Grayscale image of asteroid Bennu with large bolder
Asteroid Bennu is unexpectedly covered by large boulders, which are a hazard for the spacecraft’s touch-and-go sample retrieval. The largest boulder, seen here, is 21.7 meters tall. Credit: NASA/Goddard/University of Arizona

Update 12 December 2019: OSIRIS-REx’s primary sample site will be Nightingale and its secondary site will be Osprey. The team announced this at a press conference at AGU’s Fall Meeting 2019.

How do you pick the right spot on an asteroid to snag a handful of rocks and dust to bring back home? The mission team for NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) spacecraft has been hard at work since July weighing the limited options on asteroid 101955 Bennu.

“It’s quite a bit more rugged than what we anticipated,” said OSIRIS-REx deputy investigator Heather Enos. “When we first started to get our images back, we realized we were going to be hard-pressed to find real estate that provided a big enough site that met the criteria for us to be able to safely TAG.” (TAG is the team’s shorthand for touch-and-go.) Enos is a planetary scientist at the University of Arizona’s Lunar and Planetary Laboratory in Tucson and will discuss the selection process on 13 December during AGU’s Fall Meeting 2019.

How Do You Solve a Problem Like Bennu?

OSIRIS-REx arrived at Bennu on 3 December 2018. Since then, the craft has completed multiple detailed scans, mapping out the plethora of boulders and craters that litter the surface. The mapping campaign, aided in large part by a citizen science project, counted every rock on the surface larger than about 10 centimeters in size.

There turned out to be many more “large” rocks than initially expected, which presented a twofold problem. First, the boulders are a navigational hazard for the spacecraft’s approach and departure during the touch-and-go maneuver. The team needed to adjust its target landing radius from 25 meters across to just 10 meters, akin to striking the bull’s-eye of a dartboard.

“This is a very challenging mission, and it requires incredible integrative effort between the flight dynamics team, the spacecraft team, the science planning team, and the science team [itself].”

Second, the craft’s arm is designed to pick up material 2 centimeters or smaller in size. That material seems to be in short supply. Using the detailed map and model of the surface, the researchers digitally removed every boulder larger than 15 centimeters to locate spots that might have smaller material.

The slope of the land factored into the safety of approach and touchdown, and the reflectivity of the material factored into the likelihood that the yet-unresolved material is grabbable. The researchers considered all of these aspects when they calculated the probability that a particular site will be the right spot on which to land.

“This is a very challenging mission, and it requires incredible integrative effort between the flight dynamics team, the spacecraft team, the science planning team, and the science team [itself],” Enos said. “It’s a more integrated process tactically than any other mission I’ve ever participated on and most that I am even aware of.”

The team announced its final four candidates on 12 August: Nightingale, Kingfisher, Osprey, and Sandpiper, named after native Egyptian birds.

Room for Adjustment

OSIRIS-REx will conduct more reconnaissance campaigns in 2020, Enos said, which will help the team resolve the smaller-grained material in each of the sites and refine the landing coordinates.

“We have our TAG coordinates that we specify, but we will have the ability to tweak those a little bit by perhaps a meter,” Enos said. “Once we get the higher-resolution data, we can…slightly adjust where within our TAG site we’re going to target to an area that we think is the best and has particles that are 2 centimeters or less in the most abundance possible.”

OSIRIS-REx is scheduled to attempt sample retrieval from Bennu in late 2020 and to return to Earth with its cache in 2023. The mission aims to bring home 60 grams or more of asteroid material for extensive laboratory study.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2019), Location, location, location: The how-to’s of asteroid sampling, Eos, 100, Published on 12 December 2019.

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