It’s Time to Decide. Where Should OSIRIS-REx Take a Sample from Bennu?

NASA’s OSIRIS-REx arrived at asteroid Bennu in December 2018. During the past year, it’s been imaging the surface of the asteroid extensively, looking for a spot to take a sample from. Though the spacecraft has multiple science objectives, and a suite of instruments to meet them, the sample return is the key objective.

Now, NASA has narrowed the choice down to four potential sampling locations on the surface of the asteroid.

The spacecraft performed a month long flyover of each of the four sites. That phase was called Recon A, and it was completed in July. Now mission operators have high-resolution images of each of the sites, detailing their topography, albedo, color, and the sampling suitability.

Now it’s time to narrow the four sites down to two: a primary site and a back-up site. But unfortunately, Recon A showed that each of the four sites is tricky, and sampling might not go as smoothly as hoped. Though each of the four sites has suitable fine-grained material, smaller than 2.5 cm (1 inch), they’re also hazardous.

One of the four potential sampling sites with a parking lot overlaid for scale. Image Credit: NASA/Goddard/University of Arizona

“Sample site selection really is a comprehensive activity. It requires that we look at many different types of data in many different ways to ensure the selected site is the best choice in terms of spacecraft safety, presence of sampleable material, and science value,” said Heather Enos, OSIRIS-REx deputy principal investigator at the University of Arizona, Tucson, and chair of the sample site selection board. “Our team is incredibly innovative and integrated, which is what makes the selection process work,” she said in a press release.

Bennu is rougher than anticipated, and that’s adding some complication to the sampling site selection. OSIRIS-REx was designed to land on a beach-like surface, where there are pond-like deposits of fine-grained material suitable for sampling. But Bennu is rocky. There are no large beach-like areas, there are only small open areas surrounded by boulders.

This image shows a view across asteroid Bennu’s southern hemisphere and into space, and it demonstrates the number and distribution of boulders across Bennu’s surface. The image was obtained on Mar. 7 by the PolyCam camera on NASA’s OSIRIS-REx spacecraft from a distance of about 3 miles (5 km). The large, light-colored boulder just below the center of the image is about 24 feet (7.4 meters) wide, which is roughly half the width of a basketball court. Image Credit: NASA/Goddard/University of Arizona

When OSIRIS-REx does eventually take a sample, it will do so autonomously. The spacecraft has a Natural Feature Tracking (NFT) system that uses images of the asteroid to guide itself to the surface. By comparing its onboard images of Bennu with what its cameras are seeing in real time, it can avoid collisions.

But Bennus’s unexpected inhospitability means that system will face some challenges. It has an automatic fail-safe that will abort the sampling if an obstacle is too close. To prevent that from happening, mission operators will be working overtime. In fact, the team almost expects the spacecraft to waive its first attempt when it approaches the building-sized boulders that litter the surface of the asteroid.

“If the spacecraft executes a wave-off while attempting to collect a sample, that simply means that both the team and the spacecraft have done their jobs…”

Mike Moreau, OSIRIS-REx Deputy Project Manager.

“Bennu’s challenges are an inherent part of this mission, and the OSIRIS-REx team has responded by developing robust measures to overcome them,” said Mike Moreau, OSIRIS-REx deputy project manager at Goddard. “If the spacecraft executes a wave-off while attempting to collect a sample, that simply means that both the team and the spacecraft have done their jobs to ensure the spacecraft can fly another day. The success of the mission is our first priority.”

<Click to Enlarge> This flat projection mosaic of asteroid Bennu shows the relative locations of the four candidate sample collection sites on the asteroid: Nightingale, Kingfisher, Osprey and Sandpiper. NASA’s OSIRIS-REx spacecraft is scheduled to touch down on one of these four sites to collect a sample in summer 2020.
Credits: NASA/Goddard/University of Arizona

But NASA still has to settle on one of the four potential avian-themed sites: Sandpiper, Osprey, Kingfisher, and Nightingale.

Sandpiper

Sandpiper has the right type of fine-grained material for a good sample, but the material is trapped between larger rocks, making it problematic. Overall, the Sandpiper location is relatively flat.

Site Sandpiper is located in asteroid Bennu’s southern hemisphere, and the region of interest is visible in the center of the image (situated above the large boulder). The image was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on October 5, from a distance of 0.6 miles (1 km). The field of view is 48 ft (14.6 m). For scale, the large, light colored boulder in the bottom center of the image is 16 ft (5 m) wide, which is about the size of a box truck. Image Credit: NASA/Goddard/University of Arizona

Osprey

Initially, Osprey looked promising. Spectrometry revealed that it has dark carbon-rich material that’s desirable scientifically. Images also showed fine-grained material, but high-resolution images show that it’s scattered with larger material that’ll make sampling difficult.

Site Osprey is located near asteroid Bennu’s equator in the northern hemisphere. Because the region of interest is so large, only a portion of the crater is shown in this image. Site Osprey’s recognizable features are visible – there is a dark patch of material in the center of the crater, and a large, flat boulder on the northern crater wall (upper left). The image was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on October 12, from a distance of 0.6 miles (1 km). The field of view is 47 ft (14.2 m). For reference, the fracture in the large boulder (upper left) is 10 ft (3 m) long, which is about the length of a standing grizzly bear. Image Credit: NASA/Goddard/University of Arizona

Kingfisher

The Kingfisher site is inside a small crater, meaning it’s a younger feature than other parts of Bennu’s surface, including the large crater that holds Sandpiper. Younger craters hold fresher material, which hasn’t been subjected to as much space-weathering as older features. Kingfisher may be too rocky, but another nearby crater may be more suitable.

Site Kingfisher is located near asteroid Bennu’s equator in the northern hemisphere. The site itself is visible in the lower right of the image, located in the middle of the relatively clear space. The image was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on October 19, from a distance of 0.6 miles (1 km). The field of view is 47 ft (14.4 m). For reference, the small crater is 9 ft (2.7 m) across, which is about the length of a dolphin. Image Credit: NASA/Goddard/University of Arizona

Nightingale

This crater site has accessible fine-grained material, although a single nearby boulder could be hazardous. Also, it’s further north, meaning the lighting could make it difficult for the autonomous NFT to recognize hazards.

Site Nightingale is located near asteroid Bennu’s north pole. The crater’s center is visible in the top center of the image, which contains an accumulation of smaller rocks. The image was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on October 26, from a distance of 0.6 miles (1 km). The field of view is 47 ft (14.4 m). For reference, the light-colored boulder (far left center) is 5 ft (1.4 m) long, which is about the length of a bicycle. Image Credit: NASA/Goddard/University of Arizona

NASA is just days away from narrowing the four sites down to two. Once the primary and backup sites are known, further recon flights next Spring will prepare the team for the eventual sample-taking maneuver. That maneuver is scheduled for next Summer, with the sample arriving back here at Earth in September 2023.

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Evan Gough

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