1in the early morning of June 30, 908, there was a deafening noise in the dense forest 50 kilometers north of Vanavara in the middle reaches of Tunguska River in Siberia, Russia. Mushroom clouds rose from the ground to an altitude of nearly 20,000 meters. Animals in the forest, together with tall and straight trees, were washed away and burned by the scorching air billow. Days of raging fires devoured 2000 square kilometers of virgin forest. The burning flame turns Fiona Fang red within 800 kilometers, and it can be seen from 1500 kilometers away. Within 3000m of the central area, there are more than 200 potholes with a diameter of1~ 50m, and the surrounding trees are dumped radially outward, just like spokes of bicycle wheels. According to a resident 60 kilometers away from the explosion site, he was standing on the porch when he suddenly saw a fireball dragging a cloud of smoke passing by. He felt the heat wave and dazzling fire coming, and the powerful shock wave made him unconscious. When he woke up, he only felt that the earth was shaking, the house was shaking and there was a thunderous noise overhead. "It seems that the end of the world is coming." 160km away, a worker working on a riverbank was thrown into the river by air billow. 240 kilometers away, the strong wind blew off a layer of soil on the ground and piled up a water retaining wall on the angara River. Scientists later estimated that the energy of the explosion was about 500 times that of the Hiroshima atomic bomb.
The explosion in air billow caused a strong airflow across eastern Siberia, and the British Meteorological Center across the English Channel also monitored the violent fluctuation of atmospheric pressure for 20 minutes. Seismographs in Petersburg, 3500 kilometers away, recorded seismic waves, as well as those in Australia, Java and Washington. For two consecutive nights, the sky was exceptionally bright, and even in Scotland, the words in the newspaper could be clearly read at midnight.
1937, an asteroid named "Hermes" with a diameter less than 1 km passed the earth at a speed of 30,000 kilometers per hour, only 780,000 kilometers away from the earth. If Hermes collides with the earth, it will release the energy equivalent to 654.38+ million megatons of bombs.
1947 on the morning of February 12, Boli residents in the far east of the former Soviet Union witnessed another wonder of the sky: a fireball, dragging smoke and sparks, attacked South America in the air, and soon after it disappeared, a huge explosion occurred. The former Soviet Air Force, which was carrying out its mission, also observed this phenomenon, and they helped investigators find the scene where the meteorite fell.
On August 1972, 10, there was a rumbling noise 58 kilometers above California, and a huge fireball flashed across the sky. It turned out that an asteroid with a diameter of 10 meter and a weight of thousands of tons passed through the earth and almost hit the United States. Nasa's infrared detector recorded the event.
1976 On March 8th, at 3pm Beijing time, a rare meteorite shower appeared in the north of Jilin City, Jilin Province, China. Millions of people saw a dazzling fireball fly to the ground, and then split into three fireballs, one formed a full moon, and the other two were fragments the size of football, and then fell to the ground. The roar resounded through the sky, shaking ground waves as high as tens of meters. The clod splashed hundreds of meters away, raising a mushroom cloud-like smoke column as high as 50 meters. A large number of meteorite fragments are scattered in the area of nearly 500 square kilometers in the north of Jilin city.
Meteorite shower, Jilin Province, is scattered in a long and narrow strip area, which is more than 70 kilometers long from east to west, and the widest point in the north-south direction is no more than 10 km. According to research, Jilin meteorite may be a part of an Apollo asteroid with a diameter of 2,200 kilometers. During its interplanetary space flight, it was hit and split into fragments with a diameter of more than 10 meter 8 million years ago, and was hit again 400,000 years ago and split into fragments with a diameter of about 2 meters. This fragment caught up with the earth from behind at 1976 at 15 on March 8, 1976. After entering the earth's atmosphere, intense friction makes it burn. A large part of the meteorite was burned and gasified, and the rest was broken in the atmosphere many times, forming many fragments and falling to the ground. When it reaches the ground, the speed is only a few tens of meters per second. Scientists have done a lot of physical and chemical research and analysis on Jilin meteorite specimens, and Science Press has published a research monograph-1979 Selected Papers of Jilin meteorite shower.
At the beginning of 1989, American scientists announced a news that shocked the world: an asteroid "1989FC", which can generate the explosion energy equivalent to 20,000 hydrogen bombs, will fly about 690,000 kilometers away from the earth on March 22nd of that year. The orbit of this asteroid is quite special. The period of revolution around the sun is 1.03 (3-7 years for most asteroids period of revolution), and it will fly close to the earth once every 33 years. Astronomers in China believe that this asteroid with a diameter of about 300 meters will hit the earth, and if it explodes at high altitude, it will hit a circular impact structural pit with a diameter of about 6 kilometers due to the collision speed as high as 16 km/s, resulting in a strong earthquake with a radius of several tens of kilometers.
1989 In August, an asteroid "1989 Pb" with a diameter of 1 km flew over the earth for 4 million km.
199 1 year 1 month, an asteroid with a diameter of 5 ~ 10 km (199 1BA) passed at a distance of170,000 km from the earth, which is less than this distance.
