After months grappling with the rugged reality of asteroid Bennus surface, the team leading NASAs first asteroid sample return mission has selected four potential sites for the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft to tag its cosmic dance partner.
Since its arrival in December 2018, the OSIRIS-REx spacecraft has mapped the entire asteroid in order to identify the safest and most accessible spots for the spacecraft to collect a sample. These four sites now will be studied in further detail in order to select the final two sites a primary and backup in December.
The team originally had planned to choose the final two sites by this point in the mission. Initial analysis of Earth-based observations suggested the asteroids surface likely contains large ponds of fine-grain material. The spacecrafts earliest images, however, revealed Bennu has an especially rocky terrain. Since then, the asteroids boulder-filled topography has created a challenge for the team to identify safe areas containing sampleable material, which must be fine enough less than 1 inch (2.5 cm) diameter for the spacecrafts sampling mechanism to ingest it.
We knew that Bennu would surprise us, so we came prepared for whatever we might find, said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. As with any mission of exploration, dealing with the unknown requires flexibility, resources and ingenuity. The OSIRIS-REx team has demonstrated these essential traits for overcoming the unexpected throughout the Bennu encounter.
The original mission schedule intentionally included more than 300 days of extra time during asteroid operations to address such unexpected challenges. In a demonstration of its flexibility and ingenuity in response to Bennus surprises, the mission team is adapting its site selection process. Instead of down-selecting to the final two sites this summer, the mission will spend an additional four months studying the four candidate sites in detail, with a particular focus on identifying regions of fine-grain, sampleable material from upcoming, high-resolution observations of each site. The boulder maps that citizen science counters helped create through observations earlier this year were used as one of many pieces of data considered when assessing each sites safety. The data collected will be key to selecting the final two sites best suited for sample collection.
In order to further adapt to Bennus ruggedness, the OSIRIS-REx team has made other adjustments to its sample site identification process. The original mission plan envisioned a sample site with a radius of 82 feet (25 m). Boulder-free sites of that size dont exist on Bennu, so the team has instead identified sites ranging from 16 to 33 feet (5 to 10 m) in radius. In order for the spacecraft to accurately target a smaller site, the team reassessed the spacecrafts operational capabilities to maximize its performance. The mission also has tightened its navigation requirements to guide the spacecraft to the asteroids surface, and developed a new sampling technique called Bullseye TAG, which uses images of the asteroid surface to navigate the spacecraft all the way to the actual surface with high accuracy. The missions performance so far has demonstrated the new standards are within its capabilities.
"Although OSIRIS-REx was designed to collect a sample from an asteroid with a beach-like area, the extraordinary in-flight performance to date demonstrates that we will be able to meet the challenge that the rugged surface of Bennu presents," said Rich Burns, OSIRIS-REx project manager at NASAs Goddard Space Flight Center in Greenbelt, Maryland. "That extraordinary performance encompasses not only the spacecraft and instruments, but also the team who continues to meet every challenge that Bennu throws at us."
The four candidate sample sites on Bennu are designated Nightingale, Kingfisher, Osprey, and Sandpiper all birds native to Egypt. The naming theme complements the missions two other naming conventions Egyptian deities (the asteroid and spacecraft) and mythological birds (surface features on Bennu).
The four sites are diverse in both geographic location and geological features. While the amount of sampleable material in each site has yet to be determined, all four sites have been evaluated thoroughly to ensure the spacecrafts safety as it descends to, touches and collects a sample from the asteroids surface.
Nightingale is the northern-most site, situated at 56 degrees north latitude on Bennu. There are multiple possible sampling regions in this site, which is set in a small crater encompassed by a larger crater 459 feet (140 m) in diameter. The site contains mostly fine-grain, dark material and has the lowest albedo, or reflection, and surface temperature of the four sites.
Kingfisher is located in a small crater near Bennus equator at 11 degrees north latitude. The crater has a diameter of 26 feet (8 m) and is surrounded by boulders, although the site itself is free of large rocks. Among the four sites, Kingfisher has the strongest spectral signature for hydrated minerals.
Osprey is set in a small crater, 66 feet (20 m) in diameter, which is also located in Bennus equatorial region at 11 degrees north latitude. There are several possible sampling regions within the site. The diversity of rock types in the surrounding area suggests that the regolith within Osprey may also be diverse. Osprey has the strongest spectral signature of carbon-rich material among the four sites.
Sandpiper is located in Bennus southern hemisphere, at 47 degrees south latitude. The site is in a relatively flat area on the wall of a large crater 207 ft (63 m) in diameter. Hydrated minerals are also present, which indicates that Sandpiper may contain unmodified water-rich material.
This fall, OSIRIS-REx will begin detailed analyses of the four candidate sites during the missions reconnaissance phase. During the first stage of this phase, the spacecraft will execute high passes over each of the four sites from a distance of 0.8 miles (1.29 km) to confirm they are safe and contain sampleable material. Closeup imaging also will map the features and landmarks required for the spacecrafts autonomous navigation to the asteroids surface. The team will use the data from these passes to select the final primary and backup sample collection sites in December.
The second and third stages of reconnaissance will begin in early 2020 when the spacecraft will perform passes over the final two sites at lower altitudes and take even higher resolution observations of the surface to identify features, such as groupings of rocks that will be used to navigate to the surface for sample collection. OSIRIS-REx sample collection is scheduled for the latter half of 2020, and the spacecraft will return the asteroid samples to Earth on Sept. 24, 2023.
Goddard provides overall mission management, systems engineering, and safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator, and the University of Arizona leads the science team and the missions science observation planning and data processing. Lockheed Martin Space in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace are responsible for navigating the spacecraft. OSIRIS-REx is the third mission in NASAs New Frontiers Program, which is managed by NASAs Marshall Space Flight Center in Huntsville, Alabama, for the agencys Science Mission Directorate in Washington.
