65 million years ago, a collision between the Earth and an object about 10 kilometers in diameter would have resulted in theCretaceous extinction causing the disappearance of about 75% of all life forms. While many studies on the risk of asteroid impact have been published, few have addressed the risk posed by giant asteroids or long period giant comets (LPC), i.e. greater than 200 years. Those comets, less often cited than asteroids, represent a major threat to humanity. Indeed, for the next 100 years, the probability of impact is higher with long-period comets than for giant asteroids, because none of the hundreds of asteroids identified is on a trajectory close to that of the Earth, whereas for the LPCs we do not know their trajectory. Nevertheless, it is much stronger in the long term, with nearly one in 10 risk in the next billion years.
Jean-Marc Salotti, University Professor at Bordeaux INP, member of theInternational Academy of Astronautics and the Planète Mars association, took an interest in the risk of extinction of humanity by theimpact of a giant objectwhich could be an asteroid or a comet, whose size would be ten times larger than that of the asteroid which caused the end of the reign of the dinosaurs. If an object some 100 kilometers in diameter were to impact the Earth, the planet would become inhospitable, thus causing the extinction of many forms of life, includingspecies human. In his study, Jean-Marc Salotti, assisted by Sean Raymond from the Bordeaux Astrophysics Laboratory, focused mainly on the impact probability of giant asteroids or LPC comets, the warning time, the deviation capacity and the risk of extinction of humanity by impact.
Today, on the scale of centuries, no known giant near-Earth asteroid poses a threat. But, some of them might see their orbit change and therefore become a risk for our Blue Planet and the survival of our species. The possibility of such an event is anything but imaginary. ” The probability of a giant impact is between 0.03 and 0.3 for the next billion years. Although the deadline is very distant and uncertain, we must prepare for it… just in case.
It almost seems impossible to divert a giant asteroid from its collision course »
In the field of planetary defensealthough technological advances now make it possible to imagine certain countermeasures, the “ options considered are hardly conclusive “. As Jean-Marc Salotti’s study shows, it almost seems ” impossible to divert a giant asteroid from its collision course “. Forget the idea, popularized by literature and cinema, of destroying asteroids which ” is not possible with current technologies “. Our only weapon is the small change in orbit so that theoretically a displacement of only 12,000 kilometers (diameter of the Earth) would be enough to avoid the collision “. But nothing is less certain. If theoretically we know how to deflect an asteroid, ” in practice it’s a different story “.
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Surprisingly, being able to deflect asteroids might not be sufficient due to the uncertainty of the orbital parameters of the near-earth objects say NEO (Near-Earth Objects). Despite ” our efforts to determine them with high accuracy, there are significant uncertainties in this prediction “, in particular because ” the prediction of the risk of collision is measured in centuries and the scale of distances is in millions of kilometers “.
Deflect asteroids before they reach Earth
Concretely, if we take as an example the asteroid 2021QM1, which approached the Earth in 1961 at a distance of 255,000 kilometers and should approach again in 2052 with a nominal distance of one million kilometers “, despite efforts to determine its orbital parameters with great precision, there is ” significant uncertainties on this prediction and the impact is possible with a probability close to 10-4 “. In other words, since the same rules of celestial mechanics apply to all objects, ” we would not be able to predict ten years before the event that this asteroid would avoid or impact Earth “. This uncertainty in the knowledge of the orbital parameters has the consequence that whatever the method of deviation used, “ it is not certain that the expected result will be achieved ! Indeed, if we try to deflect a giant object long before the close encounter, and if the deviation is only a displacement of a few thousand kilometers, “ we could just reduce the probability of impact a bit, but that wouldn’t guarantee zero impact and we might as well figure out a few years later that the deviation eventually increased the probability of impact “.
In this context, whatever the diversion strategies put in place, “ these may not be sufficient, or even be ineffective “. If the mastery of deflection techniques is important, it is just as important “ improve the accuracy of orbital parameters and the model that predicts the position of Earth and NEOs years in advance “.
Another size constraint, if the object is a comet with a long period of several thousand years and even if it was detected quite early, for example at the level of Neptune“ the warning period would not exceed six years “, in particular because of its speed which for these objects is about 42 kilometers per second! ” This excludes a deflection mission due to insufficient time to complete it! »
Realistic ideas for deflecting asteroids but far from sufficient
To deflect a NEO, the latest technological advances make it possible to consider several more or less realistic intervention options. Jean-Marc Salotti retains three methods of deviation which seem to him the most effective, but which would nevertheless be insufficient. Except in very special circumstances, even if the warning period is very long, it ” seems difficult and uncertain to divert a giant asteroid from its collision course “. We therefore count thenuclear explosionthe slow and guided expulsion of matter and billiard shooting.
In detail, the use of nuclear explosions does not “ may be an option only in very specific circumstances, depending on the shape, density and structure of the asteroid “. The idea is not to “explode” the asteroid but rather to eject from ” large amounts of material more or less in the same direction “. However, and this is where the shoe pinches, the ” velocity of the ejected mass must be very high and the fraction of the ejected mass must also be significant to produce any noticeable change in orbit “. Would it be possible to excavate tens of cubic kilometers of rock using nuclear explosions? Nothing is less sure ! For example, if “ a single nuclear explosion removes 10-3 miles3 (a 100 mx 100 mx 100 m cube) and the average velocity of the ejected mass is 1,000 m/s (optimistic), one million explosions would eliminate 103 miles3or only one thousandth of the object ! The change in speed would therefore be of the order of 1 m/s, which is not sufficient to eliminate the risk of collision. This also assumes that we are able to send to the NEO several hundreds of thousands of rockets in ” very short deadlines and when Windows firing points that would only open for a few weeks and only repeat every 5-10 years, depending on the NEO’s orbit “. The feasibility of this method thus seems very uncertain.
The so-called method of mass driver (slow, guided material ejection) also involves ejecting material from the asteroid. But it has the advantage over nuclear explosions of “ provide continuous action on the object and control the speed of material ejection “. Concretely, if the matter is ” ejected at 10 km/s, which is possible using existing technologies such as thrusters ionic, 2% of the asteroid must be excavated to move its orbit to a safe zone, about 1 million kilometers away from the initial prediction “. In terms of energy efficiency, this strategy is therefore better, but 2% is still a huge amount of material. ” Assuming an asteroid diameter of 100 kilometers, this would represent about 1013 tons. Even though many nuclear center are installed on the asteroid and that many ejection systems are used at the same time, it would be necessary to eject 331 tons of matter every second for 1,000 years to obtain such a quantity “. The feasibility of this option is therefore doubtful.
The third option is rather original. This is the “billiard shot”. This option consists of deflect a smaller asteroid, putting it on a collision course with the Earth-threatening NEO “. It might seem the most realistic but, in order to deflect the giant asteroid, the ” smaller must also be very large, at least on the order of 2% of the mass of the first “. In other words, divert the smallest “ would therefore be almost as difficult as the previous options, unless it was already very close to a collision course with the threatening NEO “.
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