3.1.1. OSIRIS-REx—NASA

The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), the NASA-operated New Frontiers 3 mission, was launched on September 8, 2016, and is currently approaching 101955 Bennu, a primitive carbonaceous asteroid. The mission is being carried out in partnership with the Canadian Space Agency (CSA), as described below. The spacecraft arrived at Bennu on December 3, 2018, and began 505 days of surface mapping to study the asteroid with five suites of instruments for surface imaging and spectroscopy. A sample of Bennu's regolith will be collected by a robotic arm Touch-And-Go Sample Acquisition Mechanism (TAGSAM). Sample collection by the TAGSAM system blows compressed nitrogen onto the surface regolith of Bennu and then collects the fluidized solid materials into a ring-shaped canister. The system will collect at least 60 g (and up to 2 kg) in up to three sample collection attempts, and it is able to collect a variety of sizes and size-frequency-distributions of particles. The spacecraft is expected to return the sample to Earth in September 2023 via a sample return capsule (see Figure 3.2) that will reenter Earth's atmosphere after being released from the spacecraft.

FIGURE 3.2

Sample return capsule (SRC) of OSIRIS-REx undergoing an open spin balance test. This test spins the SRC while it is in the open configuration. The cylindrical structure on the left side is where the sample head will be stored with the sample from Bennu. (more...)

Bennu was selected because it is an easily accessed near-Earth carbonaceous asteroid, which is expected to contain primitive material, perhaps including organic molecules. In addition, Bennu is relatively large (492 m in diameter) and has a slow-enough rotation speed (4 hours for one rotation) to allow sampling operations. The samples collected will provide information about the composition of materials that built the terrestrial planets during the formation of the solar system, including sources of water and organic molecules, key components for the development of life on Earth.

Since the target asteroid is predicted to be carbon-rich, all stages of the curation require great care to prevent terrestrial contamination. Part of the early curation involved monitoring all materials that go into the design and construction of the TAGSAM system and the return capsule. The samples will be returned to a contamination-free curation environment at NASA Johnson Space Center (JSC). The sample return, curation, and characterization environments require prohibiting materials in the facilities that would create amino acid-like materials, such as latex, nylon, and so on. CSA will receive 4 percent of the returned sample of Bennu in return for their contribution of the Laser Altimeter instrument to the mission. Per international agreement, NASA will provide 0.5 percent of the returned sample of Bennu by mass to JAXA, and JAXA will provide 10 percent of the returned sample mass of Ryugu from the Hayabusa2 mission to NASA (see Section 3.1.2).

The OSIRIS-REx Bennu sample curation laboratory is currently under construction within existing space in Building 31 at JSC (Figure 3.3). The space will be entirely refurbished and cleaned, and all electrical, mechanical, security, and information technology will be upgraded. Adjacent to the OSIRIS-REx clean room, a sample preparation room with separate access will be dedicated to cutting, grinding, thin sectioning, and other types of necessary sample preparation. The clean room suite's completion date is scheduled for June 2020, allowing time for commissioning, including rehearsals of sample reception, preparation, and handling.

FIGURE 3.3

Drawing of planned new curatorial space at NASA Johnson Space Center designed to handle returned samples from OSIRIS-REx and Hayabusa2. Also shown are existing laboratories for Stardust samples. The air purity of each laboratory is shown in red; the lower (more...)

3.1.2. Hayabusa2—JAXA

JAXA spaceship Hayabusa2 was launched on December 3, 2014, and is an asteroid explorer whose target is a C-type asteroid named 162173 Ryugu. Hayabusa2 arrived at Ryugu in late June 2018 (Figures 3.4 and 3.5), and it will study the asteroid for one and a half years before leaving at the end of 2019 and returning samples to Earth at the end of 2020. Like its predecessor, the Hayabusa mission to the near-Earth asteroid 25143 Itokawa, Hayabusa2 will also use a projectile blast and funnel method to collect samples from the asteroid. However, unlike Hayabusa, Hayabusa2 will release an impactor to generate a crater on Ryugu, in which it will then land and collect samples. The reflectance spectra of C-type asteroids are similar to carbonaceous chondrites; therefore, Ryugu is thought to be a more primordial body than Itokawa and may contain organic matter, water, and hydrated minerals.

