Success! Defence Against Asteroids

Most people will know that about 64 million years ago an asteroid with a diameter of about 10 km struck the Yucatán peninsula and exterminated the dinosaurs, or at least did great damage to them from which they never recovered. The shock-wave probably also initiated the formation of the Deccan Traps, and the unpleasant emission of poisonous gases which would finish off any remaining dinosaurs. The crater is 180 km wide and 20 km deep. That was a very sizeable excavation. Rather wisely, we would like to avoid a similar fate, and the question is, can we do anything about it? NASA thinks so, and they carried out an experiment.

I would be extremely surprised if, five years ago, anyone reading this had heard of Dimorphos. Dimorphos is a small asteroid with dimensions about those of the original Colosseum, i.e.  before vandals, like the Catholic Church took stones away to make their own buildings. By now you will be aware that Dimorphos orbits another larger asteroid called Didymos. What NASA has done was to send a metallic object of dimensions 1.8 x 1.9 x 2.6 meters, of mass 570 kg, and velocity 22,530 km/hr to crash into Dimorphos to slightly slow its orbital speed, which would change its orbital parameters. It would also change then orbital characteristics of the two around the sun. Dimorphos has a “diameter” of about 160 m., Didymos about 780 m. Neither are spherical hence the quotation marks.

This explains why NASA selected Dimorphos for the collision. First, it is not that far from Earth, while the two on their current orbits will not collide with Earth on their current orbits. Being close to Earth, at least when their orbits bring them close, lowers the energy requirement to send an object there. It is also easier to observe what happens hence more accurately determine the consequences. The second reason is that Dimorphos is reasonably small and so if a collision changes its dynamics, we shall be able to see by how much. At first sight you might say that conservation of momentum makes that obvious, but it is actually more difficult to know because it depends on what takes the momentum away after the collision. If it is perfectly inelastic, the object gets “absorbed” by the target which stays intact, then we simply add its relative momentum to that of the target. However, real collisions are seldom inelastic, and it would have been considered important to determine how inelastic. A further possibility is that the asteroid could fragment, and send bits in different directions. Think of Newton’s cradle. You hit one end and the ball stops but another flies off from the other end, and the total stationary mass is the same. NASA would wish to know how well the asteroid held together. A final reason for selecting Dimorphos would be that by being tethered gravitationally to Didymos, it could not go flying off is some unfortunate direction, and eventually collide with Earth. It is interesting that the change of momentum is shared between the two bodies through their gravitational interaction.

So, what happened, apart from the collision. There was another space craft trailing behind: the Italian LICIACube (don’t you like these names? It is an acronym for “Light Italian Cubesat for Imaging Asteroids”, and I guess they were so proud of the shape they had to have “cube” twice!). Anyway, this took a photograph before and after impact, and after impact Dimorphos was surrounded by a shower of material flung up from the asteroid. You could no longer see the asteroid for the cloud of debris. Of course Dimorphos survived, and the good news is we now know that the periodic time of Dimorphos around Didymos has been shortened by 32 minutes. That is a genuine success. (Apparently, initially a change  by as little as 73 seconds would have been considered a success!) Also, very importantly, Dimorphos held together. It is not a loosely bound rubble pile, which would be no surprise to anyone who has read my ebook “Planetary Formation and Biogenesis”.

This raises another interesting fact. The impact slowed Dimorphos down relative to Didymos, so Dimorphos fell closer to Didymos, and sped up. That is why the periodic time was shortened. The speeding up is because when you lower the potential energy, you bring the objects closer together and thus lower the total energy, but this equals the kinetic energy except the kinetic energy has the opposite sign, so it increases. (It also shortens the path length, which also lowers the periodic time..)

The reason for all this is to develop a planetary protection system. If you know that an asteroid is going to strike Earth, what do you do? The obvious answer is to divert it, but how? The answer NASA has tested is to strike it with a fast-moving small object. But, you might protest, an object like that would not make much of a change in the orbit of a dinosaur killer. The point is, it doesn’t have to. Take a laser light and point it at a screen. Now, give it a gentle nudge so it changes where it impacts. If the screen as a few centimeters away the lateral shift is trivial, but if the screen is a kilometer away, the lateral shift is now significant, and in fact the lateral shift is proportional to the distance. The idea is that if you can catch the asteroid far enough away, the asteroid won’t strike Earth because the lateral shift will be sufficient.

