Two merged images from 19 January show Bennu ejecting particles from its surface.

NASA/Goddard/University of Arizona/Lockheed Martin

Mysterious asteroid activity complicates NASA’s sampling attempts

THE WOODLANDS, TEXAS—NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission to sample the asteroid Bennu and return to Earth was always going to be a touch-and-go maneuver. But new revelations about its target—a space rock five times the size of a U.S. football field that orbits close to Earth—are making the mission riskier than ever. Rather than smooth plains of rubble, Bennu’s surface is a jumble of more than 200 large boulders, with scarcely enough gaps for robotic sampling of its surface grit, the spacecraft’s team reported here today at the Lunar and Planetary Science Conference and in a series of Nature papers.

The $800 million spacecraft began to orbit Bennu at the start of this year, and the asteroid immediately began to spew surprises—literally. On 6 January, the team detected a plume of small particles shooting off the rock; 10 similar events followed over the next month. Rather than a frozen remnant of past cosmic collisions, Bennu is one of a dozen known “active” asteroids. “[This is] one of the biggest surprises of my scientific career,” says Dante Lauretta, the mission’s principal investigator and a planetary scientist at the University of Arizona in Tucson. “We are seeing Bennu regularly ejecting material into outer space.”

Ground-based observations of Bennu had originally suggested its surface was made of small pebbles incapable of retaining heat. OSIRIS-REx was designed to sample such a smooth environment, and it requires a 50-meter-wide circle free of hazards to approach the surface. No such circle exists, say mission scientists, but there are several smaller boulder-free areas that it could conceivably sample. Given how well the spacecraft has handled its maneuvers so far, “We’re going to try to hit the center of the bull’s-eye,” says Rich Burns, OSIRIS-REx’s project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

OSIRIS-REx has always been a cautious mission. Unlike the speedy Hayabusa2 mission from Japan, which sampled the near-Earth Ryugu asteroid a half-year after its arrival, OSIRIS-REx plans to sample Bennu in July 2020, a year and a half after it started to orbit. That timetable has not changed, Lauretta says. By this summer, researchers hope to have the sampling site selected. And much remains to be discovered about the spinning, top-shaped asteroid, starting with the plumes, which can shoot off penny-size particles at speeds of up to several meters per second.

Just after OSIRIS-REx entered orbit around Bennu, the asteroid reached its closest approach to the sun. The other known active asteroids, which are all located in the asteroid belt between Mars and Jupiter, have similarly spouted particles as they get closer to the sun. It’s possible that the plumes are related to this approach, perhaps driven by water ice sublimating into vapor. But there are a dozen different hypotheses to explore, Lauretta says. “We don’t know the answer right now.”

The abundance of impact craters on Bennu’s ridgelike belly suggest the asteroid is up to a billion years old, more ancient than once thought. The craters also imply that Bennu got its toplike shape early in its history, rather than later from sun-driven spinning. And there are signs that material on the asteroid’s poles is creeping toward the equator, suggesting geological activity.

Although many of these puzzles intrigue scientists, ultimately the point of the mission is to return the largest amount of asteroid material ever captured to Earth’s surface. That is expected to happen in 2023. But, Lauretta adds, “The challenge got a lot harder when we saw the true nature of Bennu’s surface.”