Developing Prototype Simulants for Surface Materials and Morphology of Near Earth Asteroid 2016 HO3

Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100029, China School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China Beijing Spacecraft Manufacturing Factory Co., Ltd., Beijing 100094, China Beijing Institute of Space System Engineering, Beijing 100094, China Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China


Introduction
Asteroid simulants are needed in supporting space missions, developing near-earth asteroids in situ resource utilization technology, and developing planetary defense techniques. It is unrealistic to create simulants that replicate all characteristics of the target asteroids due to different geological processes on asteroids and Earth [1]. Therefore, purposespecialized simulant is more useful and practical for actual space missions that always have specific purposes, such as navigation experiment and sampling test [2].
The recent asteroid missions to Itokawa, Ryugu, and Bennu have acquired numerous high-resolution images of these asteroids. Observations show that small asteroids (less than kilometer-sized) are rubble piles, and up to tens of meter-sized boulders and submicron-to millimeter-sized regolith particles are exposed on their surface [3][4][5]. Such a characteristic brings unexpected challenges to spacecraft when selecting landing and sampling sites, as what Hayabusa 2 and OSIRIS-Rex have encountered. China will launch its first asteroid mission in the coming few years aiming to first return samples back from a near-earth asteroid (469219) 2016 HO3 and then rendezvous with a main belt comet 133P/Elst-Pizarro or 311P/PANSTARRS [6]. Due to the very much unknown surface morphology and properties of 2016 HO3, it is highly likely that this mission will be facing great challenges during sampling operation. To reduce risks, evaluations should have been performed before the actual landing and sampling operations, which would ultimately provide valuable information about the surface condition. In this case, it is necessary to develop asteroid simulants that can simulate the morphology and reflectance characteristics of 2016 HO3. Meanwhile, such simulants for 2016 HO3 should be easily to modify to reflect new observations made by the spacecraft.
There are previous attempts to developing simulated materials for asteroids, most of which are concentrating on C-type asteroids. These simulants are usually based on carbonaceous chondrites plus remote observations of targeted asteroids, including a series of prototype simulants based on carbonaceous chondrites [7], simulant HCCL-1 for Bennu [8], and a simplified Ryugu simulant [2]. Simulants for S-type asteroid are relatively less reported. A new Itokawa asteroid regolith simulant (called IRS-1) as an S-type asteroid was developed for China's upcoming asteroid exploration [9]. However, this simulant only contains fine grains ranging from submillimeter to centimeter, without larger-sized boulders.
This study designed and developed the morphology equivalent prototype simulants for 2016 HO3 that are with particle sizes ranging from micrometer to at least tens of centimeters. The goal of these simulants is to simulate a possible rubble pile morphology and the reflectance feature of the surface of 2016 HO3 so that they can serve practical uses during the mission preparation and operation phases, such as test and evaluation of navigation system and sampling device before the actual touchdown or landing operation. Furthermore, when the spacecraft's initial observation reveals a different size distribution of 2016 HO3 surface than our estimation, which is always the case, our simulants can be easily modified to reflect the new findings.

Observation of (469219) 2016 HO3
The near-earth asteroid (469219) 2016 HO3 was found on 27th of April 2016 by the 1.8 m Ritchey-Chretien telescope of the Pan-STRARRS Project [10]. The current available orbital properties of this body were estimated through terrestrial observations spanning a data-arc of 5140 d or 14.07 yr, from 17th March of 2004 to 27th April of 2019. 2016 HO3 is classified as an Apollo asteroid moving in a low eccentricity (e = 0:10) and low inclination (I = 7:77). It orbits in a 1 : 1 mean motion resonance with Earth with a minimum orbital intersection distance of 0.03 AU.
The physical parameters of 2016HO3 are currently poorly constrained. One single light curve from April 2017 gives a magnitude H = 24:3 and a rotation period = 0:467 ± 0:008 hr [11], suggesting it is a typical small fast rotator. The size of 2016 HO3 has not yet been firmly established, but it is likely about 40-100 m. Based on an assumed standard albedo for S-type asteroids of 0.20 and an absolute magnitude of 24.3, it measures 41 meters in diameter. A recent study by Reddy et al. [12] established a possible shape model suggesting that 2016 HO3 is likely an elongated asteroid, which has a b∕ a ratio smaller than 0.479. Color ratios derived from a low-resolution visible spectrum of 2016 HO3 suggest an S taxonomic type [11].
The particle size distribution and porosity of the regolith can be estimated if the thermal inertia or effective thermal conductivity of asteroids is known. However, the faintness of 2016 HO3 makes such an estimation almost impossible. Observations by the Hayabusa, Hayabusa 2, and OSIRIS-Rex spacecraft revealed that the surface morphology of small-sized asteroids (less than 1 km) is mostly covered by boulders and in some areas by regolith, which is different from larger asteroids that are mostly covered by finegrained regolith [3,5,13]. 2016 HO3 is a small asteroid with a diameter of 40-100 meters. It is reasonable to assume that it has a similar rubble pile surface. Therefore, for this study, we consider the surface of 2016 HO3 is covered by boulders with sizes ranging from several centimeters to at least several tens of centimeters and fine particles with sizes ranging from micrometer and millimeter.