FIGURE 3.4

Left: Asteroid Ryugu photographed by Hayabusa2 from a distance of about 20 km. The image was taken at around 23:13 JST on June 30, 2018. Right: As the asteroid has rotated, this image is almost the reverse side of the figure on the left. SOURCE: Institute (more...)

FIGURE 3.5

The surface of asteroid Ryugu imaged by the MINERVA-II 1B rover on September 23, 2018, one of two rovers deployed from the Hayabusa2 spacecraft. SOURCE: Japan Aerospace Exploration Agency, “MINERVA-II1: Images from the surface of Ryugu,” (more...)

Once the samples are received, phase 1 curation (sample description) will be done at the JAXA curation facility. Phase 2 curation, consisting of further analysis, description, and creation of a sample database, will be done both at and outside the JAXA curation facility, all supervised by the JAXA curation staff.

A new clean chamber is being built at the JAXA facility in Sagamihira, Kanagawa, Japan, for the Hayabusa2 returned samples at a cost of $12.5 million,³ and opened in 2018.⁴ The new clean chamber includes areas designated for vacuum conditions, as well as other areas intended to be operated in an ultra-pure nitrogen atmosphere. The vacuum area has separate rooms for opening the sample container, for sampling, and for storage. The nitrogen atmosphere area has separate chambers for examining approximately micron-size particles and approximately millimeter-scale particles. The JAXA team is building on the Hayabusa curation expertise⁵ and furthering the technical development for handling and cutting the larger particles, which were not necessary for the Hayabusa samples. Preventing and monitoring contamination of the Hayabusa2 samples is a high priority, with additional engineering controls designed for all stages of sample collection, return, sample handling, curation, and characterization.

As noted above, 10 percent of the Ryugu samples will be provided to NASA for curation at JSC. Figure 3.3 shows the curation facility for Hayabusa2 at JSC, which will be co-located within the curation facilities for OSIRIS-REx described in Section 3.1.1.

3.3.1. Downselected: Comet Surface Sample Return via CAESAR—NASA

The decadal survey names the return of cryogenic comet samples an essential goal in the study of primitive bodies. However, such missions are logistically challenging and will require extensive technological development. Thus, in preparation for eventual cryogenic comet sample return, the decadal survey recommended a comet surface sample return mission capable of returning a minimum of 100 g of material. Such a mission was considered the highest priority in studying primitive materials and the origin of the solar system.

Accordingly, in December 2017, the CAESAR mission was downselected as one of two finalists for NASA's New Frontiers program. The objective of CAESAR is to collect material from the surface of comet 67P, Churyumov-Gerasimenko (Figure 3.6), a Jupiter-family comet that was previously investigated by the European Space Agency's Rosetta mission, which included sending the Philae lander to its surface for in situ analyses of organic molecules. Churyumov-Gerasimenko has spectral characteristics suggesting the presence of nonvolatile organic materials at its surface,¹³ and a suite of 16 organic compounds were detected by Philae.¹⁴ If selected, the mission would use a touch-and-go robotic arm to collect between 80 to 800 grams of regolith from the surface. The sample target includes the solid components of the particulate matter that comprises the surface of a comet, which will likely include ices and organic material, as well as rocky material.

FIGURE 3.6

Comet 67P, Churyumov-Gerasimenko. SOURCE: Courtesy of European Space Agency (ESA)/Rosetta/NAVCAM—CC BY-SA IGO 3.0.

CAESAR is not a cryogenic sample return mission and will return only solid materials and the associated gas liberated from the sublimation of the ice. The container system collects the sample, then systematically devolatilizes it, sequestering the resulting gas from the solid samples; both are returned in differentially cooled chambers of the return container (Figure 3.7). The return capsule also preserves the samples, insulating them using phase-change materials that will maintain the samples at subzero (^oC) temperatures until recovery.

FIGURE 3.7

A rendering of the prototype of proposed sample containment system for the CAESAR comet return mission, which will separate solids from gases sublimated from cometary ice. SOURCE: Courtesy of Steve Squyres, Cornell University.

