In a study released in the journal Icarus today, NASA and University of Arizona researchers used precision-tracking data from the OSIRIS-REx spacecraft to better understand movements of the potentially hazardous asteroid Bennu. The findings confirm that Bennu has a significantly decreased risk of impacting Earth late in the next century, and improve our ability to predict orbits of many other asteroids.

Using NASA's Deep Space Network and a state-of-the-art computer model, scientists pinpointed Bennu's future orbits with unprecedented accuracy, effectively reducing the asteroid's orbit uncertainties to determine the total impact probability through the year 2300.

In 2135, asteroid Bennu will make a very close approach with Earth. Although the near-Earth object will not hit 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 its risk of Earth impact in the future.

Scientists were able to significantly shrink uncertainties about Bennu's orbit, determining its 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, with an impact probability of 1 in 2,700, or about 0.037%.

"The orbital data from this mission helped us better appreciate Bennu's impact risk over the next couple of centuries and our overall understanding of potentially hazardous asteroids – an incredible result," said Dante Lauretta, OSIRIS-REx principal investigator and a professor in the Lunar and Planetary Laboratory at the University of Arizona.

Although the chances of hitting Earth are low, Bennu is still one of the two most hazardous known objects in the solar system, along with another asteroid called 1950 DA.

"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," said Kelly Fast, program manager for NASA's Near-Earth Object Observations Program. "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."

Before leaving Bennu on May 10, 2021, OSIRIS-REx spent over two years in close proximity to the asteroid, gathering information about its size – about a third of a mile, or 500 meters, wide – as well as its shape, mass, and composition, spin and orbital trajectory. The spacecraft is on its way back to Earth with a bounty of dust, pebbles and small rocks collected from the asteroid's surface. OSIRIS-REx will deliver the sample to Earth on Sept. 24, 2023, for further scientific investigation.

"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 study lead author Davide Farnocchia of the Center for Near Earth Object Studies, which is managed by NASA's Jet Propulsion Laboratory in Southern California. "We've never modeled an asteroid's trajectory to this precision before."

Gravitational Keyholes

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 close approach in the year 2135. These keyholes are areas in space that if passed through by Bennu at a certain time and location would set the asteroid on a path where Earth's gravitational pull could cause a potential future impact.

To calculate exactly where the asteroid will be during its 2135 close approach – and whether it might pass through a gravitational keyhole – Farnocchia 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 the side facing it. As the asteroid spins, the heated side will cool down as it rotates away. 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 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," said Steve Chesley, senior research scientist at JPL and study co-investigator. "The effect is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennu's future impact risks over the decades and centuries to come."

The team accounted for many other perturbing forces as well, including the gravity of the planets, their moons and over 100 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, or TAG, sample collection event on Oct. 20, 2020, to see how it might have slightly altered Bennu's orbit.

"The force exerted on Bennu's surface during the TAG event was tiny even in comparison to the effects of other small forces considered," said Rich Burns, OSIRIS-REx project manager at NASA Goddard Space Flight Center. "TAG did not alter Bennu's likelihood of impacting Earth in any meaningful way."

Tiny Risk, Huge Gain

Although a 0.057% impact probability through the year 2300 and an impact probability of 0.037% in 2182 are exceedingly low, the study highlights the crucial role that OSIRIS-REx operations played in precisely characterizing Bennu's orbit.

"The spacecraft is now returning home, carrying a precious sample from this fascinating ancient object that will help us better understand not only the history of the solar system but also the role of sunlight in altering Bennu's orbit since we will measure the asteroid thermal properties at unprecedented scales in laboratories on Earth," UArizona's Lauretta said.

The University of Arizona leads the science team and the mission's science observation planning and data processing. NASA's Goddard Space Flight Center in Greenbelt, Maryland, provides overall mission management, systems engineering and the safety and mission assurance for OSIRIS-REx. Lockheed Martin Space Systems in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx, which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer, is the third mission in NASA's New Frontiers Program. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the agency's New Frontiers Program for the Science Mission Directorate in Washington, D.C.