Habitat Bennu: Design Concepts for Spinning Habitats Constructed From Rubble Pile Near-Earth Asteroids

The authors explore the possibility that near-earth, rubble pile asteroids might be used as habitats for human settlement by increasing their rotation to produce spin gravity. Using previously published scaling by Maindl et al. and studies of asteroid populations, it is shown that there is no class of hollowed body that would survive the spin-up process on its own without additional reinforcement. Large solid-rock asteroids (diameter D > 10 km) would not have the tensile strength to withstand the required rotation rates and would fracture and break apart. Smaller asteroids, being ‘rubble piles’, have little tensile strength and would quickly disperse. The possibility of containing the asteroid mass using higher-strength materials like carbon nanofiber is instead considered. It is found that a moderate tensile strength container can maintain the integrity of a large spinning cylinder composed of dispersed asteroid regolith. The research extends the range of possible asteroid habitat candidates, since it may become feasible to construct habitats from the more numerous smaller bodies, including NEAs (Near Earth Asteroids). The required tensile strength of the container material scales with habitat radius and thickness and is ∼ 200 MPa for a starting asteroid body of radius 300 m that is spun up to provide 0.3 g⊕ while increasing its radius to 3 km and maintaining a rubble and regolith shield thickness of 2 m to protect against cosmic rays. Ambient solar power can be harvested to aid in spin-up and material processing.

Size Distributions of Bluish and Reddish Small Main-belt Asteroids Obtained by Subaru/Hyper Suprime-Cam*

We performed a wide-field survey observation of small asteroids using the Hyper Suprime-Cam installed on the 8.2 m Subaru Telescope. We detected more than 3000 main-belt asteroids with a detection limit of 24.2 mag in the r-band, which were classified into two groups (bluish C-like and reddish S-like) by the g–r color of each asteroid and obtained size distributions of each group. We found that the shapes of the size distributions of asteroids with C-like and S-like colors agree with each other in the size range of 0.4–5 km in diameter. Assuming the asteroid population in this size range is under collision equilibrium, our results indicate that compositional difference hardly affects the size dependence of impact strength, at least for the size range between several hundred meters and several kilometers. This size range corresponds to the size range of “spin barrier,” an upper limit observed in the rotation rate distribution. Our results are consistent with the view that most asteroids in this size range have a rubble-pile structure.

Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

Asteroids are young: global-scale reshaping and resurfacing by small-scale impacts

The fraction of the asteroid population that survived since the Solar System formation has experienced numerous collisional, dynamical and thermal events, which have shaped their structures and orbital properties. Small asteroids are often considered to be rubble-pile objects, aggregates held together only by self-gravity or small cohesive forces (1; 2). The artificial impact experiment of JAXA’s Hayabusa2 mission on the surface of asteroid Ryugu (3) created a surprisingly large crater (≈14 m). This unexpected result suggests that at least the near-surface of the asteroid is controlled to a large extent by its rather weak gravity rather than strength. Due to the inability to re-create these impact conditions in laboratory experiments, this observed regime of low-gravity, low-strength cratering remained largely unexplored so far. In addition, the very large times scales involved in the crater growth made it impossible to numerically simulate these impact processes up to now. Here we use a novel approach to model the entire cratering process resulting from impacts on small, weak asteroids, which uses shock physics code calculations directly. We found that small impacts can significantly deform weak asteroids, causing global resurfacing at the same time. We also show that even very low asteroid cohesions can drastically influence the outcome of an impact and that the collisional life-time of the overall asteroid shapes is significantly lower than the traditionally used life-time based on catastrophic disruption events. Consequently, we predict that NASA’s Double Asteroid Redirection Test (DART) impact on Dimorhpos (4; 5) will not lead to a cratering event, as originally anticipated (i.e., 6; 7). Rather, the impact is going to change the global morphology of the asteroid, if the surface cohesion is less than ≈ 10 Pa. Our results, together with the future observations by the ESA’s Hera mission (8) will provide constraints regarding the evolution of the shapes and structures of small asteroids by sub-catastrophic impacts.

