Massively Expanded NEA Accessibility via Microwave-Sintered Aerobrakes

The availability of a wide range of natural resources among the near-Earth asteroid (NEA) population offers the opportunity to utilize these resources in the service of making access to most of the Solar System much easier than any classical approach which relies solely upon structural, heat-shield, life support and propellant materials lifted from Earth. We have concentrated our attention on the two main factors that influence the application and utility of in situ aerobrake manufacture on near-earth asteroids. The first of these is the use of microwave sintering in the fabrication of aerocapture heatshields for retrieval of asteroidal materials into Earth orbit; the second is assessment of the performance of these aerocapture devices, including making very large numbers of NEAs accessible as sources of essential materials to support space exploration and exploitation. The ability to provide propellants, life support materials, or structural metals in space is dependent upon identifying volatile-rich carbonaceous asteroids in orbits that are energetically accessible for outbound spacecraft. They must also be accessible for retrieval of returned material into Earth orbits that are well situated for launching such missions. The general NEA population is well suited to providing these materials; the subset of NEAs with the easiest access from (and to) Earth are the small population of bodies with heliocentric orbits that are closest to Earth and have the lowest orbital eccentricity (the Aten family). These bodies are generally quite small and faint, with diameters rarely larger than 100 meters. They also typically have long synodic periods of tens of years, which make both Earth-based astronomical studies and spacecraft launch opportunities infrequent and challenging. As a result of these difficulties, Earth-based spectral characterization of these small bodies remains very incomplete; in the absence of spectral evidence for an economically attractive composition, there would be little incentive to launch exploratory spacecraft to such asteroids. These bodies also experience higher temperatures than most NEAs because they are 1) closer to the Sun, 2) are much smaller, and 3) have low-eccentricity orbits that do not provide lengthy cold-soak conditions near aphelion. There is general reason to conclude that these bodies must have experienced more severe solar heating and outgassing than other NEAs with more typical (distant and eccentric) orbits. Even producing evidence for a significant population of dark (low albedo) bodies in near-Earth orbits would not demonstrate that they are attractive sources of volatiles; convincing proof that water is present would require detection of the 3 μm water absorption feature, which requires such extreme sensitivity that tiny, faint, and rarely-visible asteroids would be unpromising observation targets. A compensatory benefit is that such bodies provide lower encounter velocities with Earth, so that capture into Earth orbit by a single lunar flyby is possible. The broader population of NEAs, typically of much larger size, much larger aphelion distances (mostly Apollo asteroids), and with much shorter synodic periods, provides thousands of attractive targets that require larger return velocities. Many of these asteroids are kilometers in diameter and come with strong spectral data for the presence of water. It is this expectation that the target asteroid masses and compositions will direct our attention to Apollo asteroids rather than Atens that makes it necessary (and profitable) to consider higher v∞ approaches to Earth. Approach velocities up to 5 km/s are considered in this report and would vastly increase the number of accessible NEAs. Such high approach velocities require a means of energy dissipation during capture that exceeds the ability of a lunar swingby to effect capture. Purely propulsive capture maneuvers become prohibitively expensive at such high approach velocities, suggesting aerobraking as an approach that minimizes propellant use and has the additional benefit of making the material of the used aerobrake available for processing in the target Earth orbit. Phase I research showed that simulated asteroid regolith could be microwave sintered into blocks that were sufficiently strong to withstand the stresses predicted by aerobraking simulations. The energy required was substantially less than if conventional sintering were used. Phase I aerocapture analysis also found that significant incoming relative velocity could be cancelled by a 10-meter aeroshell if aimed at a perigee of 64.5 km. However, the microwave sintering produced samples with relatively wide strength ranges that need to be narrowed in Phase II. In addition, the feasibility of producing a 10-meter diameter aeroshell as one unitary object is unknown, and the alternative of producing multiple plates that are sintered together to form the overall shell is seen to be an important research topic for Phase II. In addition, the modeling tools available in Phase I for aerocapture were too basic to answer important questions, such as how thick the aeroshell should be, what shape would be most effective, and how much of the return payload mass would be required by the aeroshell and how much would be available for the volatiles. These issues will be addressed in Phase II research.