WASHINGTON — NASA announced Dec. 12 that it has selected the site on a near Earth asteroid where a spacecraft will touch down and collect samples, a task that has become more difficult than mission planners originally expected.

NASA said that its Origins Spectral Interpretation Resource Identification Security – Regolith Explorer, or OSIRIS-REx, spacecraft will gather samples from a crater on the asteroid Bennu dubbed Nightingale, located in the northern latitudes of the asteroid. That sampling attempt is currently scheduled for August 2020.

Nightingale was one of four sites on the asteroid that scientists had selected as finalists to be the location where the spacecraft would approach the surface and, using a sample collection tool mounted on the end of a robotic arm, touch the surface briefly to collect rock and soil samples before moving away.

“This one really came out on top because of the scientific value,” said Dante Lauretta, principal investigator for OSIRIS-REx, during a NASA broadcast that announced the site.

The site selection process was more challenging than the mission originally expected. When OSIRIS-REx arrived at Bennu in 2018, the spacecraft’s images revealed an asteroid whose surface was littered with boulders that would be hazards for any sampling attempt, making it difficult to find a location that was both of scientific interest and safe enough for the spacecraft to approach.

Lauretta said the high latitude of the Nightingale site means that it avoids the night-and-day temperature swings of locations closer to the asteroid’s equator, preserving organic and water-bearing minerals there. “Being in those high latitudes, we think, gives us the best chance to preserve that kind of material,” he said.

The site, though, will also be a challenge for any sampling attempt. Lauretta said large boulders in the area, including one dubbed “Mount Doom,” pose obstructions to the spacecraft as it approaches. “We’re trying to get into a crater that’s on the order of a few parking lot spaces wide,” he said. “We’re aware we have hazards around us, so precision navigation to that sample material is our biggest challenge.”

That’s led to changes in the spacecraft’s flight control software. “We’ve had to make some changes to the capabilities of the spacecraft to allow us to navigate into a site like Nightingale,” said Mike Moreau, OSIRIS-REx deputy project manager. That includes creating a “hazard map” of the site based on images of it, which the spacecraft will use to determine if it is coming too close to a boulder and, if so, back away.

Should the Nightingale site prove to be too hazardous to collect a sample from, Lauretta said the mission selected another crater close to the equator, called Osprey, as a backup site. “We know we can get in there,” he said, although once into Osprey it may be more difficult to collect the sample itself.

OSIRIS-REx will depart the vicinity of Bennu in 2021 after collecting between 60 grams and two kilograms of samples. It will return to Earth in September 2023, with the sample return container landing at the Utah Test and Training Range.

The mission, launched in September 2016, is the third in NASA’s New Frontiers program of medium-class planetary science missions, after the New Horizons mission to Pluto and the Kuiper Belt and the Juno mission to Jupiter. NASA selected the fourth New Frontiers mission, a spacecraft called Dragonfly that will fly from site to site on the surface of Saturn’s moon Titan, in June for launch in 2026.