NASA Refines Threat Posed by Potentially Hazardous Asteroid Bennu

Data gathered during the years NASA’s OSIRIS-REx spent zipping around asteroid Bennu has allowed scientists to update the risk posed by this potentially hazardous near-Earth object.

The spacecraft OSIRIS-REx is currently en route to Earth, carrying surface samples it collected from asteroid Bennu. From December 2018 to May 2021, the NASA spacecraft studied the gigantic rubble pile from every angle, measuring its size, shape, mass, composition, spin, orbital trajectory, and other important characteristics. Bennu is a primitive carbonaceous asteroid, so by studying this object, scientists can make inferences about what our solar system was like during its formative period.

But there’s more to this $800 million mission than just looking for organic molecules or signs of water and heavy elements. Bennu is currently ranked second on the list of potentially dangerous asteroids, highlighting the importance of learning as much as we can about it—especially the orbital dynamics that dictate its future movements.

The new research, published in Icarus, does exactly this, providing a refined trajectory of Bennu through to the year 2300. The misanthropes among you may be pleased to learn that Bennu still has a very slight chance of hitting our planet next century. The odds of a collision through the year 2300 remain very low, however: They’re now pegged to be about 1 in 1,750, or 0.057%.

Data derived by OSIRIS-REx, NASA’s Deep Space Network, and computer models allowed the scientists to constrain uncertainties in Bennu’s orbit by a factor of 20. OSIRIS-REx is what really made this possible, as it measured Bennu’s position relative to Earth down to the scale of a few meters.

Speaking earlier today at a teleconference held for reporters, Davide Farnocchia, the lead author of the new paper and a researcher with the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory in Southern California, said it’s an “impressive” result, as “we had one of the best known orbits in the entire asteroid catalogue,” in reference to Bennu. Dante Lauretta, a co-author of the study and OSIRIS-REx principal investigator at the University of Arizona, said this “incredible precision” allowed the team to characterize the asteroid’s orbital parameters and better predict where it’ll be in the future.

Impressively, the new model allowed the researchers to eliminate 24 of 26 possible keyholes for Bennu that were predicted to exist on September 11, 2135, when the asteroid is scheduled to safely zip past Earth. Gravitational keyholes can be likened to fictional gateways that take characters into alternative timelines (fans of the new Loki series know what I’m talking about). Keyholes are very much real, however, and they’re bad news—we don’t want asteroids to pass through keyholes, as they’re gateways that take asteroids onto orbital trajectories that threaten Earth.

There’s no chance of an impact during this encounter in 2135, said Farnocchia, but Bennu will be close to Earth—about half the distance from Earth to the Moon—and this will change the asteroid’s trajectory. To know this change in trajectory, however, scientists have to consider gravitational keyholes.

As NASA describes them, keyholes are “areas in space that would set Bennu on a path toward a future impact with Earth if the asteroid were to pass through them at certain times, due to the effect of Earth’s gravitational pull.” The new research describes two keyholes still in play, including one that would involve a collision between Earth and Bennu on September 24, 2182 (mark your calendars), but the probability is slim, at 1 in 2,700, or 0.037%. As Farnocchia reminded reporters repeatedly during the press conference, “there’s no reason for concern.”

The reason for so much uncertainty has to do with all the variables in play. Sir Isaac Newton described a universe that works with clock-like precision, but the clock that is our solar system features an unspeakable number of moving parts. These perturbing influences include things like the Sun’s gravity, the planets, all the moons, hundreds of asteroids, interplanetary dust, and the solar wind.

For the new study, Farnocchia and his colleagues tried to account for as many variables as possible to predict Bennu’s future trajectory, including the masses of 343 known asteroids. They even accounted for a possible nudge exerted by OSIRIS-REx when it grabbed a sample of surface material on October 20, 2020 (it turned out to be negligible) and bits of debris that are naturally falling from Bennu (also not a factor).

There’s also the Yarkovsky effect to consider. This is what happens when an object absorbs radiation from the Sun and this radiation then leaks away. This alters an object’s momentum in space, causing it to drift slightly from the path otherwise dictated by gravity. This effect is very slight, but it becomes meaningful over vast timescales. OSIRIS-REx gathered invaluable information—information that’s hard if not impossible to collect from the ground—that was used to calculate the Yarkovsky effect as Bennu travelled around the Sun, including the object’s size, mass, shape, rotation, surface properties, and other factors, as Farnocchia explained. This “helped us to model the future motion of Bennu,” he added.

Interestingly, the samples collected by OSIRIS-REx could further our understanding of how the Yarkovsky effect might continue to change Bennu’s trajectory. Analysis of the surface samples could “expose changes to the asteroid over time, like surface weathering,” which would “further our understanding of one of the most important parameters for determining orbital trajectory,” as Lauretta explained in response to a question posed by yours truly.

The new research provides the most solid estimates of Bennu’s future to date, but there’s still room for improvement. The researchers would like to account for the gravitational influence of all the asteroids in the solar system; determining the mass of these asteroids would be “a major next step forward,” said Farnocchia. Improved measurements of Bennu’s mass and density, which are still uncertain, would also help. Bennu is a loose pile of rocks and dust that probably features empty cavities and an uneven distribution of materials beneath the surface.

And now we wait for September 24, 2023, when OSIRIX-REx is set to return to Earth with its samples. Lauretta said the mission is in “great shape right now,” which is obviously good news. There’s still much to learn about this fascinating—and possibly worrisome—asteroid.