The asteroid collision theory holds that about 65 million years ago, an asteroid with a width of 10 kilometers collided with the earth, and the violent collision rolled up a large amount of dust, filling the earth's atmosphere with dust and gathering into a dust cloud, and a thick dust cloud covered the entire earth, blocking the sunlight and making the earth a "dark" world, which lasted for decades. Hua Feng took the rare initiative to mention the content of the class after class.

Yun Meng nodded, and the thoughts of the two of them went back to a long time ago......

Because of the lack of sunlight, the photosynthesis on which plants depend for survival has been destroyed, a large number of plants have withered and died, and the huge herbivorous dinosaurs consume hundreds and thousands of kilograms of plants every day, and they simply cannot adapt to the changes in the living environment caused by this sudden event, and can only collapse in despair under the torture of hunger;

In 1991, American scientists used radioisotope methods to measure the age of a large crater (about 180 kilometers in diameter) in the Yucatan Peninsula in the Gulf of Mexico to be about 65,051,800 years old.

Judging by the discovery of surface craters, it is possible that three asteroids with a diameter of 500 meters will hit the Earth every million years. Larger asteroids are less likely to hit Earth.

At 7:17 a.m. on June 30, 1908, a large explosion occurred in the Evenki Autonomous Region of Siberia, Russia, which became known as the Tunguska explosion. The explosion was equivalent to 10-15 million tons of *****, burning and falling 60 million trees over an area of 2,150 square kilometers.

Although the cause of the explosion is still a mystery, impact theories such as meteorite impacts, comet impacts, and planetary impacts are still very popular.

Asteroid 4179 is one of the largest asteroids to date close to Earth. It is 4.46 kilometers long and 2.4 kilometers wide, and its shape looks like a multi-tumor peanut. The asteroid will be in close contact with the Earth, which will be only about 0.046 AU, or 6.9 million kilometers, from the Earth. Assuming that it unfortunately touches the Earth, the impact will cause an explosion that is equivalent to 1 trillion tons**.

Asteroid 4179 was first seen on February 10, 1934, when it was recorded as 1934CT, but was soon lost. It wasn't until January 4, 1989, that the French astronomer Christian Porras discovered it again and named it after the Celtic myth of the god of war, Tutatis.

The orbital period of 4179 is about 4 years, so it frequently approaches the Earth, and its closest distance to the Earth can reach 0.006 AU, which is only 2.3 times the distance between the Earth and the Moon. On September 29, 2004, asteroid 4179 was very close to Earth, only 0.0104 AU (four times the distance from Earth to the Moon).

The last time asteroid 4179 approached Earth occurred on November 9, 2008, at a distance of 0.0502 AU. The next time it came close to Earth was on December 12, 2012, at a distance of 0.046 AU.

Radar images of 3D models of different sides of asteroid 4179 show that it is a very irregularly shaped celestial body divided into two distinct lobules, with the largest widths of 4.6 km and 2.4 km, respectively. Presumably, it would have been two different asteroids that at some point combined to form an asteroid likened to a "giant dumbbell". Because it is so close to the Earth when it orbits, the asteroid 4179 has long been included in NASA's "list of potentially dangerous asteroids", and scientists around the world are watching its every move.

On December 12, 2012, the asteroid 4179 Toutatis (shaped like a dumbbell) will make a "close" contact with the Earth with only 0.046 AU.

(Note: The definition of an astronomical unit is determined as: 1495,9787,0700 meters, which is the average distance from the Earth to the Sun.) ）

A British space research expert once said that a huge asteroid named 2002 NT7 will hit the Earth within 17 years, and life on Earth will be devastated. The asteroid is said to be the largest threat to Earth ever detected, with a diameter of about two kilometers and a projected impact speed of 28 kilometers per second.

There are nearly 700 near-Earth asteroids on the danger list, among which, many astronomical experts are concerned about a near-Earth asteroid called "Apophis". Although the danger of "Apophis" hitting the Earth in 2029 has been ruled out, there is still the possibility of a collision with the Earth in 2036.

Scientists have accurately calculated the orbit of "Apophis" through the Arecibo Astronomical Telescope, predicting that the probability of hitting the ground in 2036 is 4 in 1 million, and the probability of hitting the ground in 2068 is 1 in 330,000. In astronomy, this is definitely a very high probability. It's just that "Apophis" is elusive, and the time that can be observed is very limited, usually two to three years, it will appear in our field of vision, and the time is only one or two nights.

