SA: after the Chelyabinsk meteorite, technologies for detecting threats from space have become more advancedThe fall of the Chelyabinsk meteorite became a loud signal for humanity about the danger coming from outer space, writes Scientific American.
Over the past ten years, methods of detecting asteroids have become more advanced, and ways to change their trajectory have appeared.
Ten years ago, on February 15, 2013, when the sun rose over the Russian city of Chelyabinsk, an explosion occurred in the sky.
An asteroid burst into the Earth's atmosphere, flying at a speed of almost 70 thousand kilometers per hour. This cosmic lump, about the size of a tennis court, left a bright glowing trail in the sky – as if a second sun had appeared there, rushing from the southeast to the northwest.
Entering the atmosphere at supersonic speed, the asteroid burst into flames, and a thick trail of evaporating rocks followed it. Due to the enormous pressure, this block began to change shape. As a result, the pressure finally took over the asteroid about 40 kilometers above the Earth's surface. It disintegrated into smaller pieces, each of which flew at a speed ten times the speed of a bullet fired from a rifle. These fragments also began to change shape, which is why a whole series of short but powerful flashes of light could be observed from the Ground when they were heated to the limit. Eventually the remaining pieces evaporated.
All this happened within a few seconds, and the last explosion occurred when the asteroid was 30 kilometers above the Earth's surface. The energy of his residual movement instantly turned into heat. The resulting giant fireball briefly eclipsed the sun, radiating energy approximately equal to the explosion of half a million tons of TNT.
A powerful shock wave from this explosion reached the center of Chelyabinsk, located 40 kilometers to the north, in about a minute. This industrial city with a population of one million people was just starting its day when a fireball swept across the sky. This stunning sight – and a long shining trail of steam – made people go outside or look out of windows to see what was going on. And it was at that moment that the shock wave reached the ground.
The strongest blow knocked out windows all over the city – the vast majority of those who suffered as a result of the meteorite fall received glass cuts. Fortunately, no one was killed, and the damage to the infrastructure turned out to be relatively small. If the asteroid had been larger, if it had been made of metal, or if it had crashed down at a sharper angle, this story could have turned out quite differently, and the consequences would have been much more serious.
The fall of the Chelyabinsk meteorite was an alarm signal for the Earth. A very loud signal.
It also served as valuable material for study – the largest event in the Earth's atmosphere since the fall of the Tunguska meteorite in 1908. The smoking trail of the asteroid was observed by satellites, as well as thousands of eyewitnesses and cameras. The earth was strewn with small fragments of the meteorite, and one giant piece measuring about one meter across and weighing half a ton fell into the lake, from which it was then extracted. There is even footage from surveillance cameras, where you can see how this piece crashed into a frozen lake, leaving behind a very impressive splash of upwelling snow and water.
The pieces of the meteorite that scientists collected after the fall told them the complicated history of this asteroid. They were riddled with grooves and narrow slits. Scientists have found out that this 19-meter block was once part of a much more massive asteroid, which also suffered from an impact, as a result of which a piece broke off from it, which crashed into the Earth and made a deep crack in it. Isotope dating has shown that the first collision could have occurred as early as 4.4 billion years ago, when the Solar System was less than 200 million years old. Cracks in the Chelyabinsk block weakened it, which is why it collapsed high above the ground and managed to create a powerful shock wave. On that day, the "ghost fingers" of an ancient collision in deep space touched the lives of thousands of Russians.
It is unclear which asteroid is the "parent" of the Chelyabinsk meteorite. Scientists traced its trajectory in the opposite direction into space and found coincidences with the asteroids 2007 BD7 and 2011 EO40.One of them may well be the "parent" of the Chelyabinsk meteorite, but there is no exact data yet.
Analysis of the fall of this block, along with smaller events, showed that similar "collision objects" crash into the Ground much more often than previously thought. Asteroids the size of Chelyabinsk crash into the atmosphere about every 25 years, but fortunately, most of them do it over the ocean or wilderness.