You might protest that asteroids do not travel in a straight line. No, they don’t, and in fact have trajectories that are part of an ellipse. However, this is still a line, and will still shift laterally. The mathematics are a bit more complicated because the asteroid will return to somewhere fairly close to where it was impacted, but if you can nudge it sufficiently far away from Earth it will miss. How big a nudge? That is the question about which this collision was designed to provide us with clues.

If something like Dimorphos struck Earth it would produce a crater about 1.6 km wide and 370 m deep, while the pressure wave would knock buildings over tens of km away. If it struck the centre of London, windows would break all over South-East England. There would be no survivors in central London, but maybe some on the outskirts. This small asteroid would be the equivalent a good-sized hydrogen bomb, and, as you should realize, a much larger asteroid would do far more damage. If you are interested in further information, I have some data and a discussion of such collisions in my ebook noted above.

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Don’t Look Up

No, I am not going to discuss the film, the merits of which you can decide for yourself. However, it might be worth considering some of the things it says about the way we consider and treat science. What the film is supposed to say is that those in society with the power to do something about a crisis wilfully avoid taking action. Consider the excuses for doing nothing.

The film presents a wipe-out event that we will be struck by a comet. The probability of this happening is assessed at 99.8%. So it is not 100%? What we have to recognize that scientific measurements have errors in them. Statistically we make lots of measurements and use a statistical analysis, and while someone in the movie says “Scientists never like to say 100%” that is wrong too. Scientists do not like or dislike; they report the mathematics, and a statistical spread cannot give a 100% because that denies the initial spread. Further, that 0.2% is not physically meaningful either because the errors due to instruments are not randomly probable, but nobody is going to waste time working out the error function for every piece of equipment. Statistical analysis takes care of that. To gain perspective, consider a bag of 1000 50 calibre bullets. You are assured two are blank. One is placed into a gun. What amount of money do you need, if you survive, to put your head in front of the barrel when it is fired?

A second problem for scientists is that long-term realities will be ignored by the public. This more relevant to something like climate change. What are you prepared to do to avoid a major problem fifty years down the track? For many, not a lot, so they ignore the problem on the grounds that it can be dealt with “later”. Related to this are the economic considerations. One response is we cannot afford to do something. When we hear that we seldom see what the costs are of not doing said something. Again, the response might be, but you do not absolutely know that will solve the problem. No, we do not, but that is because we do not think there will be one simple solution for a problem like climate change.

Another response is to rely on technological changes. For an approaching comet, there are probably no other choices. You either construct some space vehicle that will push the comet off course or it strikes you. To make that work, a major investment in development work would be required, since we do not have such a vehicle now. As it happens, for this scenario NASA is doing work, and around the end of September a space vehicle weighing 550 kg will slam into an asteroid called Dimorphos. This is part of a double asteroid system, and we will be able to follow the effect of the impact in fine detail because it will alter the orbital characteristics of Dimorphos as paired with Didymos, the larger companion. The problem with something like climate change is that while technology might fix it, we are not doing the research and development needed to make it work.

Society seems to work against science, simply because people do not trust it. Over 5 million have died with Covid 19, yet we have many very active antivaxxers trying to persuade others not to be vaccinated. The interesting question is why? It is one thing to refuse to be vaccinated yourself, but why impose these views on others?  In their effort το persuade others they spread completely stupid stories. Recall the story that Bill Gates was inserting nano-trackers into the vaccine so he could know what everyone was doing? There are also stories with an element of truth but with no comprehension of relevance. Like our 98.8% above, they focus on the 0.2%. There is a tiny segment of the populations that respond adversely to certain vaccines. The medical profession knows this, and can look out for them and treat them properly if such an event occurs. These stories totally ignore what would happen to these far more sensitive people if the virus struck them. Finally, there is a tendency for navel-gazing. Consider our experiment on Dimorphos. There is a view, “What right have we to change the solar system?” If we took this view to the limit, we would still be hunter-gatherers and our biggest problem would be that lion in the shrubbery planning on eating us. Dimorphos is a lump of rock. It does not have feelings. It is not planning its future. The allied question, do your sensitivities about the Universe and the pristine nature of rocks in it give you the right to prevent the killing of billions of innocent people who do not share your view?