Effect Of Equivalent Ratio (ER) On the Flow and Combustion Characteristics in A Typical Underground Coal Gasification (UCG) Cavity

Underground Coal Gasification (UCG) is thought to be the most favourable clean coal technology option from geological-engineering-environmental viewpoint (less polluting and high efficiency) for extracting energy from coal without digging it out or burning it on the surface. UCG process requires only injecting oxidizing agent (O2 or air with steam) as raw material, into the buried coal seam, at an effective ratio which regulates the performance of gasification. This study aims to evaluate the influence of equivalent ratio (ER) on the flow and combustion characteristics in a typical half tear-drop shape of UCG cavity which is generally formed during the UCG process. A flow modeling software, Ansys FLUENT is used to construct a 3-D model and to solve problems in the cavity. The boundary conditions are- (i) a mass-flow-inlet passing oxidizer (in this case, air) into the cavity, (ii) a fuel-inlet where the coal volatiles are originated and (iii) a pressure-outlet for flowing the product Syngas out of the cavity. A steady-state simulation has been run using k-? turbulence model. The mass flow rate of air varied according to an equivalent ratio (ER) of 0.16, 0.33, 0.49 and 0.82, while the fuel flow rate was fixed. The optimal condition of ER has been identified through observing flow and combustion characteristics, which looked apparently stable at ER 0.33. In general, the flow circulation mainly takes place around the ash-rubble pile. A high temperature zone is found at the air-releasing point of the injection pipe into the ash-rubble pile. This study could practically be useful to identify one of the vital controlling factors of gasification performance (i.e., ER impact on product gas flow characteristics) which might become a cost-effective solution in advance of commencement of any physical operation.

Experiments on rebounding slow impacts under asteroid conditions

<p class="p1"><span class="s1">The<span class="Apple-converted-space">  </span>surfaces<span class="Apple-converted-space">  </span>of<span class="Apple-converted-space">  </span>rubble-pile<span class="Apple-converted-space">  </span>asteroids<span class="Apple-converted-space">  </span>are<span class="Apple-converted-space">  </span>covered<span class="Apple-converted-space">  </span>in<span class="Apple-converted-space">  </span>regolith of a variety of sizes.<span class="Apple-converted-space">  </span>In some cases like for the asteroid Itokawa, the size distribution of regolith is not uniform across the surface [1]. Some areas are dominated by finer grains, while other areas are covered by larger rocks.<span class="Apple-converted-space">  </span>There are a number of competing explanations for this observed size segregation [2–4]. One approach is the so called ballistic-sorting-effect [2], where impacting particles sort themselves through different rebound behavior.</span></p> <p class="p1"><span class="s1">In our work we want to set practical limits on the role ballistic sorting can play in shaping an asteroids surface. To this end we conduct a series of drop tower experiments examining the impact kinetics of slow (cm/s)<span class="Apple-converted-space">  </span>3 mm sized projectiles into a regolith surface under conditions realistic for asteroid surfaces, i.e. vacuum and low gravity. We track the impactor with high-speed cameras and determine its velocity in 3 dimensions before and after the impact. From these velocities, we can then compute a coefficient of restitution (COR).<span class="Apple-converted-space">  </span>We then repeat the experiment for surfaces composed of differently sized material.<span class="Apple-converted-space">  </span>We find that for a regolith bed made from particles of similar size as the impactor we get a lower COR (0,1) than for beds made up of significantly larger (0,5) or smaller particles (0,8). The more elastic collisions for larger sized targets follows from conservation of momentum. For the finer material we suggest that the higher COR is a function of interparticle adhesion.</span></p> <p class="p1"><span class="s1">[1] A. Fujiwara, J. Kawaguchi, D.K. Yeomans, M. Abe, T. Mukai, T. Okada, J. Saito, H. Yano, M. Yoshikawa, D.J. Scheeres et al., Science 312, 1330 (2006)</span></p> <p class="p1"><span class="s1">[2] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo, P. Chakraborty, Phys. Rev. Lett. 118, 111101 (2017)</span></p> <p class="p1"><span class="s1">[3] S. Matsumura, D.C. Richardson, P. Michel, S.R. Schwartz, R.L. Ballouz, Mon. Not. the R. Astron. Soc. 443, 3368 (2014)</span></p> <p class="p1"><span class="s1">[4] A.J. Dombard, O.S. Barnouin, L.M. Prockter, P.C. Thomas, Icarus 210, 713 (2010)</span></p>

