Bennu, already considered the second most dangerous asteroid in the Solar System, has a slightly increased chance of hitting Earth in the coming centuries, NASA said this week.
There is, according to the latest calculations, a 1-in-2,700 chance (0.037 per cent) that the half-kilometre-wide space rock will collide with Earth on September 24, 2182, and a 1-in-1,750 (0.057 per cent) chance of a strike by 2300 – a tiny bit higher than previous estimates.
The spinning top-shaped space rock is one of the two most hazardous known asteroids in the Solar System – hazardous to Earth, that is – with the other being 1950 DA, which has an estimated 0.3 per cent chance of hitting our planet on March 16, 2880. Astronomers are much more familiar with Bennu, however, thanks to NASA’s OSIRIS-REx mission during which a spacecraft orbited the asteroid for years.
The probe briefly touched down on the surface to obtain a sample of Bennu's regolith. The craft is heading back to Earth, where it is expected to arrive on September 24, 2023. Data already collected by that mission was used to recalculate Bennu's path and thus its chance of whacking into Earth.
“The OSIRIS-REx data give us so much more precise information, we can test the limits of our models and calculate the future trajectory of Bennu to a very high degree of certainty through 2135,” said Davide Farnocchia, of the Center for Near-Earth Object Studies (CNEOS), which is run by NASA's Jet Propulsion Laboratory (JPL). “We’ve never modeled an asteroid’s trajectory to this precision before.”
OSIRIS-REx has provided scientists with the most accurate data yet on Bennu’s size, shape, mass, spin, chemical composition, and orbital motion. There are multiple factors to consider when trying to map its path as it travels through the Solar System. The team had to model the gravitational interactions between the asteroid and the Sun, and other planets and their satellites, as well as more than 300 other space rocks, and pressure from the solar wind.
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Although the researchers had a good idea of the asteroid’s path, it can be nudged by something called the Yarkovsky effect. As Bennu spins, the side of its body facing the Sun absorbs heat and rotates away. The heat is then radiated back into space as it cools down, releasing infrared energy that generates a small push on the asteroid. The effects are minuscule though can adjust Bennu’s motion over long-enough periods of time. Yarkovsky effect measurements from the OSIRIS-REx probe were thus used to fine-tune Bennu's trajectory.
NASA has a good explanation of the Yarkovsky effect and its impact on Bennu traveling through gravitational keyholes in the video below.
“The Yarkovsky effect will act on all asteroids of all sizes, and while it has been measured for a small fraction of the asteroid population from afar, OSIRIS-REx gave us the first opportunity to measure it in detail as Bennu traveled around the Sun,” explained Steve Chesley, co-author of the paper and a senior research scientist at JPL.
“The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennu’s future impact chances over the decades and centuries to come.”
Also, NASA did not significantly alter Bennu’s motion when its probe ejected a puff of nitrogen gas to kick up dirt to collect and return to Earth.
“The force exerted on Bennu’s surface during the Touch-and-Go (TAG) event were tiny even in comparison to the effects of other small forces considered,” Rich Burns, the project manager of the OSIRIS-REx mission, who was not directly involved in the research, said. “TAG did not alter Bennu’s likelihood of impacting Earth.” ®