Assessing the Sampleability of Bennu’s Surface for the OSIRIS-REx Asteroid Sample Return Mission

Space Science Reviews (2022) 218:20 https://doi.org/10.1007/s11214-022-00887-2 S P E C I A L C O M M U N I C AT I O N Assessing the Sampleability of Bennu’s Surface for the OSIRIS-REx Asteroid Sample Return Mission Kevin J. Walsh · Edward B. Bierhaus · Dante S. Lauretta · Michael C. Nolan · Ronald-Louis Ballouz· Carina A. Bennettet al. [full author details at the end of the article] Accepted: 28 March 2022 © The Author(s) 2022 Abstract NASA’s first asteroid sample return mission, OSIRIS-REx, collected a sample from the surface of near-Earth asteroid Bennu in October 2020 and will deliver it to Earth in September 2023. Selecting a sample collection site on Bennu’s surface was challenging due to the surprising lack of large ponded deposits of regolith particles exclusively fine enough (≤ 2 cm diameter) to be ingested by the spacecraft’s Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Here we describe the Sampleability Map of Bennu, which was constructed to aid in the selection of candidate sampling sites and to estimate the probability of collecting sufficient sample. “Sampleability” is a numeric score that expresses the compatibility of a given area’s surface properties with the sampling mechanism. The algorithm that determines sampleability is a best fit functional form to an extensive suite of laboratory testing outcomes tracking the TAGSAM performance as a function of four observable properties of the target asteroid. The algorithm and testing were designed to measure and subsequently predict TAGSAM collection amounts as a function of the minimum particle size, maximum particle size, particle size frequency distribution, and the tilt of the TAGSAM head off the surface. The sampleability algorithm operated at two general scales, consistent with the resolution and coverage of data collected during the mission. The first scale was global and evaluated nearly the full surface. Due to Bennu’s unexpected boulder coverage and lack of ponded regolith deposits, the global sampleability efforts relied heavily on additional strategies to find and characterize regions of interest based on quantifying and avoiding areas heavily covered by material too large to be collected. The second scale was site-specific and used higher-resolution data to predict collected mass at a given contact location. The rigorous sampleability assessments gave the mission confidence to select the best possible sample collection site and directly enabled successful collection of hundreds of grams of material. Keywords Asteroid exploration · Bennu · Landing site selection · Surface topography · Spacecraft safety Note by the Editor: This is a Special Communication linked to the Topical Collection on OSIRIS-REx published in Space Science Reviews. In addition to invited review papers and topical collections, Space Science Reviews publishes unsolicited Special Communications. These are papers linked to an earlier topical volume/collection, report-type papers, or timely papers dealing with a strong space-science-technology combination (such papers summarize the science and technology of an instrument or mission in one paper). 20 Page 2 of 34 K.J. Walsh et al. 1 Introduction The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) launched in 2016 as NASA’s first asteroid sample return space mission (Lauretta et al. 2017, 2021). The mission’s target, near-Earth asteroid (101955) Bennu, was characterized in numerous ways prior to the launch (Lauretta et al. 2015), and many of these characterizations were verified when OSIRIS-REx arrived at Bennu in late 2018 (Lauretta et al. 2019). Consistent with expectations, Bennu is a small rubble-pile asteroid (Walsh 2018) with a diameter of 492 m and a bulk density of 1190 kg m−3 (Barnouin et al. 2019; Scheeres et al. 2019). It has a top-like shape with an equatorial bulge that was evident in pre-launch radar observations (Nolan et al. 2013; Lauretta et al. 2015; Barnouin et al. 2019). Its global average albedo and thermal inertia were well-characterized before arrival finding a disk-averaged value thermal inertia of 310 ± 70 J m−2 K−1 s−1/2 that was interpreted to suggest dominant regolith grain sizes of a few millimeters up to a centimeter (Emery et al. 2014; Lauretta et al. 2015). On arrival at Bennu globally averaged thermal inertias were measured at 350 ± 20 J m−2 K−1 s−1/2 but variations across the surface became evident with observations from the spacecraft (DellaGiustina et al. 2019; DellaGiustina et al. 2020; Rozitis et al. 2020). A significant surprise during the spacecraft encounter was the large number of sizeable boulders ( 30 m) on Bennu’s surface and the lack of any significant surficial deposits, or ponds, of centimeter- to millimeter-scale regolith (where “regolith” is loose unconsolidated material that composes the upper portions of the asteroid; Robinson et al. 2001) (Lauretta et al. 2019; DellaGiustina et al. 2019; Walsh et al. 2019). These latter properties of Bennu were particularly important because OSIRIS-REx carried a sampling device, the Touch-andGo Sample Acquisition Mechanism (TAGSAM), whose performance varies as a function of the regolith properties (Bierhaus et al. 2018). The TAGSAM head is a cylindrical container that sits at the end of an extendable arm connected to the spacecraft (Fig. 1). It was designed to be pressed into the surface of Bennu for a few seconds, whereupon the release of high-pressure nitrogen gas would mobilize regolith and redirect it into the annular sample collection chamber inside TAGSAM (Bierhaus et al. 2018). TAGSAM was expected to only collect particles ∼ 2 cm and smaller – larger particles could frustrate sampling or pose a risk to the spacecraft – which made the selection of a suitable sample collection site challenging (Lauretta et al. 2021). To achieve its objective of collecting a sample from Bennu, the OSIRIS-REx mission developed a rigorous process for selecting a sample collection site (Lauretta et al. 2021). This process involved the construction of a hierarchy of maps designed to maximize the probability of returning the best possible sample to Earth. The highest-priority map was the Deliverability Map, which represented how precisely the spacecraft could be navigated to a targeted point on the asteroid surface. Next was the Safety Map, which identified hazards such as boulders that could endanger the spacecraft. Third was the Sampleability Map, which used observable surface properties to estimate how much sample mass could be collected at a given location. Last was the Science Value Map, which considered the potential scientific return of a sample from a specific location based on spectral, geologic, and thermal studies, where properties such as the presence of hydrated minerals and carbon-bearing compounds were considered important. These maps were determined as if they were separable quantities, though there were unavoidable interdependencie (...truncated)