Development of 2016 HO3 Simulants
Minerals are the basic building blocks of planetary materials, and thus, mineral-based simulants offer the closest potential match to the properties of actual asteroid materials [7]. Both telescopic observations and analyses on the returned asteroid Itokawa sample indicate that most S-type bodies have mineralogy similar to those of ordinary chondrites [14,15]. Ordinary chondrite meteorites (OCs) are silicate-rich meteorites and by far the most abundant meteorites (80% of all falls) [16]. According to their iron content, these meteorites are divided into H (high total Fe), L (low total Fe), and LL (low total Fe, low metal) groups, with 42.8%, 47.4%, and 9.8% of OC falls belonging to each group, respectively. Composition measurements of H, L, and LL group OCs provide a reference model from which to develop simulated asteroid 2016 HO3 materials for current observations suggests an S-type asteroid. The modal mineral abundance results obtained by X-ray diffraction analysis on 18 H, 17 L, and 13 LL unbrecciated OC falls show that olivine and low-Ca pyroxene are the most abundant phases present, while plagioclase, high-Ca pyroxene, troilite, and metal comprise the remaining XRD-measured mineralogy [17]. It is not clear that which OC group 2016 HO3 belongs to, and thus, we developed three prototype simulants QLS-1, QLS-2, and QLS-3 corresponding to H, L, and LL OCs, respectively.    Table 1 lists the average modal mineralogy of the aforementioned H, L, and LL OC falls. The mineral recipes for the prototype simulants are based on these data, with some modification because of various trades involved in sourcing raw materials (Table 2). To develop asteroid 2016 HO3 simulant, we obtained different types of raw rock or mineral materials from terrestrial mines, including olivine from Hannuoba area in Hebei Province, low-Ca pyroxene from Alashan area in Inner Mongolia Province, high-Ca pyroxene from Qingdao area in Shandong Province, and plagioclase from Shijiazhuang area in Hebei Province. Some raw materials are from commercial supplies, including powdery pyrite, metal iron and nickel, apatite, ilmenite, and chromite.
We first crushed the raw materials in chunk or other nonpowder forms into fine powders. Pyrite, metal iron, and nickel came from commercial suppliers as micronsized powders and require no further processing. The average grain size of crushed powder materials is 50-100 microns. After having all raw materials processed, we mixed the powder material in the appropriate weight ratios (Table 2), resulting in homogeneous medium to dark gray powders (Figure 1(a)). Due to the absence of natural binders (clays) in the OC composition, we added sodium metasilicate pentahydrate (a.k.a. water glass) into the mixture, which polymerizes into a rocky binder under modest heat and the "pentahydrate" is released during polymerization. We dissolved sodium metasilicate pentahydrate in pure water and then mixed this solution with the dry mixture (in a 1 : 5 ratio). The concentration of sodium metasilicate pentahydrate is 4-5% of the dry mixture. Then, the wet paste was dried in an oven under temperature at 150-160°C. When the paste lost all its water and formed solid blocks, we crushed them using hand tools, such as a wooden club, which results in a power-law particle size distribution [19]. The size distribution could be adjusted by sieving and mixing.

Characteristics of 2016 HO3 Simulants
We developed three prototype simulants with different abundance of iron and metal contents (QLS-1, QLS-2, and QLS-3), which accordingly represent asteroids linking to H, L, and LL type OCs (Figure 2). These asteroid simulant prototypes were characterized using scanning electron microscopy, X-ray fluorescence spectrometer, and VNIR reflectance spectroscopy.
The three S-type asteroid prototype simulants were imaged using a Zeiss Super 55 field emission scanning electron microscope (SEM) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), with 15 kV accelerating voltage. As shown in Figure 3, particles are subrounded to highly angular, with elongated, platy, and square shapes. Very fine particles (<5 μm) adhere to the surfaces of larger ones. Larger particles include individual minerals and agglomerate of smaller particles.
The major element chemistry of the prototype simulants was measured and analyzed using XRF-1500 X-ray  Table 3. Compared to H, L, and LL OCs, the simulants have significantly lower TFe 2 O 3 (Table 3), a consequence of using terrestrial low-Fe pyroxene and olivine as the raw material.
Reflectance spectra of the prototype simulants were acquired at the China University of Geosciences (Wuhan), using a VERTEX 70v Fourier transform infrared spectrometer at ambient condition. The scattering configuration was approximately 45°-45°biconical. The resulting spectra (0.500-2.5 μm) are shown in Figure 4, which offsets them for clarity. As shown in Figure 4, the measured spectrum of the three prototype simulants is broadly similar to the spectrum for asteroid Itokawa and S-type asteroids. Both samples have two strong absorption bands at~0.9-1 μm  Figure 3: VNIR reflectance spectra of 2016 HO3 prototype simulants compared with asteroid 25143 Itokawa and S-type asteroids [21].

Application
Our primary purpose for developing 2016 HO3 simulants is to reproduce the optical properties and a possible rubble pile structure of the surface. Since the morphology of 2016 HO3 is still unknown, asteroid Itokawa was chosen here as a reference. We sieved and mixed large boulders (several to several tens of centimeters) and smaller particles to match the size distributions of particles roughly following the powerlaw index as measured on Itokawa [4,22]. The simulants were distributed in a Styrofoam container with the size of 1 m * 0:8 m. Comparison between images of our simulated topography and Itokawa surface demonstrates similar morphological characteristics (Figure 4). When the optical properties of 2016 HO3 are revealed by the initial observations of the spacecraft during the phase mission, our prototype simulants could be easily adjusted to match possible new and unexpected findings. Subsequently, the simulants could be used in optical navigation experiment and to study the efficiency of the sampler before its touchdown or landing.

Conclusion
We used remote observations of asteroid 2016 HO3 and modal mineralogy data from ordinary chondrites to develop three prototype simulants of S-type asteroids for engineering evaluation of the optical navigation system and the sampling device of the spacecraft. The size of the simulant particles ranges from tens of centimeter to micrometer. The characterizing results of the prototype simulants show similarity to their analogue meteorites despite some discrepancies in bulk chemistry and reflectance spectra due to unavoidable factors in choosing terrestrial raw materials. These prototype simulants are easy to make and modify according to need.

Data Availability
The data used to support the findings of this study are included within the article.