Scientists fine-tune odds of asteroid Bennu hitting Earth

If the possibility of an asteroid called Bennu slamming into Earth a lifetime from now was keeping you up at night, NASA scientists think you can rest a little easier.

The agency's OSIRIS-REx spacecraft spent more than two years closely orbiting the space rock. And with that incredibly detailed view of the asteroid, experts studying  potential space rock impacts with Earth have been able to fine-tune their existing models of Bennu's future.

As a result, scientists behind new research now say they're confident that the asteroid's total impact probability through 2300 is just 1 in 1,750. Estimates produced before OSIRIS-REx arrived at the space rock tallied the cumulative probability of a Bennu impact between the years 2175 and 2199 at 1 in 2,700, according to NASA. While a slightly higher risk than past estimates, it represents a minuscule change in an already minuscule risk, NASA said.

Related: NASA spacecraft leaves asteroid Bennu to bring pieces to Earth
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Technically, that's a small increase in risk, but the scientists behind the new research say they aren't worried about a potential impact. And besides, the lessons the research offers for asteroid trajectory calculation could reduce concerns about potential impacts by other asteroids more than enough to compensate.

"The impact probability went up just a little bit but it's not a significant change, the impact probability is pretty much the same," lead author Davide Farnocchia, who works at NASA's Center for Near Earth Object Studies in California, said during a news conference held Wednesday (Aug. 11). "I think that, overall, the situation has improved."

Precision counts 

OSIRIS-REx (formally known as Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) launched in September 2016 and slipped into orbit around the asteroid Bennu in December 2018.

After its arrival, the spacecraft spent nearly two and a half years studying the space rock from orbit, swooping in, hovering overhead and inspecting the rock in every way possible.

"The trajectory of the spacecraft was really amazing — I compare it to a hummingbird," Dante Lauretta, a planetary scientist at the University of Arizona, the principal investigator for OSIRIS-REx and an author on the new research, said during the news conference. "We were able to go into orbit, we were able to leave orbit, we were able to get into a variety of imaging angles."

Throughout that time, the spacecraft constantly logged its location with respect to Earth and to the asteroid. Using that data, the team behind the new research were able to fine-tune their understanding of the space rock's location and movement for nearly two and a half years.

The spacecraft's visit to the asteroid culminated in October 2020, when OSIRIS-REx snatched a sample of Bennu to stash away. In May, the spacecraft and its precious cargo bid farewell to the asteroid and turned back for Earth, where OSIRIS-REx will deposit the sample capsule in September 2023. The delivery will give scientists the rare opportunity to study asteroid material using all the sophisticated equipment terrestrial laboratories have to offer.

But scientists don't need to get their hands on that sample material to dig into the spacecraft's two and a half years of observations. Add in astronomers' work observing Bennu from the ground before OSIRIS-REx was even an idea and scientists have about 20 years of data tracking a single space rock.

Scientists' models of asteroid trajectories always include some degree of uncertainty, since a multitude of forces tug at a space rock as it rattles around the inner solar system. That uncertainty remains even in the new and improved models of Bennu's future path, but with the OSIRIS-REx observations, scientists were able to dramatically reduce their uncertainty about Bennu's fate.

Among other factors, the researchers were able to account for the tiny orbital changes that forces like radiation from the sun, the impact of relativity and the gravity of hundreds of other relatively large asteroids whizzing through the neighborhood.

In addition to factors like these that affect all solar system objects (albeit to a tiny degree), the team was also able to check the impact of two unusual characteristics of Bennu specifically: the plumes of dust that regularly shoot off the asteroid and the rock's interaction with the spacecraft itself.

One factor that scientists were particularly concerned about is called the Yarkovsky effect, which is triggered by the constant temperature fluctuations that occur as regions of the asteroid pass in and out of daylight, gently pushing the asteroid.

"The Yarkovsky effect acting on Bennu is equivalent to the weight of three grapes," Farnocchia said. "That's what's really driving the motion of Bennu into the future, because this acceleration is persistent, its effect builds up over time, and it becomes very significant by the time you get to 2135."

However, despite the scientists' best efforts, predicting Bennu's course after 2135 is still tricky. In September of that year, Bennu will swing past Earth — not close enough that there's any risk of an impact, but certainly close enough that Earth's gravity could nudge the asteroid a bit on its path. How precisely that dance plays out will shape Bennu's trajectory over the next decades and centuries.

With that remaining uncertainty, and other similar events potentially occurring in the future, the scientists now say that Bennu's total impact probability through 2300 is about 1 in 1,750, according to a NASA statement about the new research. Within that time period, the most concerning date is Sept. 24, 2182; even on that day, Bennu's impact probability is only 1 in 2,700.

Beyond Bennu 

Of course, Bennu isn't the only space rock that scientists worry about. Planetary defense is dedicated to identifying all asteroids that could potentially impact Earth in a meaningful way. One aspect of that work is finding as many near-Earth asteroids as possible — scientists have cataloged more than 26,000 to date, according to NASA.

But to accurately identify which asteroids might actually pose a risk, scientists need to be able to calculate as accurately as possible where a space rock's trajectory will take it. That trajectory is based on existing observations of where an asteroid has been, of course, but scientists have known for years that factors like the Yarkovsky effect may be muddling their calculations.

The new data from OSIRIS-REx quantifying that three-grape-heavy Yarkovsky effect should help scientists understand how the same effect shapes the trajectories of other asteroids as well, according to the researchers.

"We have been able for the first time to test our models on an asteroid trajectory to levels really never tried before," Farnocchia said. "The results are valid in general for every other asteroid, we can apply these models and be confident that they're extremely accurate."

The research is described in a paper published Tuesday (Aug. 10) in the journal Icarus.

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.