An impact (impact, impact, from the Latin impactus = struck ) or impact describes the collision of two celestial bodies at a very high speed. Numerous impacts from small bodies ( meteoroids , asteroids and comets ) have been documented on the earth , the moon and other celestial bodies. An impact crater forms on the mainland . The rock remnants of the impacted small body are the meteorites .
Impact effects in the history of the earth
The earth's history, which is around 4.6 billion years old, is largely shaped by the effects of meteorite impacts. The formation of the earth and its present form is inconceivable without the initial collisions with asteroids of any size, because these events not only possibly caused the origin of the earth's water in the form of the oceans , but could also happen until around 3.9 billion years ago the hypothetical "Late Heavy Bombardment" - also prevented the formation of a stable earth crust .
Much of the matter in the solar system was captured by the gravity of the earth and the other planets at this early stage . Every year, however, around 20,000 meteorites fall to the earth, mostly without leaving any significant traces in the landscape. The natural disasters of the past triggered by the largest impactors can often only be detected indirectly, for example through a mass extinction or global climate change triggered by them , since on earth - unlike on the moon, for example - the erosive effects of wind and water are the real ones Impact crater erodes again within geologically short periods of time.
Another method to prove an impact is the geochemical and mineralogical investigation of the earth's rocks and the deposited meteor dust . Under suitable conditions, large impacts leave characteristic, thin rock layers which, in addition to rare earths (iridium, platinum, osmium) with non-terrestrial isotope distributions (which correspond to the isotope distributions in meteorites), can also contain shocked quartz rocks or melted impact glasses (tektites). Since these minerals are distributed over large parts of the earth's surface and ocean floors due to the great energy released during the impact, these traces are often the only evidence of an impact, while the actual crater has long been eroded.
Impact marks on the earth
All small bodies that would leave visible traces in the form of craters on the moon, Mars or other (almost) atmospheric celestial bodies, burn up because of the friction with the particles of the earth's atmosphere before they can reach the earth's surface. Larger bodies, on the other hand, can hit the surface, but there is a 71% chance that they would fall into one of the oceans that cover most of the earth. In this case, too, they do not leave any permanent evidence on earth in the form of impact craters .
The traces of the celestial bodies hitting the mainland will sooner or later also be erased: craters of larger meteorites are made unrecognizable over the course of a few decades or centuries by vegetation and deformed beyond recognition by atmospheric weathering over millennia ( geologically a short time). Over the course of several hundred million to billions of years, tectonic processes bring about a renewal of almost the entire surface of the earth. Terrace- like subsidence, such as those that occur in some burglar basins , can also blur traces of impact.
Only the impact craters of the largest and therefore most serious impacts of the last millions of years are still visible in the landscape today. The rule of thumb for the ratio of the diameter of the impact body to the diameter of the resulting crater is 1:20 for stone meteorites and 1:40 for iron meteorites (for large known impact craters, see the article on impact craters ).
If material ejected by a large impact during an impact is distributed over a large area, this material can be detected in the geological stratification of the areas concerned over very long periods of time. A well-known example is the evidence of the material ejected by the Chicxulub impact 66 million years ago and distributed globally on the basis of its iridium content. Such a layer is called an impact layer .
Objects with a diameter of more than 500 m are globally dangerous. Scientists in New Mexico ( USA ) counted more than 1,100 asteroids with a diameter of more than 1 km that are in an orbit that could bring them dangerously close to Earth . Impacts from bodies of this diameter would have a number of devastating consequences: Billions of people could become victims of flood disasters and a rapid cooling of global temperatures as a result of the strong clouding of the atmosphere from aerosols (“ impact winter ”, comparable to a nuclear winter ). The probability that such a meteorite would hit the sea would be relatively high, because 71% of the earth's surface is covered by water . The result would be a megatsunami with a wave height in shallow water areas of 100 m and above, which would flood entire coastal landscapes and their hinterland over a large area . An impact could also affect the planet's ionosphere and magnetosphere .
From a purely statistical point of view, such an impact must be expected every 500,000 to 10 million years. Events such as the so-called KP impact on the boundary between the Cretaceous and Paleogene ( Tertiary ) periods are said to take place approximately every 100 million years. The impactor of Chicxulub crater (an asteroid or comet) is estimated to be about 10 to 15 km in diameter. Comparatively smaller impacts occur more frequently. The impact in the Nördlinger Ries (with an impactor diameter of around 1.5 km), accompanied by a second impact in the Steinheim Basin , devastated large parts of Europe around 14.6 million years ago.
