Preparing to Collect a Pristine Sample of a Carbonaceous Asteroid
After a nearly two-year journey since its launch in September 2016, the OSIRIS-REx spacecraft arrived at its target asteroid Bennu in December 2018. The initial images sent back to Earth revealed a rocky surface with unexpected roughness. The spacecraft is currently characterizing the asteroid in preparation for sample collection in August 2020.
The OSIRIS-REx mission was selected in May 2011 in the third New Frontiers Program. Pristine carbonaceous asteroid material is not part of any collection on Earth; carbonaceous chondrite meteorites are heated upon entering the atmosphere and undergo chemical and mineralogical transformation on Earth. These Earth effects hinder the understanding of which organic compounds in meteorites originate from space and which are formed on Earth, and whether any become unstable and disappear after arrival at Earth’s surface. Getting a pristine piece of a carbonaceous asteroid would eliminate this uncertainty and allow scientists to better understand the organic chemistry of the early solar system.
This idea led mission planners to choose near-Earth asteroid (101955) Bennu as the mission target. Bennu is accessible, volatile-rich, sufficiently large, and sufficiently characterized to allow fundamental navigation and encounter planning, and among the bodies most likely to collide with Earth in the future. The mission has five main goals, which are reflected in its name: OSIRIS-REx stands for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer. First, as noted, pristine samples will allow scientists to understand the origin of organic compounds present in carbonaceous asteroids and their potential contribution to life on Earth. Second, the characterization of spectral features combined with detailed analyses of returned samples will allow evaluation of connections between asteroids and meteorites. Third, the volatile-rich nature of asteroids may offer important resources for space exploration, and OSIRIS-REx will provide a better understanding of the type and amount of volatiles present on a typical low-albedo asteroid. Fourth, the mission will better characterize the variables that control Bennu’s orbital evolution and, thus, its potential for Earth impact. Finally, the head of the Touch and Go Sample Acquisition Mechanism (TAGSAM) has a ring of specially designed collector pads that will sample the upper layer of regolith material at the surface.
The mission is a partnership between the University of Arizona, NASA Goddard Space Flight Center, and Lockheed Martin. University of Arizona planetary scientists Michael J. Drake and Dante Lauretta were the Principal Investigator (PI) and Deputy PI when the mission was selected, and Lauretta serves as the current PI. Scientific payloads onboard the spacecraft include the OSIRIS-REx Laser Altimeter (OLA), provided by the Canadian Space Agency; the OSIRIS-REx Thermal Emission Spectrometer (OTES), built by Arizona State University; the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS), built by NASA Goddard; the OSIRIS-REx Camera Suite (OCAMS), built by the University of Arizona; and the Regolith X-Ray Imaging Spectrometer (REXIS), built by the Massachusetts Institute of Technology (Fig. 1). The returned sample will be stored and curated at the NASA Johnson Space Center (JSC) in Houston.
Construction and Launch
Building the spacecraft began in 2015 with the Assembly, Testing, and Launch Operations (ATLO) phase of the mission. Because one of the mission goals is to study the organic geochemistry of the collected sample, special attention was given to building and assembling in a “clean” environment. (In this context, a clean environment refers to one that is as free from any kind of biological contaminants as humanly possible.) Mission scientists and curators made a considerable effort to acquire data and materials that allow a complete understanding of the ATLO environments as well as the materials composing the spacecraft and its components. Monitoring of the ATLO environments was focused on both particle and volatile characterization. Witness plates (materials that have defined open and closed periods at points throughout the mission, exposing and protecting them from environmental conditions) were deployed in the various ATLO environments in Denver and Florida. In addition, materials are archived at JSC from various spacecraft and instrument components that will have direct or indirect contact with collected samples.
As ATLO activities shifted from the Denver cleanrooms to Florida, so did monitoring for contamination, including efforts focused on the payload fairing containing the spacecraft. The most challenging aspects involved the last days before launch, as the fairing (the nose cone used to protect the spacecraft against the impact of dynamic pressure and aerodynamic heating during launch through an atmosphere) had to move through a number of environments on the way to assembly on the launchpad. This collection of witness plates and materials is being stored and curated at JSC in Houston. These items will be available indefinitely with the purpose of allowing evaluation of any possible contamination of the collected sample by the spacecraft hardware or assemblies.
The launch of OSIRIS-REx was carried out with tremendous efficiency. The 21-day launch window for OSIRIS-REx afforded flexibility in the launch schedule. However, OSIRIS-REx, under the guidance of United Launch Alliance (ULA) at Cape Canaveral, lifted off from launch complex 41 at only 100 milliseconds into the launch window at 7:08 p.m., September 8, 2016. This portion of the mission was described in Issue 146 of this publication (see suggested reading below).
Activities Since Launch
In February 2017, the spacecraft undertook a search for the enigmatic class of near-Earth objects known as Earth-Trojan asteroids. Trojan asteroids are trapped in stable gravity wells, called Lagrange points, which precede or follow a planet. OSIRIS-REx traveled through Earth’s fourth Lagrange point, located 60° ahead in Earth’s orbit around the Sun. The mission team took multiple images in this region with the OCAMS MapCam camera in the hope of identifying Earth-Trojan asteroids. Although none were discovered, the spacecraft’s camera operated flawlessly and demonstrated that it could image objects two magnitudes dimmer than originally expected.
