communication satellites, and commercial asteroid resource utilization missions. This wide applicability of asteroid retrieval technology is, in many ways, is just as surprising as was the initial finding about the feasibility of ARM. The current Asteroid Redirect Mission concept consists of two major parts: the development of an advanced Solar Electric Propulsion (SEP) capability and the retrieval of a near-Earth asteroid. The improvement in SEP technology required by ARM provides an extensible path to support human missions to Mars, is applicable to all planetary defense techniques, could reduce the time required for the LEO-to-GEO transfer of large commercial or military satellites, would enable new deep space robotic science missions, and could enable affordable removal of large orbital debris objects. The asteroid retrieval part of ARM would greatly improve the understanding of the structure of rubble-pile asteroids necessary to evaluate the effectiveness of primary asteroid deflection techniques, demonstrate at least one secondary asteroid deflection technique, greatly accelerate the use of material resources obtained in space to further space exploration and exploitation, and further planetary science.