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Asteroid Bennu from the perspective of the NASA probe "Osiris Rex"
Photo: AFP / NASA / Goddard / University of Arizona
When NASA sent its probe "Osiris Rex" to the asteroid Bennu on a multi-year mission in 2016, it did so for good reason: The observations and calculations of the chunk's trajectory had shown that it would cross the earth's orbit again and again and would cross us in around 150 years could get pretty close.
The lump, with an average diameter of around 500 meters and a mass of more than 60 million tons, is one of the so-called near-Earth asteroids.
In order to counter the danger of an impact, Nasa has mapped Bennu and collected numerous data: The previously available researchers led by Davide Farnocchia from NASA's Jet Propulsion Laboratory have now evaluated and calculated the probability of an impact more precisely than before. According to the study published in the journal »Icarus«, the risk is 1: 1750 that Bennu could collide with the earth by 2300. Earlier calculations were somewhat more reassuring and came to a probability of 1: 2700 for an impact up to 2200.
The decisive factor for the impact risk is the influence of the earth on a flyby of Bennu in the year 2135. Then he will be closer to us than the moon and about half the distance from our satellite, which is around 380,000 kilometers away.
For the trajectory of future passages, the decisive factor is how close Bennu is to the earth at this moment - and whether the gravity of the much more massive earth pulls on the comparatively tiny chunk and deflects it.
To do this, Bennu would have to pass certain tiny areas near our planet, astronomers speak of so-called gravitational keyholes.
Bennu 2135 avoids 24 of these 26 keyholes that are larger than a kilometer.
Nevertheless, Bennu could collide with earth at some point later with a probability of 1: 1750 or 0.057 percent because his course is deflected accordingly. The researchers even calculated a possible impact date. It is September 24, 2182. The probability that this event will occur is 1 in 2700 (0.037 percent).
However, based on the new study, there is no need to panic. Astronomer Farnocchia does not speak of any significant change to the New York Times. He is no more worried than before, as the likelihood of impact remains very low, it is said. However, the researchers cannot rule out an impact by 2300. On the Palermo scale, which shows how high the risk of an impact is, Bennu is at the top, together with the asteroid (29075) 1950 DA, which also crosses the orbit of Mars.
For Bennu, who is named after the ancient Egyptian god of the dead and was discovered in 1999 by the Arecibo telescope in Puerto Rico, which has since collapsed, not all data are available for a better collision estimate. For example, "Osiris Rex" is currently still taking the soil samples and will probably not reach Earth until 2023. The results of Bennu's surface quality are quite relevant. The so-called Jarkowsky effect, named after a Russian engineer, plays a role in the changes in trajectories.
The different degrees of warming of the asteroid surfaces by the sun have an influence on the deflection of the asteroid's movement. Sunlight is first absorbed by an asteroid or a meteorite and then released back into space as infrared radiation with a slight delay. But because the rotation of the body warms up its sides to different degrees, there is a difference in radiation that Bennu can easily deflect inwards or outwards. How strong the effect is depends, for example, on the reflectivity and thus on the surface properties of the celestial body. That is why one of the goals of "Osiris Rex" is to determine Bennu's Jarkowsky effect as precisely as possible.This effect has an influence on many asteroid trajectories and probably directs large chunks from the asteroid belt into the interior of the solar system more frequently than is the case with collisions with other clumps.
It is quite possible that Bennus' trajectory will be modeled more precisely in the future. According to estimates by researchers, other of these remnants from the early phase of our solar system have not yet been discovered anyway - or are already flying past Earth to within a few thousand kilometers. The asteroid »2020 QG«, about the size of a small car, passed the southern Indian Ocean in August 2020 at an altitude of almost 3,000 kilometers.
Sometimes collisions also occur: a few years ago the explosion of a 20-meter chunk over the Russian metropolis of Chelyabinsk caused devastation. Hundreds of people were injured by the meteor shower from the wreckage of the estimated 16,000 ton celestial body, most of them from shattering windows. Researchers estimate that asteroids larger than 500 meters on Earth cause supraregional damage when impacted. The size of the crater in such events is usually ten to 20 times the size of the object. So Bennu would not threaten the existence of the earth, but he could cause massive damage and claim many victims.
In any case, experts have long been working on techniques to keep dangerous, near-Earth asteroids away from Earth. China calculated, for example, that the use of 23 missiles of the type "Long March 5" could dissuade Bennu from terrestrial course. So far, most experts have assumed that the use of such a so-called kinetic impactor, which shoots the approaching boulder off the track like a billiard ball, is the only chance for the earth in an emergency.