Lead image: This mosaic of Bennu was created using observations made by NASA’s OSIRIS-REx spacecraft that was in close proximity to the asteroid for over two years. Credit: NASA/Goddard/University of Arizona
In a study released on August 11, 2021, NASA researchers used precision-tracking data from the agency’s Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (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.
The study, titled “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” was published in the journal Icarus.
“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 the Near-Earth Object Observations Program at NASA Headquarters in Washington. “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, asteroid Bennu will make a close approach with Earth. Although the near-Earth object will not pose a danger to our 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 September 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 our solar system, along with another asteroid called 1950 DA.
Before leaving Bennu May 10, 2021, OSIRIS-REx spent more than two years in close proximity to the asteroid, gathering information about its size (it is about one-third of a mile, or 500 meters, 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 September 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 Davide Farnocchia, of the Center for Near Earth Object Studies (CNEOS), 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 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 – 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 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 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 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.”
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 (TAG) sample collection event on October 20, 2020, to see if it might have slightly altered Bennu’s orbit, ultimately confirming previous estimates that the TAG event had a negligible effect.
“The force exerted on Bennu’s surface during the TAG event were tiny even in comparison to the effects of other small forces considered,” said Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “TAG did not alter Bennu’s likelihood of impacting Earth.”
Tiny risk, huge gain
Although a 0.057% impact probability through the year 2300 and an impact probability of 0.037% on September 24, 2182, are low, this study highlights the crucial role that OSIRIS-REx operations played in precisely characterizing Bennu’s orbit.
“The orbital data from this mission helped us better appreciate Bennu’s impact chances 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 professor at the University of Arizona. “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’s thermal properties at unprecedented scales in laboratories on Earth.”
More about OSIRIS-REx
Goddard provides overall mission management, systems engineering and the safety and mission assurance for OSIRIS-REx. Lauretta is the principal investigator, and the University of Arizona also leads the science team and the mission’s science observation planning and data processing. Lockheed Martin Space Systems in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace in Tempe, Arizona are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx 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 agency’s Science Mission Directorate in Washington.
Precision counts
OSIRIS-REx launched in September 2016 and slipped into orbit around the asteroid Bennu in December 2018.
After its arrival, the spacecraft spent nearly two and a half years studying the space rock from orbit, swooping in, hovering overhead and inspecting the rock in every way possible.
“The trajectory of the spacecraft was really amazing — I compare it to a hummingbird,” Dante Lauretta, a planetary scientist at the University of Arizona, the principal investigator for OSIRIS-REx and an author on the new research, said during the news conference. “We were able to go into orbit, we were able to leave orbit, we were able to get into a variety of imaging angles.”
Throughout that time, the spacecraft constantly logged its location with respect to Earth and to the asteroid. Using that data, the team behind the new research were able to fine-tune their understanding of the space rock’s location and movement for nearly two and a half years.
The spacecraft’s visit to the asteroid culminated in October 2020, when OSIRIS-REx snatched a sample of Bennu to stash away. In May, the spacecraft and its precious cargo bid farewell to the asteroid and turned back for Earth, where OSIRIS-REx will deposit the sample capsule in September 2023. The delivery will give scientists the rare opportunity to study asteroid material using all the sophisticated equipment terrestrial laboratories have to offer.
But scientists don’t need to get their hands on that sample material to dig into the spacecraft’s two and a half years of observations. Add in astronomers’ work observing Bennu from the ground before OSIRIS-REx was even an idea and scientists have about 20 years of data tracking a single space rock.
Scientists’ models of asteroid trajectories always include some degree of uncertainty, since a multitude of forces tug at a space rock as it rattles around the inner solar system. That uncertainty remains even in the new and improved models of Bennu’s future path, but with the OSIRIS-REx observations, scientists were able to dramatically reduce their uncertainty about Bennu’s fate.
Among other factors, the researchers were able to account for the tiny orbital changes that forces like radiation from the sun, the impact of relativity and the gravity of hundreds of other relatively large asteroids whizzing through the neighborhood.
In addition to factors like these that affect all solar system objects (albeit to a tiny degree), the team was also able to check the impact of two unusual characteristics of Bennu specifically: the plumes of dust that regularly shoot off the asteroid and the rock’s interaction with the spacecraft itself.
One factor that scientists were particularly concerned about is called the Yarkovsky effect, which is triggered by the constant temperature fluctuations that occur as regions of the asteroid pass in and out of daylight, gently pushing the asteroid.
“The Yarkovsky effect acting on Bennu is equivalent to the weight of three grapes,” Farnocchia said. “That’s what’s really driving the motion of Bennu into the future, because this acceleration is persistent, its effect builds up over time, and it becomes very significant by the time you get to 2135.”
However, despite the scientists’ best efforts, predicting Bennu’s course after 2135 is still tricky. In September of that year, Bennu will swing past Earth — not close enough that there’s any risk of an impact, but certainly close enough that Earth’s gravity could nudge the asteroid a bit on its path. How precisely that dance plays out will shape Bennu’s trajectory over the next decades and centuries.
Beyond Bennu
Of course, Bennu isn’t the only space rock that scientists worry about. Planetary defense is dedicated to identifying all asteroids that could potentially impact Earth in a meaningful way. One aspect of that work is finding as many near-Earth asteroids as possible — scientists have cataloged more than 26,000 to date, according to NASA.
But to accurately identify which asteroids might actually pose a risk, scientists need to be able to calculate as accurately as possible where a space rock’s trajectory will take it. That trajectory is based on existing observations of where an asteroid has been, of course, but scientists have known for years that factors like the Yarkovsky effect may be muddling their calculations.
The new data from OSIRIS-REx quantifying that three-grape-heavy Yarkovsky effect should help scientists understand how the same effect shapes the trajectories of other asteroids as well, according to the researchers.
“We have been able for the first time to test our models on an asteroid trajectory to levels really never tried before,” Farnocchia said. “The results are valid in general for every other asteroid, we can apply these models and be confident that they’re extremely accurate.”
The research is described in a paper published Tuesday (Aug. 10) in the journal Icarus.
Reference: “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data” by Davide Farnocchia, Steven R. Chesley, Yu Takahashi, Benjamin Rozitis, David Vokrouhlický, Brian P. Rush, Nickolaos Mastrodemos, Brian M. Kennedy, Ryan S. Park, Julie Bellerose, Daniel P. Lubey, Dianna Velez, Alex B. Davis, Joshua P. Emery, Jason M. Leonard, Jeroen Geeraert, Peter G. Antreasian and Dante S. Lauretta, 10 August 2021, Icarus.
DOI: 10.1016/j.icarus.2021.114594
