OSIRIS-REx sample collection site selected on asteroid Bennu

A close-up view of the Nightingale sample collection site on asteroid Bennu with an overlaid graphic showing the relative size of NASA’s OSIRIS-REx spacecraft. The mound of boulders casting a shadow to the right rises 30 metres from the floor of the depression. Image: Goddard Space Flight Center, University of Arizona

Out of four finalists, NASA has chosen a promising site on the asteroid Bennu where the agency’s OSIRIS-REx spacecraft will attempt to snag rock and soil samples next summer. If all goes well, the samples will be returned to Earth in 2023.

The preferred site, known as Nightingale, provides the best balance between risk and scientific reward, researchers say, offering an opportunity to collect organic and water-bearing material from the early solar system.

But it will be a challenge for spacecraft navigators. A mound of boulders dubbed “Mount Doom” rises to one side of the sample site, an obstruction that OSIRIS-REx must avoid to execute a “touch-and-go” collection.

“It’s a substantial, building-size obstruction, and we’re trying to get into a crater that’s on the order of a few parking lot spaces wide,” said Dante Lauretta, the OSIRIS-REx principal investigator at the University of Arizona.

“So we are doing a really tight job parking. We’re aware that we have hazards around us. So precision navigation to that sample material is our biggest challenge.”

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer — OSIRIS-REx — was launched from Cape Canaveral in September 2016 and braked into orbit around Bennu two years later.

In case of problems reaching the Nightingale sample collection site, the OSIRIS-REx spacecraft will target a backup site known as Osprey. Image: Goddard Space Flight Center, University of Arizona

The spacecraft is equipped with a “Touch-and-Go Sample Acquisition Mechanism,” or TAGSAM, device on the end of a 10-foot-long robot arm. The collect samples, OSIRIS-REx will slowly move in toward Bennu, briefly pressing the pie pan-shaped TAGSAM collector against the asteroid’s surface.

A jet of nitrogen gas will stir up soil and small rocks that will then be snagged by TAGSAM’s collector. The material will be placed in a sample return container for a two-and-a-half-year flight back to Earth.

The challenge for mission planners is Bennu’s rubble-pile surface. The asteroid is covered in rocks and rocky fragments, most too large to be collected by OSIRIS-REx.

“When we first got there, the most obvious feature that we saw were these abundant, very large boulders and overall a rough and rugged surface very different than what we designed the spacecraft to sample,” Lauretta said.

Asteroid Bennu. Image: University of Arizona

“We were (planning for) sampling areas that were 25 metres (80 feet) across, and quite honestly, I thought it was going to be obvious from the first images where the sample regions were and that it was going to be a straightforward site selection. And it was nothing of the sort.”

Nightingale was selected from four finalist sample sites.

“We recognise that this does have some hazards around it, and so we are doing a lot of work to make sure that we’re targeting the safe regions,” said Lauretta. “But this one really came out on top because of the scientific value.”