If selected as the New Frontiers 4 mission, the projected launch date is August 2024; the craft will arrive at the comet in March 2029 and depart in November 2033. The Earth return is estimated to be November 2038. After retrieval from the spacecraft in the Utah desert, the samples will be transported to JSC for curation and initial characterization. The curation team will leverage JSC's experience with OSIRIS-REx samples, as well as the cold-curation expertise for the Tagish Lake meteorite samples at the University of Alberta.¹⁵

3.3.2. Martian Moons Exploration—JAXA

MMX is a mission concept currently under consideration by the Institute of Space and Astronautical Science/JAXA to investigate the martian moons Phobos and Deimos, with sample return from Phobos as a major scientific goal. JAXA proposes to fly a Hayabusa-like spacecraft to the martian moons, spend about 1 year studying these objects, then sample the regolith of one moon and return this sample to Earth. The scientific rationale for the mission centers on whether these moons are captured asteroids or coalesced impact ejecta from a large impact on Mars. A primary test for these models is the overall nature of the material; captured asteroids would be expected to be primitive chondritic material, perhaps akin to CM2 chondrites, whereas coalesced impact ejecta would be expected to be heavily shocked or impact melted material of primarily igneous origin. The mission, if funded, would launch in the mid-2020s, perhaps in 2023, with a return date in the late 2020s or early 2030s. The stated sample return objective is approximately 10 g of material.

3.3.3. Mars Sample Return

Mars sample return was identified as the highest priority large strategic mission by the decadal survey and will require multiple missions to accomplish, spanning into the next decadal survey.¹⁶ There are also sample return missions to Mars being explored by Russia (Mars-Grunt Mission, mid-2020s), and China (2030).

With the recent publication by Eigenbrode et al.¹⁷ showing evidence for the presence of organic molecules on Mars, the demand for Mars sample return to Earth continues to escalate. Whereas martian meteorites are readily available for study on Earth, they are presumably not fully representative of any plausible martian biosphere or its fossil remnants. By contrast, the Mars Science Laboratory rover Curiosity has encountered exclusively sedimentary martian rocks and now has demonstrated that at least one of these rocks contained some sedimentary organic matter, released as small volatile organic molecules when heated. On Earth, one finds the vestiges of ancient life (kerogen) primarily in shales; so naturally these same lithologies on Mars would be prime targets for future sampling.

One of the primary objectives of the Mars 2020 mission is to identify and collect and cache optimum martian samples of sedimentary rock.¹⁸ The plan will be for a future mission(s) to be sent to retrieve and transport these samples back to Earth. From the perspective of future sample handling, the design of the cache containers and transport canister will dictate the design and operation of the sample handling facility back on Earth. From the standpoint of planetary protection standards (Section 3.4.2), it is likely that the technological complexity of the Mars sample return facility will require a significant investment long before the costs of subsequent analysis by the broader community are assessed.

3.3.4. Lunar Sample Return

Several types of lunar sample return missions are currently being explored by NASA, including the SPA basin sample return mission,¹⁹ new commercial sample return missions from the Lunar Exploration and Discovery Program, and the potential for using the proposed NASA Gateway (see Section 3.3.4.3) to facilitate sample return. Additional missions to return samples from the Moon may be explored through Discovery class missions. The decadal survey highlights potential targets for such sample returns that could explore “the nature of polar volatiles [where cryogenic sample return would be enabling], the significance of recent lunar activity at potential surface vent sites, and the reconstruction of both the thermal-tectonic-magmatic evolution of the Moon and the impact history of the inner solar system through the exploration of better characterized and newly revealed lunar terrains.”²⁰ A lunar polar volatiles explorer concept is described in Appendixes D and G of the decadal survey.

3.3.4.2. Commercial Lunar Sample Return

On July 12, 2017, the privately funded company Moon Express²¹ unveiled a lunar robotic sample return architecture (Figure 3.8). With the first mission to land on the Moon (Lunar Scout) by the MX-1E lander approved by the Federal Aviation Administration in 2016, Moon Express plans to launch this inaugural flight in 2019, with plans to return lunar samples to scientists by 2021. There are also plans by another private company, Astrobotic,²² to offer sample return, but the architecture has not yet been made public.

