What are we going to do on Bennu? Why invest so much energy and money to land a probe on an asteroid that is currently over 120 million km away from us?
A mission, among other things, apparently completely analogous to the one that Hayabusa2 is carrying out on Ryugu, another asteroid from which samples will be taken to bring back to Earth? Let's put it this way: landing on Bennu will be an excellent investment for our great-grandchildren.
Discovered in 1999, due to its trajectory and size, Bennu is in fact a potentially dangerous asteroid for our planet. Let's start with its size: half a kilometer. More than enough, in the unfortunate event of an impact, to cross the Earth's atmosphere while maintaining a threatening mass to the point of devastating a large region of our planet.
And how long is this unfortunate circumstance? In the most at-risk era of the near future, between 2175 and 2196, orbital calculations estimate a probability of impact of 1 in 2700. In other words, there is a 99,963 percent chance that the asteroid will not hit us. Is this a reassuring percentage? Or is it good to worry? The question scientists are asking is another: how can we make the calculation of Bennu's future trajectories more precise and reliable?
The answer is far from simple. Although every time the asteroid passes “close” to Earth – and it happens more or less every six years – an array of optical, infrared and radio telescopes tracks and photographs it far and wide to reconstruct its trajectory and behavior as best as possible, and although the orbital models fed to computers take into account every possible gravitational interaction (with the Sun, the Moon, the planets and other asteroids), uncertainty continues to remain high.
A bit like with weather, the problem is with long-term forecasts. If we know Bennu's current position with a margin of error of a few kilometers, and even in the medium term the error window - 30 km for the 2060 passage - is more than acceptable, trying to calculate its position more than half a century from now, the uncertainty increases dramatically: 14 thousand km in 2080, 160 thousand km (almost half the distance between the Earth and the Moon) in 2135.
"Bennu is the asteroid with the best orbital information in our database right now. And yet, if we look beyond 2135, we have no idea where it's headed," admits Steven Chesley , a researcher at NASA's Center for Near-Earth Object Studies and a member of the Osiris-Rex team tasked with predicting Bennu's future trajectory.
There are more than one reason, but the contribution to the uncertainty of the individual factors that make long-term forecasts so difficult is actually quite limited. Except for one: the Yarkovsky effect .
Described for the first time by the Polish engineer Ivan Osipovich Yarkovsky in 1901, it is an effect due to the action of sunlight on the asteroid: by heating the illuminated side, it causes the absorbed heat to be subsequently radiated. And it is this irradiation that alters its trajectory in a way that is not easy to predict, based on the direction and speed of rotation of the asteroid itself. In the case of Bennu, we are talking about an alteration of the orbit – starting from 1999, the year of its discovery – of about 284 meters per year in the direction of the Sun.
An apparently small alteration, but one that propagates over time. And, above all, much more unpredictable than that due to gravitational forces. This is because it depends on multiple factors - such as the exact morphology and composition of the asteroid's surface - too complex to develop adequate models. Scientists hope that the in-depth knowledge that Osiris-Rex's in situ analyses will allow - and subsequently also those here on Earth, thanks to the samples collected - will allow us to improve the predictions of Bennu's trajectory by 60 times.
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