Impact winter

An impact winter is a hypothesized period of prolonged cold climate due to an impact event upon the Earth's planetary surface by a large asteroid or comet. If such meteors were to survive atmospheric entry and successfully impact land or a shallow body of water, it would eject an enormous amount of debris, dust, ash and other particulate materials into Earth's atmosphere, blocking off irradiance from the Sun. This would lead to global dimming and cause the climatic temperature to decrease drastically. If an asteroid or comet with the diameter of about 5 km (3.1 mi) or more were to hit in a deep body of water or explode mid-air before hitting the surface, there would still be an enormous amount of debris released into the atmosphere. It has been proposed that the rapid climate shifts associated with impact winters could cause severe floral disruptions and subsequently cascading collapse of food chains, leading to mass extinctions of the Earth's biosphere that wipe out many of the world's extant species. An example of such an event would be the Cretaceous–Paleogene extinction event, which probably involved a worldwide impact winter caused by the 10 km (6.2 mi) diameter Chicxulub impactor, and led to extinction of nearly all tetrapods weighing more than 25 kilograms (55 lb), which included all non-avian dinosaurs, pterosaurs and Mesozoic marine reptiles.

Possibility of impact Each year, the Earth is hit by 5 m (16 ft) diameter meteoroids that deliver an explosion 50 km (31 mi) above the surface with the power equivalent of one kiloton of TNT. The Earth is hit every day by a meteor less than 5 m (16 ft) 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. Another study in 1994 found a 1-in-10,000 chance that the Earth will be hit by a large asteroid or comet with a diameter of about 2 km (1.2 mi) during the next century. This object would be capable of disrupting the ecosphere and would kill a large fraction of the world's population. One such object, Asteroid 1950 DA, currently has a 0.038% chance of colliding with Earth in the year 2880, though when first discovered the probability was 0.3%. The probability goes down as orbits are refined with additional measurements. Over 300 short-period comets pass near larger planets, such as Saturn and Jupiter, which can change the comets' trajectories and could potentially put them into an Earth-crossing orbit. This could happen for long-period comets also but the chance is highest for short-period comets. The chance of these directly impacting Earth is far lower than a near-Earth object (NEO) impact. Victor Clube and Bill Napier support a controversial theory that a short-period comet in an Earth-crossing orbit does not need to impact to be hazardous, as it could disintegrate and cause a dust veil with possibilities of a "nuclear winter" scenario with long-term global cooling lasting for thousands of years (which they consider to be similar in probability to a 1 km impact).

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. Thus, not every object that hits the Earth will cause an extinction-level event or even cause any real harm. Objects release most of their kinetic energy in the atmosphere and will explode if they experience a column of atmosphere greater than or equal to their mass. Extinction-level impacts on the Earth occur about every 100 million years. Although extinction events happen very rarely, large projectiles can do severe damage. This section will discuss the nature of the hazards posed by projectiles as a function of their size and composition.

Size A large asteroid or comet could collide with the Earth's surface with the force of hundreds to thousands of times the force of all the nuclear bombs on the Earth. For example, the Cretaceous–Paleogene extinction event has been proposed to have caused extinction of all non-avian dinosaurs 66 million years ago. Early estimates of this asteroid's size put it at about 10 km (6.2 mi) in diameter. This means it hit with nearly a force of 100,000,000 MT (418 ZJ). 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 180 km (110 mi) 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 4 km (2.5 mi) deep. An asteroid, meteor, or comet would remain intact through the atmosphere by virtue of its sheer mass. However, an object smaller than 3 km (1.9 mi) 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 10 Mt (4×1016 J) or about 50 m (160 ft) diameter to breach the lower atmosphere (this is for a stony object hitting at 20 kilometres per second (40,000 mph)). 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 (≤1 g/cm3 (60 lb/cu ft)).

Possible mechanisms

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