The alarming thing is that astronomers did not see this asteroid flying towards Earth until it crashed into the atmosphere. But asteroids are usually very dark celestial objects, and it is very difficult to notice small representatives of this class, even if they are close to the Earth. Just a few years earlier, asteroid 2008 TC3, four meters wide, became the first asteroid that people noticed before it crashed into the Earth's atmosphere. Since then, astronomers have managed to detect only six asteroids before they struck, including 2023 CX1, which lit up the sky over the English Channel on February 13, 2023, as if deciding to mark the decade of the Chelyabinsk meteorite fall. All of them were small and did not pose any danger to people.
Now that I've scared you enough with my stories about asteroids falling, I want to tell you the good news: people are rapidly improving their skills in detecting asteroids in space. In the ten years that have passed since the fall of the Chelyabinsk meteorite, scientists have discovered about 20 thousand near–Earth asteroids - this is more than in the entire history of observations until 2013. We have new survey telescopes at our disposal, such as Pan-STARRS and the Zwicky Transient Facility, in addition, more advanced methods of detection and data analysis have been developed. Very soon, the huge Vera Rubin Observatory and NASA's NEO Surveyor space mission will significantly expand the list of asteroids known to people that threaten the Earth.
But discovering them is only the first step. The next stage involves the adoption of certain measures. To this end, in September last year, NASA launched the DART (Double Asteroid Redirection Test) mission, which hit a half-ton projectile into the 170-meter asteroid Dimorph, which is a satellite of the larger asteroid Didymus. The momentum from the collision changed the asteroid's rotation period by more than half an hour. The results of that mission turned out to be even more impressive than scientists had hoped: a huge plume of rocks separated from the asteroid's surface gave it an additional boost. This proved that people can use similar spacecraft to change the trajectory of asteroids.
More powerful explosions can significantly change the trajectory of a space block approaching the Earth. The explosion of a nuclear charge near a small asteroid will vaporize a significant part of its surface. The hot steam will expand rapidly, acting like rocket exhaust, and push the asteroid onto a new and hopefully safer trajectory. Now there are some rather intractable problems with the use of this method – for example, in accordance with the terms of the Outer Space Treaty, it is strictly prohibited to detonate nuclear devices outside the Earth. However, if a dangerous asteroid is flying towards us at high speed, it may force people to make changes to the current laws and requirements.
After the fall of the Chelyabinsk meteorite, two spacecraft managed not only to approach small asteroids, but also to take samples from their surface (the Hayabusa-2 spacecraft has already delivered samples to Earth, and the OSIRIS-Rex spacecraft will do this a little later this year). Both asteroids – "Ryugu" with a diameter of about a kilometer and "Bennu" with a diameter of about 500 meters – are piles of boulders, which are held together by their own rather weak force of attraction. Probably, all small asteroids are piles of boulders, and this property of them determines our tactics of confrontation: their rather fragile structure will simplify our task of striking them. Imagine that you are trying to punch a box of peanuts, and you will understand what I mean. But the DART mission also showed that a huge amount of rock is ejected upon impact, and that momentum transfer can actually enhance the impact effect.
The fall of the Chelyabinsk meteorite took us by surprise, and although such small space objects can still slip past us without being noticed, we are confidently improving technologies for detecting potential threats from space and developing an action plan in case one of them decides to rush straight to Earth. Large and dangerous asteroids are very rare, but we just need to look at the Arizona meteorite crater to understand that we still need to take this threat seriously. About 50 thousand years ago, an explosion with a capacity of 10 megatons made a hole in the desert with a diameter of more than a kilometer, most likely destroying all the plants and animals that lived there at that time. It may have been one of the most recent direct collisions of meteorites with the Earth, but it definitely won't be the last.
Of course, unless we take measures to prevent it.
Author of the article: Phil Plait, professor of astronomy and popularizer of science from Colorado.