FIGURE 3.8

Artist's rendition of the Moon Express sample return architecture launching back to Earth from the Moon. SOURCE: Moon Express, see http://www.moonexpress.com/.

The Planetary Science Division of NASA's Science Mission Directorate sponsored a workshop on January 10-12, 2018, to identify landing sites on the Moon that have high scientific interest.²³ The workshop developed a list of potential landing sites that take advantage of these new commercial capabilities to get to the lunar surface for in situ science, as well as for sample return. As seen in Figure 3.9, there are many sites that could address fundamental lunar science questions defined by National Academies documents and Lunar Exploration Analysis Group reports.

FIGURE 3.9

Potential landing sites (indicated with yellow stars) for lunar science including sample return outlined in the Lunar Science for Landed Missions workshop report. SOURCE: E.R. Jawin, S.N. Valencia, R.N. Watkins, J.M. Crowell, C.R. Neal, and, G Schmidt, (more...)

The new Lunar Exploration and Discovery Program is directed to utilize commercial lander and sample return capabilities as public-private partnerships for payloads ≤200 kg at the cadence of approximately one per year for 10 years on the notional architecture. This will be realized through a Commercial Lunar Payload Services call. Initial missions will not involve sample return, but the later ones will. Returned sample caches could be up to 2 kg. This represents a new paradigm for lunar exploration, as NASA will be a customer and will not be responsible for building the lander or sample return capsule. While some private companies have proposed architectures for landing assets on the lunar surface, none have actually demonstrated that capability to date.

3.4.1. Missions Returning Thermally Unstable Samples

The return of cryogenic and atmospheric samples to Earth is essential in order to answer questions about presolar and nebular cosmochemistry, as well as to evaluate potential habitable environments in the solar system. The 2013-2022 decadal survey, applying the Aerospace Corporation's cost and technical evaluation methodology, did not select cryogenic sample return missions, as they were considered unachievable in the scope of this decadal survey. As discussed in Section 3.3.1, comet surface sample return is part of the current New Frontiers mission competition, but CAESAR is not a cryogenic sample return mission and is designed to return rocky materials and sublimated ices in the form of gases.

Successful return of cryogenic samples requires significant development of sample return technologies, an ability to pay the high costs of sample curation (which would require cryogenic storage and characterization technologies), and adherence to planetary protection principles. There are numerous challenges for cryogenic sample return missions involving sample collection, return, curation, and analysis. Each of these steps needs to be achieved without affecting the original state of the sample by chemical, thermal, or mechanical reactions, while at the same time adhering to planetary protection requirements, which vary according to target body. For example, cryogenic ice return from Mars or Enceladus would require a dual-pressure enclosure (i.e., returned samples cannot be allowed to contaminate the environment, and the environment cannot be allowed to contaminate the samples), whereas cryogenic sample return from a comet or lunar polar ice would not.

Cryogenic sample collection can be performed from the subsurface of an ice-bearing region of a comet, or through the capture of ice and gas plume material—for example, on satellites of the outer gas giants, such as the plumes imaged by the Cassini spacecraft on Enceladus (Figure 3.10). Cold curation involves the preservation of samples at or below the ambient temperature of collection, which can be categorized on the basis of current knowledge of the maximum temperatures such materials would see (Figure 3.11).

FIGURE 3.10

Image from Cassini shows backlighting from the Sun spectacularly illuminating Enceladus's jets of water ice. SOURCE: Courtesy of NASA/JPL-Caltech/SSI.

FIGURE 3.11

A schematic diagram of approximate surface temperatures for selected solar system bodies as a function of their distance from the Sun (in astronomical units, AU). Horizontal lines show the 1 atmosphere pressure freezing points or condensation temperatures (more...)

