The asteroid Bennu is one of the most “threatening” in our solar system. Thanks to a visit from a NASA spacecraft, scientists now have a much greater understanding of the asteroid’s next moves closer to Earth – and whether it could impact our planet.
In more than two years orbiting Bennu, NASA’s OSIRIS-REx mission was able to gather unprecedented information as well as a sample that is returning to Earth. The sample will arrive in September 2023.
The data collected by the “Regolito, Origins, Spectral Interpretation, Resource Identification and Security Explorer” – a free translation for the acronym in English that gives the mission its name – allowed the precise tracking of the asteroid’s movements up to 2300 – discoveries that reduce the uncertainties scientists had about the asteroid’s future orbit.
The asteroid has 1 chance in 1,750 to impact Earth up to 2,300.
Bennu will make its next approach to Earth in 2135. Although the asteroid doesn’t come close enough to pose a threat to Earth, knowing its exact trajectory can help scientists better understand how our planet’s gravity will change its future orbit around the sun . This could also affect the chances of Bennu impacting Earth after 2135.
It is further known that the celestial body will make its most significant closing approach on September 24, 2182, with a 1 in 2700 chance of impacting Earth that day. The researchers agree that the risk of Bennu impacting Earth is low, and NASA will continue to observe the asteroid’s orbit for years to come, says a study based on the findings and published in the journal Icarus.
“NASA’s planetary defense mission is to find and monitor asteroids and comets that could approach Earth and pose a danger to our planet,” said Kelly Fast, manager of the near-Earth object observation program at Washington headquarters, in a demo.
“We carry out this effort through ongoing astronomical surveys that collect data to discover previously unknown objects and refine our orbital models for them. The OSIRIS-REx mission provided an extraordinary opportunity to refine and test these models, helping us better predict where Bennu it will be when it approaches Earth more than a century from now.”
Studying an asteroid up close
The OSIRIS-REx mission arrived in Bennu in December 2018 and departed in May this year, loaded with the sample it collected from the asteroid’s surface. Even though the spacecraft is a few years away from returning to Earth, it has been sending data that reveal what it has learned about Bennu all along. This allows scientists to learn that it is a pedestrian-shaped asteroid made up of rocks held together by gravity, about 500 meters wide.
“The OSIRIS-REx data gives us much more accurate information, we can test the limits of our models and calculate Bennu’s future trajectory with a high degree of certainty to 2135,” said lead author of the study, Davide Farnocchia, scientist at Near Earth Object Studies Center from NASA, in a statement. “We’ve never modeled an asteroid’s trajectory with this precision before.”
The Center, based at the Jet Propulsion Laboratory in Pasadena, Calif., can use data from asteroids to calculate their trajectories, predict their future movement and assess whether or not an impact is likely.
Asteroids’ orbits around the Sun change over time and the smallest things can change them.
Specifically, the researchers wanted to determine whether Bennu would experience a “gravitational keyhole” during its first approach to Earth in 2135. A “keyhole” is a small region of space where a planet’s gravity can alter an asteroid’s orbit that passes.
If Bennu passes one of them at any given time due to Earth’s gravity, it could put him on course for a future impact on our planet. “Earth’s gravity would adjust the motion just the right amount to put it on a collision course at a later date in the 22nd century,” said Farnocchia.
Prior to this study, scientists were concerned that Bennu might have 26 potential gravitational keyholes to pass. Now, they’re only concerned with two of them.
“But we must keep in mind that the probability of impact, in general, is very small,” said Farnocchia. “In fact, there is a 99.94% probability that there is no impact trajectory. So there is no specific cause for concern.”
Bennu on the move
Heat from the sun can cause something called the Yarkovsky effect on asteroids. As they orbit the Sun, they repeatedly heat and cool as they rotate. As the asteroid rotates during this temperature change, it releases energy and the asteroid, in turn, receives a small boost.
“The Yarkovsky effect will work on all asteroids of all sizes and although it was measured for a small fraction of the asteroid population from afar, the OSIRIS-REx gave us the first opportunity to measure it in detail as Bennu traveled around of the Sun,” study co-author Steve Chesley, a senior research scientist at NASA’s Jet Propulsion Laboratory, said in a statement.
“The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant in determining the chances of Bennu’s future impact in the decades and centuries to come.”
Other forces can alter the motion of asteroids, including the gravity of the sun, other planets, moons, and other asteroids. Dust in the solar system and the sun’s charged stream of particles, called the solar wind, can also affect Bennu’s orbit. And during the OSIRIS-REx mission, the team was surprised to see Bennu’s surface ejecting particles into space, which could also play a role.
Although the objective of the mission was to collect a sample of Bennu and return it to Earth, the insight gained from near-Earth asteroids and obtaining better calculations of their orbits are crucial.
“The orbital data from this mission helped us better assess the chances of Bennu’s impact over the next two centuries and our overall understanding of potentially dangerous asteroids – an incredible result,” said Dante Lauretta, principal investigator at OSIRIS-REx and professor at the University of Arizona, in a statement.