1989, an astronomer predicted that the moon, the only satellite of the earth, would be hit by a big meteor in 1992, and it was estimated that the moon would lose more than half. Although we didn't experience this disaster in 1992, the devastation on the surface of the moon tells us that people should not only worry about the "sky" of the earth, but also the "sky" of neighboring stars, because their collision will also bring disaster to the earth. In Antarctica and other places, Chinese and foreign scientists have discovered some meteorites that are thought to have reached the ground from moonfall.
The collision of celestial bodies is a normal phenomenon in space. Some scientists even think that meteorites are one of the sources of life on earth. However, the threat of large meteorites to the earth's environment is also obvious. Since the 1980s, people of insight in various countries have been calling on people to face up to the threat from near-earth space.
At present, the near-earth asteroids that threaten human beings are mainly cometary nuclei and asteroids, with cometary nuclei accounting for only a few percentage points. Therefore, asteroids with higher density are more likely to visit the earth than comets. However, it should not be underestimated that although there are not many comets that can pass through the earth's atmosphere and hit the earth, compared with small celestial bodies with the same diameter, comets are faster (the speed of asteroids hitting the earth is 20 km/s, the speed of short-period comets is 30-40 km/s, and the speed of long-period comets is 50-60 km/s), and they release more energy when they hit, which will pose a greater threat to the earth. Some scholars estimate that comet impact accounts for about 25% of impact disasters. In addition, the comet has a loose structure and low strength, just like a bullet hitting a high-speed cotton. It's hard to say how effective it is, which increases the difficulty of defense, at least for now.
Asteroids are wanderers between the orbits of Mars and Jupiter. At present, more than 10000 asteroids have been discovered in the world. Asteroids are small in size and mass, and each has its own orbit. Occasionally, some asteroids will break into the earth's orbit and become near-earth asteroids, becoming the most likely dangerous "whisperd" to visit the earth. According to the statistics of relevant scholars, during the 80 years from 1898 to 1977, * * discovered 43 near-earth asteroids; 28 were found in1978 ~1982; 45 were found in 1983 ~ 1989; 1990, 1 year, there are 14 asteroids discovered. The largest near-earth asteroid discovered so far is "1627 1var", with a diameter of about 8 kilometers, of which 77 are 1 ~ 5 kilometers, and most of them are over 200 meters in diameter. In addition to these near-earth asteroids, astronomers estimate that more than 95% near-earth asteroids and cometary nuclei with diameters larger than 1 km have not been discovered. What is even more worrying is that the search discovery rate of near-earth asteroids with diameters less than 100 meters is less than 0. 1%. It can be seen that human beings know so little about near-earth asteroids and comet nuclei that the defense against space threats is tantamount to automatically giving way to "God rolls the dice". The real threat to mankind comes from ignorance of near-earth space.
On a starry summer night, we often see meteors streaking across the sky. They are all small colored near-earth objects with a diameter less than 50 meters. When they break into the earth's atmosphere, they rub against the atmosphere and glow, and they disappear in an instant. The part of the meteoroid that burns out after passing through the atmosphere and falling to the ground becomes the meteorite displayed in the exhibition hall, such as the Jilin meteorite in China. Once the near-earth asteroids with a diameter of more than 50 meters break into the earth's space, the atmosphere, the natural barrier of human beings, can't stop these arrogant and rampaging intruders, and they will cause disastrous harm to the earth like the "Tunguska Big Bang". The greater impact even produced global disasters such as the extinction of dinosaurs at the end of Cretaceous, which led to the end of human civilization. John, a British scientist, calculated that the 1908 Tunguska explosion, even if it happened in rural areas of the United States, would have killed 68,000 people and caused property losses worth $4.5 billion. If it happens in a country with a large population, such as a densely populated area in China, the consequences will be unimaginable.
The current scientific knowledge is that as long as the diameter of the impactor is between 0.6 km and 5 km, it is possible to cover the whole world in the huge smoke generated by the impact, the sun will be completely blocked, and the earth will be covered in the dark and cold winter for months or even years. According to research, the sudden extinction of dinosaurs may be caused by a meteorite hitting the earth.
180 1 year 1 month 1 day, Italian astronomer Piazi discovered a new planet between the orbits of Mars and Jupiter, which opened the curtain for human discovery and study of asteroids. Since the first Ceres, Pallas Athena, Vesta and so on, more than 400 asteroids have been discovered in the whole19th century. In the 20th century, the discovery of asteroids became more frequent. So far, astronomers have found as many as 5000. Among them, nearly 3000 have been measured. It is estimated that the asteroids discovered by modern astronomical telescopes are less than a few thousandths of the total.
Although they are numerous, these asteroids are very small in size and mass. The largest ceres is only 770 kilometers in diameter, which is smaller than the moon diameter 1/4 and the earth volume 1/450. If you board an asteroid, you can realize at a glance that you are on a planet, and the farther you go around, the more you bend over, and the spherical feeling arises spontaneously. The gemes asteroid discovered in 1937 is less than 1 km in diameter and only half as high as Mount Tai. So up to now, asteroids are all gathered together, and at best they are only as big as a medium-sized satellite, which is really far from the size of a big planet.
