Impact event

An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, as the impacting body is usually traveling at several kilometres per second (km/s), with a minimum impact speed of 11.2 km/s (25,054 mph; 40,320 km/h) for bodies striking Earth. While planetary atmospheres can mitigate some of these impacts through the effects of atmospheric entry, many large bodies retain sufficient energy to reach the surface and cause substantial damage. This results in the formation of impact craters and structures, shaping the dominant landforms found across various types of solid objects found in the Solar System. Their prevalence and ubiquity present the strongest empirical evidence of the frequency and scale of these events.

Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Major impact events have significantly shaped Earth's history, and have been implicated in the formation of the Earth–Moon system. Interplanetary impacts have also been proposed to explain the retrograde rotation of Uranus and Venus. Impact events also appear to have played a significant role in the evolutionary history of life. Impacts may have helped deliver the building blocks for life (the panspermia theory relies on this premise). Impacts have been suggested as the origin of water on Earth. They have also been implicated in several mass extinctions. The prehistoric Chicxulub impact, 66 million years ago, is believed to be the cause not only of the Cretaceous–Paleogene extinction event but acceleration of the evolution of mammals, leading to their dominance and, in turn, setting in place conditions for the eventual rise of humans.

Throughout recorded history, hundreds of Earth impacts (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage, or other significant localised consequences. One of the best-known recorded events in modern times was the Tunguska event, which occurred in Siberia, Russia, in 1908. The 2013 Chelyabinsk meteor event is the only known such incident in modern times to result in numerous injuries. Its meteor is the largest recorded object to have encountered the Earth since the Tunguska event. The Comet Shoemaker–Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. An extrasolar impact was observed in 2013, when a massive terrestrial planet impact was detected around the star ID8 in the star cluster NGC 2547 by NASA's Spitzer Space Telescope and confirmed by ground observations. Impact events have been a plot and background element in science fiction.

In April 2018, the B612 Foundation reported: "It's 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent certain when." Also in 2018, physicist Stephen Hawking considered in his final book Brief Answers to the Big Questions that an asteroid collision was the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched. On 26 September 2022, the Double Asteroid Redirection Test demonstrated the deflection of an asteroid. It was the first such experiment to be carried out by humankind and was considered to be highly successful. The orbital period of the target body was changed by 32 minutes. The criterion for success was a change of more than 73 seconds.

Several major impact events have significantly shaped Earth's history, having been implicated in the formation of the Earth–Moon system, the evolutionary history of life, the origin of water on Earth, and several mass extinctions. Impact structures are the result of impact events on solid objects and, as the dominant landforms on many of the System's solid objects, present the most solid evidence of prehistoric events. Notable impact events include the hypothesized Late Heavy Bombardment, which would have occurred early in the history of the Earth–Moon system, and the confirmed Chicxulub impact 66 million years ago, believed to be the cause of the Cretaceous–Paleogene extinction event.

Small objects frequently collide with Earth. There is an inverse relationship between the size of the object and the frequency of such events. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater's diameter, which is on average proportional to the diameter of the impactor. Asteroids with a 1 km (3,280 ft) diameter strike Earth every 500,000 years on average. Large collisions – with 5 km (3.1 mi) objects – happen approximately once every twenty million years. The last known impact of an object of 10 km (6.2 mi) or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.

The energy released by an impactor depends on diameter, density, velocity, and angle. The diameter of most near-Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of two, by basing it on the asteroid's brightness. The density is generally assumed, because the diameter and mass, from which density can be calculated, are also generally estimated. Due to Earth's escape velocity, the minimum impact velocity is 11 km/s with asteroid impacts averaging around 17 kilometres per second (38,028 mph; 61,200 km/h) on the Earth. The most probable impact angle is 45 degrees.

Impact conditions such as asteroid size and speed, but also density and impact angle determine the kinetic energy released in an impact event. The more energy is released, the more damage is likely to occur on the ground due to the environmental effects triggered by the impact. Such effects can be shock waves, heat radiation, the formation of craters with associated earthquakes, and tsunamis if bodies of water are hit. Human populations are vulnerable to these effects if they live within the affected zone. Large seiche waves arising from earthquakes and large-scale deposit of debris can also occur within minutes of impact, thousands of kilometres from impact.