A common body in our solar system is the asteroid. We have a lot of them. Thousands of them, in fact. And recent missions have been focused on understanding these tiny worlds, in part because some of them have a chance of hitting Earth. Unlike the dinosaurs, we have scientists working on this problem, which is good for us. That’s really what NASA’s OSIRIS-REx mission is about — understanding just how the near-Earth asteroid Bennu moves in its orbit, what it is composed of, how it is structured, etc. All of these characteristics are tied together to determine what kind of threat Bennu poses for Earth.
At a press conference this week, NASA announced that after analyzing Bennu’s orbit with the Deep Space Network and some advanced computer modeling, they have calculated the change of an impact between now and the year 2300 as being about 1 in 1,750 or 0.057%. The single most significant date in that time period is September 24, 2182, when the impact probability increases to a whopping 0.037% or a 1 in 2,700 chance. Yeah, not the big threat a few dozen early articles made Bennu out to be.
This doesn’t mean that Bennu isn’t a hazard. It still is. NASA referred to it in their press release as “one of the two most hazardous known asteroids in our solar system.” The other asteroid is 1950 DA, whose current risk assessment is between 0 and 0.33% probability of causing an impact with Earth, and that threat is greatest in the year 2880. Yes, you heard that correctly — 2880. I’m not going to lose sleep over either of these asteroids.
The fascinating part of this new calculation is just how detailed the measurements and factors considered were. The team considered how the Sun heats up the asteroid’s dayside, which then affects the spin of the asteroid as the heated surface rotates away and cools down. Infrared energy gets released during the cooling process, and that energy actually generates a small amount of thrust for the asteroid, called the Yarkovsky effect. Now, on a day-to-day basis, this effect is negligible, but over the span of centuries, the change in orbit can add up.
So how much of an effect does this amount to for Bennu? Steve Chesley, a senior research scientist at JPL and study co-investigator says: 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.
Three grapes. I’m really not feeling threatened here, everyone.
Oh, but wait. Didn’t we sort of smack the asteroid while we were sampling it last year? It was supposed to be a gentle boop but instead turned out to be a half-a-meter plunge into the asteroid. How much of an effect did that event have on the orbit of Bennu? Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center answers: The force exerted on Bennu’s surface during the TAG event [was] tiny even in comparison to the effects of other small forces considered. TAG did not alter Bennu’s likelihood of impacting Earth.
Good. We didn’t make things worse for us. And at a 0.057% probability, I’m still not feeling concerned about Bennu. Overall, this result is an impressive use of data collected by the OSIRIS-REx mission. As principal investigator Dante Lauretta explains: 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.
Now we wait for the spacecraft to return to Earth and bring us that precious sample it collected, which we hope helps us further understand not only the composition of Bennu itself but also the history of our solar system. Go go, little spacecraft!
NASA press release
“Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” Davide Farnocchia et al., 2021 August 10, Icarus