Mechanical properties of fine-coarse grain mixtures of asteroid regolith

<p>Several lines of evidence indicate that most of the smaller asteroids (< 1 km) consist of granular material loosely bound together primarily by self-gravity; these are commonly called rubble piles [1]. While the strength of these rubble piles is valuable information on their origin and fate, it is still debated in the literature [2]. Therefore, we have started a laboratory measurement campaign on simulated asteroid regolith, studying the impact of several factors on material strength, such as grain size, size mixtures, and surface properties. In the work presented here, we focus on fine-coarse mixtures and the influence of the fraction of fines on the sample strength. Computer simulations suggest that the increase in the ratio of fine grains to coarse grains will increase the strength of the sample in all configurations [3].  In a series of table-top measurements, we have determined sample compression and shear strengths for various fine-coarse mixtures. We used confined setups (less than 10cm in length) to measure the strength of the material in constricted environments such as an asteroid’s core and unconfined setups (greater than 10cm in length) to simulate open environments such as the surface of an asteroid.</p> <p>Using CI Orgeuil high fidelity asteroid soil simulant [4], we performed three measurement types to determine the strength of our samples. Samples of regolith were created by measuring percentage by volume amounts of both coarse and fine grains into the measurement container. We prepared coarse grains in two size distributions, mm-sized (Figure 1) and cm-sized. The fine fraction was composed of grains sieved between 100 and 250 µm. A shear box setup was used to obtain shear yield measurements which in turn provided values for the Angle of Internal Friction (AIF), bulk cohesion, and tensile strength of the samples. A compression setup was used to measure values for the Young’s Modulus (YM) in both confined and unconfined samples. The third setup measured the Angle Of Repose (AOR), the steepest angle of descent relative to the horizontal plane to which a material can pile before collapse. From the AOR, we determined the coefficient of friction of each sample.</p> <p>For compression and AOR measurements, we find that the strength of the coarse grain samples increases with the addition of a fine fraction (Figure 2, left). These findings are intuitive and support the results from computer simulations. However, we find that the increase of the fine fraction in a sample of coarse grains does not consistently increase the sample shear strength. With increasing fine fractions, the AIF and bulk cohesion (Figure 2, right) of the mixed samples decrease (until a point of saturation). This could be indicative of the fine grains acting as a lubricant as the larger grains move across each other, aiding rolling and reducing interlocking strength.</p> <p>Our findings suggest that in the case of the surface of an asteroid, the presence of fine grains does indeed increase the strength of coarse regolith material.  However, fine grains in the regolith sublayers or the asteroid interior will reduce material strength due to grain interlocking and ease disruption. Therefore, rubble piles that are depleted in fine grains will have higher internal strength compared to those composed of grain size distributions that include sub-mm sized particles.</p> <p>[1] Walsh, K.J., 2018. Rubble pile asteroids. Annual Review of Astronomy and Astrophysics, 56, pp.593-624.</p> <p>[2] Holsapple, K., 2020. Main Belt Asteroid Histories: Simulations of erosion, cratering, catastrophic dispersions, spins, binaries and tumblers. arXiv preprint arXiv:2012.15300.</p> <p>[3] Sánchez, P. and Scheeres, D.J., 2014. The strength of regolith and rubble pile asteroids. Meteoritics & Planetary Science, 49(5), pp.788-811.</p> <p>[4] Metzger, P.T., Britt, D.T., Covey, S., Schultz, C., Cannon, K.M., Grossman, K.D., Mantovani, J.G. and Mueller, R.P., 2019. Measuring the fidelity of asteroid regolith and cobble simulants. Icarus, 321, pp.632-646.</p>