Optical navigation enables sample collection from asteroid

NASA took on a Herculean precision positioning task that culminated Oct. 20 with a spacecraft sampling the surface of an asteroid from a 5-meter area — a NASA first.

The OSIRIS-REx spacecraft, launched on Sept. 8, 2016, spent two years mapping the Bennu asteroid to determine the best site for removing the sample. Bennu was selected because of its near-Earth position (a mere 200 million miles away) as well as its age. It’s considered a primitive remnant from the formation of the solar system. It’s the smallest body a spacecraft has ever orbited.

The infrared spectrometer on OSIRIS-REx confirmed Bennu was sandy, but photos showed it to be an unrelenting rockscape dominated by boulders. Because of this difficult terrain, the planned sample site was reduced from 50 meters to 5 meters, an area smaller than a parking lot. “This required us to rethink how to navigate to the surface and come up with new ideas,” said Coralie Adam, TAG navigation manager for KinetX Aerospace. TAG stands for Touch-And-Go, the sample-collecting procedure.

The mapping method changed from using a lidar-based technique to an optical-based technique. “When we saw what Bennu looked like, we realized we had to switch to a vision-based approach, and that’s the NFT system,” explained Mike Moreau, Osiris Rex deputy project manager, Goddard Space Flight Center.

The Natural Feature Tracking (NFT) was added to the mission at the critical design review stage. NFT is an onboard optical navigation system that compares observed images to a set of asteroid terrain models rendered in real-time from a catalog stored in the flight computer’s memory. Onboard knowledge of the spacecraft state is then updated by a Kalman filter using the measured residuals between the rendered reference images and the actual observed images. (Read a technical paper on NFT.)

The asteroid terrain models used by NFT are built from a shape model generated from observations collected during earlier phases of the mission and include both terrain shape and albedo information about the asteroid surface.

“The KinetX navigation team spent last two years to learning how to navigate around Bennu using optical navigation techniques,” Adam said. She explained that every few minutes, a navigation camera takes an image of the features below and sends it to the NFT. The system identifies features in the image and sends a signal back to Earth.” The KinetX team worked with the Lockheed Martin team to map the surface of Bennu to a resolution of 2 centimeters per pixel.

With this observational data, the team created a hazard map. With position uncertainty down to half a meter, a sample site dubbed Nightingale was successfully contacted, and 2 ounces of regolith collected.

The team included mission managers from the University of Arizona, Lockheed Martin, and NASA’s Goddard Space Flight Center.

The Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (Osiris-REx) spacecraft
is scheduled to depart Bennu in 2021, and to deliver the collected sample to Earth on Sep. 24, 2023. It will be the first U.S. mission to carry samples from an asteroid back to Earth, and the largest sample returned since the Apollo missions.

Artist’s concept of the OSIRIS-REx spacecraft orbiting the Bennu asteroid. (Image: NASA)