The spacecraft used an array of small rocket thrusters to match Bennu’s velocity and rendezvous with the asteroid. After nearly 20 months of instrument checkouts and testing along the way, the OSIRIS-REx spacecraft entered the approach phase of the mission in August 2018. As the spacecraft closed in on Bennu, surface features began to sharpen, and upon arrival in December the spacecraft sent back beautiful images of the entire asteroid (Fig. 2). One remarkable observation is that the shape model for Bennu derived from previous Earth-based observational data was an excellent match to the actual shape as imaged by the mission. By early 2019, several exciting discoveries had already been made. First, OVIRS measured an absorption peak near the 2.7-micrometer (0.0001-inch) wavelength, which is a key signature of OH in minerals and in this case a confirmation that Bennu has hydrated phyllosilicates, like CM chondrite meteorites (Fig. 3). Second, as JAXA’s Hayabusa2 mission found at asteroid Ryugu, the surface of Bennu is rougher than expected and even contains very large boulders. Some of the boulders appear to have embedded clasts (Fig. 4). The paucity of finer-grained material, as observed at other asteroids such as Itokawa (by Hayabusa in 2006) or Eros (by the Near-Earth Asteroid Rendezvous mission in 1999), was puzzling, as were particle ejection events that were observed almost immediately upon entering orbit in January 2019. Initial assessments indicate that the shape of Bennu is consistent with a rubble-pile structure. Many of the boulders and large rocks exposed at the surface contain evidence for brecciation, and the loose material on the surface is diverse with respect to color, reflectance, and grain size (Fig. 5). Another important finding is that Bennu is spinning faster over time. The images and findings from the mission have captured the interest of meteoriticists who study brecciated carbonaceous chondrites and planetary scientists who study orbital and dynamic aspects of asteroids.
The detailed imaging initially led to the recognition of 50 regions of interest (ROIs) for sampling on the surface that were explored in more detail using the spacecraft instrumentation. From these 50 ROIs, the mission downselected to 16, and then to a final 4, named after Egyptian birds: Kingfisher, Osprey, Nightingale, and Sandpiper (Fig. 6). These four sites were selected in part due to their sampleability (the presence of centimeter-scale particles that can be ingested by the TAGSAM) and relative safety (presenting minimal hazards to the spacecraft). All four sites show the hydration feature that is ubiquitous across Bennu.
After a thorough evaluation of all four candidate sites, the mission team selected Nightingale as the site with the greatest amount of sampleable material that is safely accessible. Nightingale is located in a 20-meter (66-foot) crater within a larger crater in the northern hemisphere of Bennu. Its northern location means that it experiences lower temperatures than elsewhere on the asteroid, and the presumably young crater is well preserved. These characteristics support the possibility that the site will allow for a pristine sample of the asteroid, giving the team insight into Bennu’s history. The Osprey site was selected as a backup sample collection site, as it appears to have less sampleable material but is safer. The spacecraft is designed to autonomously “wave off” the sampling attempt if its predicted position is too close to a hazardous area. During this maneuver, the exhaust plumes from the spacecraft’s thrusters could potentially disturb the surface of the site, due to the asteroid’s microgravity environment. In any situation where a follow-on attempt at Nightingale is not possible, the team will try to collect a sample from the Osprey site instead.
The primary and backup sites will be the focus of even more detailed characterization during lower flyovers in early 2020. This new information will be used to plan the detailed approach and maneuvers required for sample acquisition. For example, the original mission plan envisioned a sample site with a diameter of 50 meters (164 feet), and while the Nightingale crater is larger than that, the area safe enough for the spacecraft to sample is much smaller: 16 meters (52 feet). This means that the spacecraft has to very accurately target Bennu’s surface. After the detailed characterization, the mission will rehearse the sampling maneuvers without touching down. The actual sampling attempt is nominally scheduled for August 25, 2020.
The TAGSAM head is at the end of a long arm with an elbow and pogo-stick-like mechanism for flexibility at the surface. The head is roughly 38 centimeters (15 inches) in diameter and has an internal circular cylindrical cavity where the sample will be collected and stored. The sampling mechanism utilizes a jet of nitrogen gas to mobilize loose material into the bulk sample collector. Collection tests in Earth-gravity and low-gravity environments have resulted in the collection of up to 1.5 kilograms (3.3 pounds) of material. In addition, asteroid material will be trapped in surface contact pads that are on the side of the sampler head that touches the asteroid surface. In September 2023, the spacecraft will make a close approach to Earth and release the Sample Return Capsule (SRC), which will be recovered in a parachute landing at the Utah Test and Training Range near Salt Lake City.
Upon landing in Utah, the SRC will be recovered and safely kept in a portable cleanroom before being transported to JSC — the home of all of NASA’s astromaterials, including Apollo moon rocks, Antarctic meteorites, cosmic dust particles, Stardust comet particles, and Genesis solar wind collectors. The curation cleanroom for the storage and handling of Bennu samples is currently under construction at JSC and will be completed in 2020.
The mission science team looks forward to unraveling the geologic history of Bennu, including its origins, impact record, and more recent dynamic history. JAXA’s Hayabusa2 mission to the carbonaceous asteroid Ryugu, occurring in parallel with OSIRIS-REx, offers an enhanced understanding of the volatile-bearing and carbonaceous asteroids that make up more than 50% of the asteroid belt. We look forward to the many new discoveries these missions will make and the corresponding advancements in understanding the solar system.
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