But even smaller meteorites can cause immense damage locally or regionally. According to historical reports, more than 10,000 people were killed in a meteorite impact in China in 1490 . The Tunguska event , in which an area of about 2,000 km² in Siberia was devastated in 1908, was perhaps a meteorite that exploded in the atmosphere. It is also believed that the North American Clovis culture perished as a result of the explosion of a celestial body.
Prevention of future mass extinctions
Impact events can lead to mass extinctions . Although humans are the cause of today's mass extinction, on the other hand, evolution has produced a species with them that has the potential to ward off threats such as impact events in the foreseeable future, which in turn can cause mass extinctions. Corresponding research programs have already been started.
Michael Schmidt-Salomon is aware that it seems strange, even downright crazy, to present mankind, which has already caused great damage, not as a destroyer, but as a savior of biodiversity. But there are plausible arguments that give reason to hope that humanity can get a better grip on ecological problems. He believes that, analogous to biological selection processes, a similar one takes place on the cosmic level, and that only those planets receive higher forms of life in the long term that produce species that can protect biodiversity from external threats.
Possible defense methods
Slow distraction via reflectors
The US space agency NASA announced in summer 2007 that a special space probe could be used to steer asteroids out of their orbit. This probe would carry a large solar sail that would concentrate solar radiation on a small area of the asteroid. The heat generated by this would vaporize matter in the asteroid, causing a recoil that would deflect the asteroid from its orbit. NASA estimates that this method is suitable for asteroids up to 500 m in diameter.
Slow distraction via gravity
The most accurate way to deflect an asteroid is through the use of gravity. It is sufficient to let a 20-ton satellite hover over an asteroid at a distance of 150 m from the center of an asteroid for a year in order to deflect the asteroid sufficiently and thereby protect the earth from an impending impact 20 years later. Without rocket propulsion, the satellite hovering over the asteroid would crash on it within a short time. Little continuous propulsion is therefore required to keep the satellite in suspension. Since the satellite pulls the asteroid just as strongly as the asteroid pulls the satellite, the satellite pulls the asteroid behind it accordingly (extremely slowly, but sufficient to distract within decades). Such drives are commercially available as ion drives ; they can be fed with electrical energy via solar panels or nuclear reactors.
Due to the exact controllability of the satellite drive and the precisely known effect of gravity, this deflection method is the most precise.
Impulse-like deflection via impactors
The ESA is working on a defense project called " Don Quixote ". The two probes "Sancho" and "Hidalgo" could fly to the asteroid, where "Hidalgo" would ram it as a four-ton impactor, while "Sancho" in the orbit of the asteroid collects data on its speed, composition and success of "Hidalgo". Even if four tons seem small compared to an asteroid, just a few arc seconds can be enough to divert the asteroid from its collision course. According to the ESA, this method is effective for objects up to 1 km in diameter and the mission would be started if the impact probability of an asteroid like Apophis rises above 1%.
In January 2012 the international research project " NEOShield " was founded, which also deals with possibilities for planetary defense. On May 22, 2013, the European warning system for dangerous asteroids was opened.
Blasting the asteroid
In films such as Deep Impact and Armageddon, spaceships land on the surface of the impact body in order to detonate them with the help of nuclear weapons. In the case of objects several hundred kilometers in diameter, as shown in the last-named film, an atom bomb would not be strong enough to have any effect at all. In addition, all asteroids of this size are considered fully known and all have stable orbits, so the scenario shown in Armageddon is also unrealistic in this respect . The effect of a realistic mission on an object crossing the earth orbit with a diameter of less than a few kilometers has not yet been examined in more detail.
Nuclear explosion diversion
To protect the earth, not only would the body have to be completely blown up, but most of the mass of the original body would have to be accelerated sufficiently that it would miss the earth. But then it is easier to forego the blast and to deflect the asteroid as a whole with one blast so that it misses the earth. For this purpose, the explosion of a nuclear weapon at a relatively short distance from the asteroid is considered to be practicable. The radiation released during the explosion would suddenly evaporate matter from the surface of the body, from which a ball of fire would then form. The pressure building up in the hot gas would then accelerate the asteroid in the direction of the side facing away from the explosion.
Due to numerous uncertainties, such as the exact amount of energy released by the nuclear weapon, the material-dependent absorption behavior of the asteroid surface and the exact dynamics of the fireball generated, the diversion via a nuclear weapon explosion is the least precise of all the methods mentioned. But it is also the most powerful and may be the only useful method when an asteroid is discovered on a hitting course too late to use the other methods.
- Penetration force of meteorites, projectiles and other Newtonian impactors
- Global killer
- Databases of earthly impact structures
- Asteroid # Impact probability and impact
- Asteroid Day
- Near earth object
- Impact metamorphosis
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