An asteroid codenamed 2000SG344 is likely to hit the Earth in 2071, and the probability of it "meeting" the Earth is about 1 in 1,000, and the impact energy is equivalent to 100 Hiroshima***. This asteroid is indeed the most dangerous asteroid ever discovered. Its orbit is very similar to that of the Earth, with a rotation of 354 days around the Sun (the Earth's cycle is 365 days). The asteroid's turn is consistent with that of Earth, and although it will not "collide head-on", it is possible that its orbit will coincide in 2071.

On February 4, 2018, a huge asteroid, numbered "2002 AJ129", will pass by the Earth at a speed of 107826 kilometers per hour at a distance of 4208641 kilometers – a fairly close distance in space. This means that it is nearly 15 times faster than the world's fastest manned aircraft, the North American X-15, which travels 7,300 kilometers per hour.

The asteroid is about 1.1 kilometers wide, longer than the Dubai Tower, which is 800 meters high, and is considered "potentially dangerous" by NASA. NASA does not believe that the asteroid will collide with Earth.

According to astronomers, the probability of an asteroid with a diameter greater than 1 km hitting the Earth is 1 time every 100,000 years, but this alone could destroy the Earth. The probability of a celestial body with a diameter of nearly 10 meters hitting the Earth is only once every 3,000 years. Some scientists believe that the risk of an asteroid hitting the Earth is grossly underestimated.

Sept. 3, 2003 -- Research departments in the United Kingdom and the United States have warned that an asteroid could hit Earth in 2014, albeit with a 1 in 900,000 chance. The UK government's Centre for Near-Earth Object Research said astronomers in the United States had discovered a "massive" and fast-moving asteroid that could hit the ball on March 21, 2014.

In the face of possible asteroid impacts, astronomers from all over the world attach great importance to them and issue wanted warrants for these naughty ghosts.

The PS1 astronomical telescope, which was launched in the United States, is responsible for mapping asteroids with diameters ranging from 300 meters to 1 kilometer near the Earth. A 300-meter asteroid would cause significant regional damage if it hit an inhabited area on Earth, and a 1-kilometer asteroid would cause a global catastrophe.

The PS1 takes a 1,400-megapixel picture of the 36-moon-sized sky every 30 seconds, and collects enough data each night to fill 1,000 DVDs, each of which can be printed as a 300-dpi image that is enough to cover half a basketball court.

While no asteroids attempting to hit Earth have been discovered, astronomers with the PS1 telescope have discovered more astronomical explosions (such as supernova explosions) in a month than the entire astronomical community has discovered in a year.

In fact, astronomers from all over the world have not given up on close observation of near-Earth asteroids.

In 2001, the United Kingdom announced the construction of a new research centre dedicated to the probability of near-Earth asteroids and comets colliding with Earth, in order to provide accurate and objective information to the public. The Centre's mission includes providing information on the number and location of NEOs and assessing their chances of hitting the Earth and causing disasters.

In 2009, NASA launched a new telescope to search for undiscovered objects in the universe, including asteroids and comets that could pose a threat to Earth. The telescope, called the Wide Field Infrared Detector (WISE), will use an infrared camera to detect luminescent and heat-generating objects that other in-orbit telescopes such as Hubble may miss.

Russia has discovered that an asteroid may hit the Earth, the government is considering launching a special spacecraft into space to knock it out of orbit, and Russia is also preparing to invite space agencies from the United States, Europe and China to participate in the "Save the Earth Program."

The Purple Mountain Observatory of the Chinese Academy of Sciences has built a near-Earth object detection telescope, China's first telescope dedicated to searching for near-Earth asteroid killers, and its observation capability ranks first in the country and fifth in the world. With the help of this "discerning eye", the experts of the Observatory have discovered nearly 800 asteroids and obtained international provisional numbers.

Professor Zhou Haizhong, a well-known Chinese scholar, believes that first of all, it is necessary to establish a global information, analysis, and early warning system (an observation network alone is not enough) Second, we should develop a contingency plan for disaster risks, so as to take precautions and prevent disasters before they occur; third, we should be equipped with more advanced observation equipment, train more high-level professionals, and increase the efforts to popularize science. Finally, prepare for disaster prevention and mitigation to reduce the threat of disasters.

Once any institution or individual discovers a "killer" NEO, it should report it to the International Astronomical Union (IAU) in a timely manner, and after verification and confirmation, the International Astronomical Union will report to the relevant departments of the United Nations, and then the United Nations will notify the member states and organize the global scientific and technological forces to take defensive measures.

Although the power of an asteroid impact is comparable to that of a major earthquake or a severe meteorological disaster, it is a major natural disaster that can be avoided by mankind.