A historic moment is on the horizon for NASA’s OSIRIS-REx mission. In just a few weeks, the robotic OSIRIS-REx spacecraft will descend to asteroid Bennu’s boulder-strewn surface, touch down for a few seconds and collect a sample of the asteroid’s rocks and dust – marking the first time NASA has grabbed pieces of an asteroid, which will be returned to Earth for study.
On Oct. 20, the mission will perform the first attempt of its Touch-And-Go (TAG) sample collection event. This series of maneuvers will bring the spacecraft down to site Nightingale, a rocky area 52 ft (16 m) in diameter in Bennu’s northern hemisphere, where the spacecraft’s robotic sampling arm will attempt to collect a sample. Site Nightingale was selected as the mission’s primary sample site because it holds the greatest amount of unobstructed fine-grained material, but the region is surrounded by building-sized boulders. During the sampling event, the spacecraft, which is the size of a large van, will attempt to touch down in an area that is only the size of a few parking spaces, and just a few steps away from some of these large boulders.
Because the spacecraft and Bennu are approximately 207 million miles (334 million km) from Earth during TAG, it will take about 18.5 minutes for signals to travel between them. This time lag prevents the live commanding of flight activities from the ground during the TAG event, so the spacecraft is designed to perform the entire sample collection sequence autonomously. Prior to the event’s start, the OSIRIS-REx team will uplink all of the commands to the spacecraft and then send a “GO” command to begin.
To autonomously navigate to site Nightingale, OSIRIS-REx uses the Natural Feature Tracking (NFT) navigation system. The spacecraft begins collecting navigation images about 90 minutes after orbit departure. It then compares these real-time images to an onboard image catalog, using identified surface features to make sure that it’s on the right course toward the site.
To ensure that the spacecraft touches down on a safe area that avoids the region’s many boulders, the navigation system is equipped with a hazard map of site Nightingale, which delineates areas within the sample site that could potentially harm the spacecraft. If the spacecraft’s NFT system detects that it is on course to touch one of these hazardous zones, the spacecraft will autonomously wave off its approach once it reaches an altitude of 16 ft (5 m). This keeps the spacecraft safe and allows for a subsequent sample collection attempt at a future date.
As the spacecraft performs each event in the sample collection sequence, it will send telemetry updates back to the OSIRIS-REx team, albeit at an extremely slow data rate. The images and other science data collected during the event will be downlinked after the spacecraft has backed away from the asteroid and can point its larger antenna back to Earth to transmit at higher communication rates.
OSIRIS-REx is charged with collecting at least 2 oz. (60 grams) of Bennu’s rocky material to deliver back to Earth – the largest sample return from space since the Apollo program – and the mission developed two methods to verify that this sample collection occurred. On Oct. 22, OSIRIS-REx’s SamCam camera will capture images of the TAGSAM head to see whether it contains Bennu’s surface material. The spacecraft will also perform a spin maneuver on Oct. 24 to determine the mass of collected material. If these measures show successful collection, the decision will be made to place the sample in the Sample Return Capsule (SRC) for return to Earth. If sufficient sample has not been collected from Nightingale, the spacecraft has onboard nitrogen charges for two more attempts. A TAG attempt at the back-up Osprey site would be made no earlier than January 2021.
The mission team has spent the last several months preparing for the sample collection event while maximizing remote work as part of its COVID-19 response. On the day of TAG, a limited number of team members will monitor the spacecraft from Lockheed Martin Space’s Mission Support Area, taking appropriate safety precautions. Other members of the team will also be at other locations on-site to cover the event, while also observing safety protocols.
The spacecraft is scheduled to depart Bennu in 2021 and it will deliver the collected sample to Earth on Sep. 24, 2023.