Impact winter

Each year, the Earth is hit by diameter meteoroids that deliver an explosion above the surface with the power equivalent of one kiloton of TNT. The Earth is hit every day by a meteor less than in diameter that disintegrates before reaching the surface. The meteors that do make it to the surface tend to strike unpopulated areas and cause no harm. A human is more likely to die in a fire, flood, or other natural disaster than to die because of an asteroid or comet impact. though when first discovered the probability was 0.3%. ==Necessary impact factors==

The Earth experiences a never-ending barrage of cosmic debris. Small particles burn up as they enter the atmosphere and are visible as meteors. Many of them go unnoticed by the average person even though not all of them burn up before they hit the Earth's surface. Those that strike the surface are known as meteorites. Although extinction events happen very rarely, large projectiles can do severe damage. That is over six billion times larger than the atomic bomb yield (16 kilotons, 67 TJ) that was dropped on Hiroshima during WW2. This impactor excavated the Chicxulub crater that is in diameter. With an object this size, dust and debris would still be ejected into the atmosphere even if it hit the ocean, which is only deep. An asteroid, meteor, or comet would remain intact through the atmosphere by virtue of its sheer mass. However, an object smaller than would have to have a strong iron composition to breach the lower atmosphere — the troposphere or the lower levels of the stratosphere. Composition There are three different composition types for an asteroid or comet: metallic, stony and icy. The composition of the object determines whether or not it will make it to the Earth's surface in one piece, disintegrate before breaching the atmosphere, or break up and explode just before reaching the surface. A metallic object tends to be made up of iron and nickel alloys. These metallic objects are the most likely to impact the surface because they stand up better to the stresses of ram pressure induced flattening and fragmentation during deceleration in the atmosphere. The stony objects, like chondritic meteorites, tend to burn, break up, or explode before leaving the upper atmosphere. Those that do make it to the surface need a minimum energy of about or about diameter to breach the lower atmosphere (this is for a stony object hitting at ). The porous comet-like objects are made up of low-density silicates, organics, ice, volatile and often burn up in the upper atmosphere because of their low bulk density (≤). ==Possible mechanisms==

Although the asteroids and comets that impact the Earth hit with many times the explosive force of a volcano, the mechanisms of an impact winter are similar to those that occur after a mega-volcanic eruption-induced volcanic winter. In this scenario massive amounts of debris injected into the atmosphere would block some of the Sun's radiation for an extended period of time and lower the mean global temperature by as much as 20°C after a year. This is controversially purported to have occurred following the Toba eruption. These pulverized rock particles would remain in the atmosphere until dry deposition and due to their size, they would also act as cloud condensation nuclei and would be washed out by wet deposition/precipitation, but even then, about 15% of the sun's radiation might not reach the surface. After the first 20 days, the land temperature might drop quickly, by about 13°C. After about a year, the temperature could rebound by about 6°C, but by this time about one-third of the Northern Hemisphere might be covered in ice. This volcanism could alone therefore create a volcanic winter, irrespective of the other impact effects. Multiple firestorms In combination with the initial debris ejected into the atmosphere, if the impactor is extremely large ( or more), like at the Cretaceous–Paleogene extinction event (estimated ), there might be the ignition of multiple fire storms, possibly with a global reach into every dense and therefore firestorm-prone forest. These wood fires might release enough amounts of water vapor, ash, soot, tar and carbon dioxide into the atmosphere to perturb the climate on their own and cause the pulverized rock dust cloud blocking the sun to last longer. Alternatively it could cause it to last for a much shorter time, as there would be more water vapor for the rocky aerosol particles to form cloud condensation nuclei. If it causes the dust cloud to last longer, it would prolong the Earth's cooling time, possibly causing thicker ice sheets to form. == Past events==

In 2016, a scientific drilling project drilled deep into the peak ring of the Chicxulub impact crater to obtain rock core samples from the impact itself. This crater is one of the best known impact craters and was the impact responsible for the extinction of the non-avian dinosaurs. The discoveries were widely seen as confirming current theories related to both the crater impact and its effects. They confirmed that the rock comprising the peak ring had been subjected to immense pressures and forces, and had been melted by immense heat and shocked by immense pressure from its usual state into its present form in just minutes. The fact that the peak ring was made of granite was also significant, since granite is not a rock found in sea-floor deposits — it originates much deeper in the earth and had been ejected to the surface by the immense pressures of impact. Gypsum, a sulfate-containing rock that is usually present in the shallow seabed of the region, had been almost entirely removed and must therefore have been almost entirely vaporized and entered the atmosphere, and that the event was immediately followed by a huge megatsunami (a massive movement of sea waters) sufficient to lay down the largest known layer of sand separated by grain size directly above the peak ring. These strongly support the hypothesis that the impactor was large enough to create a 120-mile peak ring, eject molten granite from deep within the earth, create colossal water movements, and eject an immense quantity of vaporized rock and sulfates into the atmosphere, where they would have persisted for a long time. This global dispersion of dust and sulfates would have led to a sudden and catastrophic effect on the climate worldwide by causing large temperature drops, devastating the food chain. ==Impact on humans==

, circa 74,000 years ago. Some scientists believe this eruption led to a population collapse and subsequent genetic bottleneck in humans. An impact winter would have a devastating effect on humans, as well as the other species on Earth. With the sun's radiation being severely diminished, the first species to die would be plants and animals who survive through the process of photosynthesis. This lack of food would ultimately lead to other mass extinctions of other animals that are higher up on the food chain and possibly kill up to 25% of the human population. These factors would make life on Earth, for humans, extremely difficult. Agriculture With the Earth's atmosphere full of dust and other material, radiation from the sun would be refracted and scattered back into space and absorbed by this debris. The first effect on the Earth, after the blast wave and potential multiple fire storms, would be the death of most, if not all, of the photosynthetic life forms on Earth. Those in the ocean that survive would possibly become dormant until the sun came out again. Economics The cost to clean up after an asteroid or comet impact would cost billions to trillions of dollars, depending on the location impacted. or 96.5% of an estimated total of about 920. ==See also==