The actual surface temperatures of solar system bodies likely deviate from the calculated equilibrium surface temperature due to contributions from internal heating, the blanketing effects of atmosphere, and details of the orbit and surface morphology. In addition, surface temperatures may show significant variations in time (day versus night; summer versus winter) or geography (equator versus poles; exposed versus permanently shadowed regions). For example, the average temperature of the lunar surface is approximately 200 K, but it can be over approximately 300 K during the long lunar day (~13.5 Earth days) near the equator and drop to below 100 K in the high latitudes during the lunar night. Surface albedo and geometry also play a role, with temperatures in the permanently shadowed cratered regions expected to stay below 40 K.²⁶ Mars, like the Moon, experiences large temporal and spatial variations in temperature, with average surface temperatures also around 200 K. Europa is estimated to have an average surface temperature of about 100 K, and Titan's surface will likely be somewhat warmer due to its atmosphere. The temperatures of comet nuclei are unknown at this time.

Figure 3.11 shows that an estimate of average equilibrium surface temperatures of many solar system bodies of interest lie at or above the condensation temperature for liquid nitrogen. The technology for long-term storage of materials at liquid nitrogen temperatures is well developed, with many applications in medical and biochemical storage, and is less costly than storage at lower temperatures. The test facilities at NASA Glenn Research Center (Cleveland, Ohio) have a –180°C (93 K) chamber. In addition, freezing points and condensation points are pressure-sensitive; therefore, the pressure of the sample return and curation environment will be an important variable. Cold curation will require strict monitoring of temperatures, pressures, and also sample reactivity.

Maintaining the 40 K temperatures of permanently shadowed regions of the Moon and other bodies will be technologically and fiscally challenging, and the temperatures for true preservation will be untenable. Thus, the temperature at which these samples will be transported back to Earth and curated will need to be a compromise between what can be reliably maintained and the available funding. Assuming that this temperature will be higher than ambient, it will be important to quantify what is lost. For example, at 1 atm, 80 K will preserve ices of CO and CO₂, but above 110 K, CO ice will be lost and water ice will sublime.

3.4.2. Planetary Protection Requirements

Preparations for eventual curation and characterization of extraterrestrial soft matter samples (e.g., volatile, organic, and high surface area samples) have to be done in a way that is mindful of the considerations and constraints of planetary protection guidelines. Planetary protection is defined as “the practice of protecting solar system bodies . . . from Earth life . . . and of protecting Earth's inhabitants and environment from harm that could be caused by possible extraterrestrial life forms.”²⁷ This dual-directional requirement, in concert with the potentially high stakes of failure in either direction, makes planetary protection a complicated and costly engineering constraint for both curation and characterization of extraterrestrial materials.

Consideration of issues related to planetary protection for extraterrestrial sample return has, over the years, led to development of extensive NASA and international guidelines regarding the proper handling of returned extraterrestrial samples.²⁸ It is clear that the degree of stringency regarding planetary protection issues differs with the nature of the object being sampled. Missions like OSIRIS-REx and Hayabusa2 to asteroids and the CAESAR mission to a comet will sample regolith from small primitive bodies that lack any features consistent with the sustenance of “life as we know it”—namely, liquid water and some type of atmosphere. Furthermore, there has been no credible evidence of life discovered in any meteorite, and meteorites have continuously rained upon Earth over 4.5 billion years of Earth history. In these cases, focus is principally stringent design for sample containment and, ultimately, transfer of a given containment canister into an appropriately designed Earth-based sample handling chamber, as was the case for the Stardust samples, Hayabusa, and now (in progress) Hayabusa2 and OSIRIS-REx.

In the future, there may be sample return missions that pose a risk for the capture of extraterrestrial microorganisms. For example, any sample return from Mars or from its moons (e.g., MMX sampling Phobos regolith), or even a mission to return high-altitude martian dust, could contain a contribution from Mars, and will be subject to planetary protection protocol. In addition, proposed missions to retrieve particles from the plumes of Enceladus (see Figure 3.10) have the possibility for returning extraterrestrial life. These missions will require considerable effort in the development of fail-proof containment. It is assumed that such sample containment facilities will require all of the functionality that current facilities have (e.g., those for Stardust samples, Hayabusa, Hayabusa2, and OSIRIS-REx), but, in addition, operate at the highest level of biohazard protection. The complexity and costs of such facilities are expected to be substantial.²⁹