Bolides are space objects that collide with Earth or explode in the atmosphere, including meteorites, asteroids, and comets. They can enter Earth’s atmosphere at speeds up to 44 miles per second and cause impact craters. Large collisions can have devastating effects on the planet, such as reducing sunlight and killing green plants. The best-known example is the Chicxulub impact, which wiped out dinosaurs 65 million years ago. The Tunguska Event in 1908 was the largest bolide fireball witnessed by humans.
Bolide is a general term for any object originating in space that collides with Earth or explodes in the atmosphere. Covers meteorites, asteroids and comets. In astronomy, the term is often used to describe a meteor that is brighter than magnitude -4 – as bright as the planet Venus – and is used synonymously with “fireball”. In geology, bolide is generally used to describe a meteorite or asteroid leaving an impact crater and tends to be employed where there is not enough information for a more specific description.
The Solar System contains many objects, most of them small, in orbits that may intersect Earth’s orbit. These objects have various compositions. Some can be described as “dirty snowballs” – dust and small rock fragments held together by ice – while some can consist of dense rocks with a high metal content, mainly iron and nickel. They orbit at speeds of up to 26 miles per second (42 km/s), compared to Earth’s 18 miles per second (29 km/s).
Depending on the direction from which they approach, they can enter Earth’s atmosphere at up to 44 miles per second (71 km/s). At these speeds, friction with the atmosphere rapidly heats the object to very high temperatures, causing it to glow brightly. Very small objects will burn completely high in the atmosphere, but larger objects could, depending on their size and composition, strike the surface or explode in the lower atmosphere.
Most objects that enter Earth’s atmosphere are the size of a grain of sand. These burn rapidly, but are visible from the surface as meteors. Bludgers that reach the surface can leave impact craters. Numerous such craters can be seen on the Moon; however, on Earth, erosion and geological activity tend to erase the evidence relatively quickly, geologically speaking. There are only a few impact craters on Earth that are well preserved enough to be apparent as such; the best example is Barringer Crater in Arizona, USA, a roughly circular crater more than 1,000 yards (1 km) wide.
A large collision of bludgers can have a devastating effect on the planet. Aside from the immediate effects on the impact area, large amounts of vaporized rock would be propelled into the upper atmosphere, where it would condense into fine dust. This could greatly reduce the amount of sunlight reaching the surface over the entire planet for an extended period, killing green plants and dramatically reducing temperatures.
Perhaps the best-known example of a large bolide event is one that occurred at the end of the Cretaceous geological period 65 million years ago. Evidence of a large crater, 110 miles (180 km) in diameter, dating from this period has been found in Mexico’s Yucatan peninsula near the town of Chicxulub. The object responsible is thought to have been about 6 miles (10 km) in diameter.
This object is believed to have caused the mass extinction event that occurred at the time, wiping out dinosaurs and many other classes of animals. Not all scientists, however, agree that the Chicxulub impact was solely responsible for this extinction event, and some think there may have been a series of large impacts. There have been several mass extinction events during Earth’s history, and it’s possible that some or all of these were caused by large fireballs.
The largest bolide fireball known to have been seen by humans was the Tunguska Event in 1908, when a huge explosion took place in the Tunguska area of Siberia, causing extensive devastation. Fortunately, the area was uninhabited and there were no known human fatalities, but the object’s light path and detonation were witnessed by a number of people in villages some distance away. Computer simulations based on the destruction model calculated the size, angle of approach and possible composition of the object, which is thought to have been a small asteroid or comet fragment.
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