Mark the date: 24 September 2182. That’s the day, according to a study released today, that a half-kilometer-wide asteroid called Bennu—recently visited by NASA’s OSIRIS-REx spacecraft—has the greatest chance of colliding with Earth in the next 300 years. The researchers behind the NASA-sponsored study emphasize that the risk of an impact remains very small—one in 2700, or 0.037%—and that, armed with the wealth of data from OSIRIS-REx’s 2 years orbiting the asteroid, they now know much more about it and the risk it poses.
“I don’t think we need to do anything about Bennu,” planetary scientist Lindley Johnson of NASA’s Planetary Defense Coordination Office said at a press briefing today. But Bennu and another asteroid known as (29075) 1950 DA remain the two most hazardous asteroids to Earth, so NASA will keep monitoring them closely.
As part of its mission to return asteroid samples to Earth, OSIRIS-REx reached Bennu in 2018 and trailed it until October 2020, studying the asteroid’s composition, structure, mass, and temperature. Then, the spacecraft swooped down to grab 1 kilogram of dust and pebbles before heading home. Bennu is thought to be a fragment of a larger planetesimal—a building block of planets—that formed beyond Jupiter in the early days of the Solar System, so it could hold important clues about how planets form. The probe is scheduled to drop its treasure by parachute in the Utah desert in September 2023.
Bennu was the chosen target partly because its orbit around the Sun is like Earth’s, so the two bodies occasionally approach each other making it easier for a spacecraft to reach the asteroid and get home again. As a bonus, NASA’s planetary defenders could also gather a wealth of data about the potential threat that Bennu poses to Earth. Although the asteroid is only about one-twentieth the size of the one that killed the dinosaurs, a direct hit would still be devastating on a continental scale.
From earlier observations by telescopes on Earth, researchers know Bennu is due to make its next closest flyby of Earth—inside the Moon’s orbit—in 2135. But beyond that, its path is unclear. That’s because Earth’s gravity could give it a tug during the flyby, altering the asteroid’s trajectory. Researchers have identified 26 so-called “gravitational keyholes” around Earth, areas of space where, if the asteroid passes through them, Earth’s gravity could deflect Bennu onto a collision course with our planet.
To get a better idea of how the 2135 encounter would play out, researchers led by Davide Farnocchia of NASA’s Jet Propulsion Laboratory analyzed those data and the new information from OSIRIS-REx. With OSIRIS-REx and its range-finding instruments so close to Bennu, the researchers could pin down its orbit to a few meters. But to calculate its future trajectory, they had to consider the drag on Bennu from solar wind and the gravitational effect of 343 nearby asteroids and other bodies. And they also had to estimate the impact of the Yarkovsky effect, a tiny amount of thrust caused when the Sun-facing side of an asteroid is warmed and then, after it has rotated, emits thermal photons in a different direction. Farnocchia says this force is about the same as that exerted on a plate by the weight of three grapes. “We improved our knowledge of Bennu’s trajectory by a factor of 20,” he says.
With this better fix on Bennu, which the team reports today in Icarus, researchers were able to eliminate almost all the keyholes as likely places Bennu would pass through in 2135; two still remain as possibilities. With that knowledge, they were able to mark the calendar for that September day in 2182 as the greatest risk to Earth from Bennu. “In 2135, we’ll know for sure,” Farnocchia says, as Bennu will be close enough to track with ground radar and map out its future path.
The new NASA “results are definitely significant, as the Yarkovsky effect … is often the largest remaining source of uncertainty on the orbit prediction and whether [a near-Earth object] will have any close encounters with the Earth in the future,” says astronomer Tim Lister of Las Cumbres Observatory. “This, in turn, allows the predictions of the future path of Bennu during its future close encounters with the Earth to be much more accurate, shrinking the uncertainty on the miss distance in the 2135 Earth encounter considerably.”
And if the news is bad in 2135? Johnson says “multiple kinetic impactors” could divert Bennu away from a collision. “That’s feasible in a 50-year timeframe,” he says. “This data set [from OSIRIS-REx] will be enormously valuable in assessing deflection technologies.” NASA’s planetary defense office is also studying other options, such as gravity tractors and ion beams, he says. “100 years from now, who knows what the technology will be.” That’s some comfort, at least.