Concept art of Bennu approaching Earth. Image: dzika_mrowka via Getty Images
The odds that one of the most hazardous known asteroids might collide with Earth in the coming centuries just went up, though they are still extremely low. There is now a 0.057 percent chance of an impact before 2300, according to a new study that was discussed in a NASA teleconference on Wednesday. Scientists have been tracking asteroid 101955 Bennu ever since it was discovered in 1999, in part because its orbit occasionally brings it close to Earth, raising the risk of a future crash. At about 500 meters (1,600 feet) wide, the asteroid could wipe out a city if it impacted a populated area of our planet, though it would not wreak the kind of planetary damage caused by the Chicxulub impactor, which spanned at least 10 kilometers (6 miles) and ended the age of the dinosaurs.
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To better understand the ominous Bennu, NASA sent a spacecraft called the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) to snatch some samples from its surface. OSIRIS REx is now headed back to Earth with a treasure trove of rocky material from the asteroid, along with precise new orbital observations, that have enabled scientists to compile an updated “hazard assessment” of Bennu, which was published on Wednesday in the journal Icarus. Whereas previous assessments had placed the chances of Bennu impacting Earth in the 22nd century at about 1 in 2,700, the new study places the risk of a collision at about 1 in 1,750 during the 2100s. The most dangerous single date in this updated timeframe is September 24, 2182, when Bennu has a 1 in 2,700 chance of hitting Earth. (It’s a coincidence that the previous estimate of a 22nd century impact and the new estimate of an impact in 2182 worked out to the same 1 in 2,700 figure). “Now, we have a much better knowledge of what the pathways of impact can be, and the most significant one would lead to an impact in 2182,” said lead author Davide Farnocchia, a navigation engineer at NASA’s Center for Near Earth Object Studies (CNEOS), at the teleconference.
“But we should keep in mind that the impact probability overall is really small,” he added. “In fact, the overall impact probability is 0.06 percent, which, put another way, means that there is a 99.4 percent probability that Bennu is not on an impact trajectory. So, there is no particular reason for concern.”Moreover, Farnocchia noted that observations from OSIRIS REx have tightly constrained Bennu’s possible trajectories until 2135, when the asteroid will pass extremely close to Earth. Though an impact won’t occur at this time, Bennu will travel about twice as close to our planet as the Moon, which means it could pass through what’s known as a “gravitational keyhole” that would place it on a collision course with Earth in the 2100s. These keyholes are small stretches of space around Earth where the gravitational force of our planet can nudge asteroids into more dangerous trajectories.OSIRIS REx has captured a range of observations during its voyage that the team used to “model the trajectory of Bennu to a level that was never tried before for any asteroid,” Farnocchia said. Whereas scientists had previously identified 26 keyholes measuring over one kilometer (3,280 feet) that could send Bennu onto a collision course with Earth, the new study narrows the risk down to only two keyholes. “By tracking the spacecraft over two years of proximity operations, we obtained fantastic positional constraints on the position of Bennu,” Farnocchia said. “We measured the distance between the Earth and Bennu, which at times was as large as the distance between the Earth and the Sun, with a precision of two meters. That's the height of a basketball player. That's how well we can constrain the position of Bennu.” Though the odds of Bennu hitting these keyholes are low, the new study still distinguishes the asteroid as one of the most dangerous objects in space. Its only rival in this category is asteroid 1950 DA, which is about twice Bennu’s size and has about a 0.17 chance of hitting Earth in the year 2880.“The OSIRIS REx mission has provided exquisitely precise data on Bennu’s position and motion through space to a level never captured before on any asteroid,” said Lindley Johnson, NASA’s Planetary Defense Officer, during the teleconference. “Not only does this allow us to better understand Bennu’s future path and if it could ever be an impact threat to Earth, but it also allows us to test and improve the overall modeling techniques that we can use on all near-Earth asteroids.”