In a study released Aug. 11, NASA researchers used precision-tracking data from the agency’s OSIRIS-REx spacecraft to better understand movements of the potentially hazardous asteroid Bennu through the year 2300, significantly reducing uncertainties related to its future orbit and improving scientists’ ability to determine the total impact probability and predict orbits of other asteroids.
“NASA’s Planetary Defense mission is to find and monitor asteroids and comets that can come near Earth and may pose a hazard to our planet,” Kelly Fast, program manager for the Near-Earth Object Observations Program at NASA headquarters, said. “We carry out this endeavor through continuing astronomical surveys that collect data to discover previously unknown objects and refine our orbital models for them. The OSIRIS-REx mission has provided an extraordinary opportunity to refine and test these models, helping us better predict where Bennu will be when it makes its close approach to Earth more than a century from now.”
In 2135, Bennu will make a close approach with Earth. Although the near-Earth object will not pose a danger to the planet at that time, scientists must understand Bennu’s exact trajectory during that encounter in order to predict how Earth’s gravity will alter the asteroid’s path around the Sun – and affect the hazard of Earth impact.
Using NASA’s Deep Space Network and state-of-the-art computer models, scientists were able to significantly shrink uncertainties in Bennu’s orbit, determining its total impact probability through the year 2300 is about 1 in 1,750 (or 0.057%). The researchers were also able to identify Sept. 24, 2182, as the most significant single date in terms of a potential impact, with an impact probability of 1 in 2,700 (or about 0.037%).
Although the chances of it hitting Earth are very low, Bennu remains one of the two most hazardous known asteroids in the solar system, along with another asteroid called 1950 DA.
Before leaving Bennu May 10, OSIRIS-REx – short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer – spent more than two years in close proximity to the asteroid, gathering information about its size (it is about one-third of a mile wide), shape, mass, and composition, while monitoring its spin and orbital trajectory. The spacecraft also scooped up a sample of rock and dust from the asteroid’s surface, which it will deliver to Earth on Sept. 24, 2023, for further scientific investigation.
The precision measurements on Bennu help to better determine how the asteroid’s orbit will evolve over time and whether it will pass through a “gravitational keyhole” during its 2135 close approach. These 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.
To calculate exactly where the asteroid will be during its 2135 close approach – and whether it might pass through a gravitational keyhole – study lead Davide Farnocchia of the Center for Near Earth Object Studies and his team evaluated various types of small forces that may affect the asteroid as it orbits the Sun. Even the smallest force can significantly deflect its orbital path over time, causing it to pass through or completely miss a keyhole.
Among those forces, the Sun’s heat plays a crucial role. As an asteroid travels around the Sun, sunlight heats up its dayside. Because the asteroid spins, the heated surface will rotate away and cool down when it enters the nightside. As it cools, the surface releases infrared energy, which generates a small amount of thrust on the asteroid – a phenomenon called the Yarkovsky effect. Over short timeframes, this thrust is minuscule, but over long periods, the effect on the asteroid’s position builds up and can play a significant role in changing an asteroid’s path.
The team considered many other perturbing forces as well, including the gravity of the Sun, the planets, their moons, and more than 300 other asteroids, the drag caused by interplanetary dust, the pressure of the solar wind, and Bennu’s particle-ejection events. The researchers even evaluated the force OSIRIS-REx exerted when performing its Touch-And-Go sample collection event Oct. 20, 2020, to see if it might have slightly altered Bennu’s orbit, ultimately confirming previous estimates that the event had a negligible effect.
NASA’s Goddard Space Flight Center provides overall mission management for OSIRIS-REx, which is the third mission in NASA’s New Frontiers Program. Marshall Space Flight Center manages New Frontiers for the agency’s Science Mission Directorate.