Most of such a large asteroid corps are concentrated in the asteroid belt between the orbits of Mars and Jupiter, and few of them are beyond this range. However, there are also a few people who run along elliptical orbits and can go far away from Jupiter and even surpass Saturn's orbit. Recently, they strode into the inner side of the Earth's orbit and even into the orbit of Venus. They are "near-Earth asteroids", which have become unstable elements of the solar family and are likely to be the main "killers" who will harm the earth in the future.
The orbital eccentricity of near-earth asteroids is generally large. Judging from their distance from the earth, the nearest ones are usually hundreds of kilometers, and few are close to a million kilometers. 1937 10 October asteroid Herm, passing around 800,000 kilometers outside the earth, only twice the distance between the moon and the earth. 1in March, 989, an asteroid flew 750,000 kilometers away from the earth and left us. From a broad spatial scale, it may be an exaggeration to say that they are close to the earth. So many asteroids are passing through the space near the earth, which really makes people sweat.
According to experts, asteroids with a diameter of more than 1km and comets with a diameter of more than 600 meters may, in principle, become potential enemies of the earth. According to astronomers' calculations, there are about 1200 ~ 2000 "dangerous molecules" with a diameter of 1 km in the universe.
So what are the chances of a near-earth asteroid hitting the earth? Estimates vary greatly. Some people estimate that it only happens once every hundreds of thousands or tens of millions of years on average, which can be described as "routine" for the long years of the earth of more than 4.6 billion years.
The possibility of it happening every year is one in five hundred thousand.
The probability of 100 in the future is one in 100,000.
-the possibility of one in 200,000 in a person's life.
Like a coma collision, every 1 000 ~ 80 million years1time.
According to the analysis of Ji Chuan, the impact probability of an asteroid with a diameter above 1km is 20000 1 time. In the next 2600 years, there will be five or six asteroids that are relatively close to the Earth, with the nearest distance of10.5 million kilometers, which is about half of the distance between the moon and the earth. Therefore, the so-called alarmism is not unreasonable, and the so-called collision between heaven and earth is not alarmist, which should attract the attention of astronomers and the public.
At present, from this point of view, even if there is a one-in-a-million chance, once a small celestial body attacks the earth, human beings must first predict and calculate the orbit. In this regard, Chinese astronomers announced a scientific prediction through the media: in the next 100 years, the earth will be at peace. Li Qibin, a researcher at Beijing Observatory, and his colleagues found that there will be three asteroids "approaching" the Earth in 2 1 century. The asteroid with the number of 4 179 passed by for the first time at a distance of1.5000 km from the earth on September 29th, 2004. The second time was in 2069. Asteroid 2340 met the Earth at 1 10,000 km, and then it will visit the Earth again at 1.05 million ~1.654,380+100,000 km in 2086.
Such a huge threat exists now, and so on, we can know that such a risk existed in prehistoric times, and this threat is likely to become a reality, which will lead to the destruction and ashes of prehistoric civilizations that amaze people today.
Although most craters on the earth have been destroyed by the hands of nature or swallowed up by seawater, scientists have found more than 120 surviving craters on the earth, and now they are still identifying several new craters every year.
Arizona crater. It was in 1905 that barringer, an American engineer and entrepreneur, first confirmed that it was a crater. Therefore, this crater is also known as Balinger Crater. Not only large, but also strange, it is a good place for local tourism. Balinje Crater has a diameter of about 1.200m, a depth of about 1.80m and a side height of 30-40m, which is almost square. It takes at least a few hours to walk around such a huge crater. Balinje Crater is formed by a "big iron block", with an estimated diameter of 60 meters and a mass of about 654.38+100,000 tons. It hit the earth at a speed of about 20 kilometers per second 20 thousand years ago, causing a huge explosion, thus leaving the earth with a "wound" that is still difficult to heal.
-The Villefort Basin in azania. The basin is located near 27℃ south latitude with a diameter of 70 kilometers. The survey results show that it was formed about 300 million years ago.
-Humbury Crater Group in central Australia. The climate in central Australia is dry, and the population in Henbury is sparse. There are 13 pits, of which the largest 1 is oval, with a maximum diameter of 220m and a depth of12m. The discovery of Henbury Crater was caused by a meteor shower on1930+065438+1October 25th.
-Lake Kali in Salma, Estonia. In the late 1920s, the lake was identified as a crater with a diameter of110m and a depth of 22m. At least six pits were found within 0.75 km around the lake. Salma Island is located on the east side of the Baltic Sea, covering an area of more than 2,600 square kilometers. It is also rare to have a group of craters on an island. The meteor shower that caused the island's crater group broke out about 3500 years ago.
-Lake around Quebec, Canada. At first, an American plane found a particularly round lake in Ngawa, Quebec, and later it was found to be a crater. The diameter of the crater is three times larger than that of the Risana meteorite, and the maximum depth is over 500 meters. It is estimated that the crater is less than 200 million years old.
-China scholars Xu and Yan Gang thought Taihu Lake was also a meteorite impact crater in the 1980s. Some craters were found in China. Duolun crater at the junction of Inner Mongolia and Hebei, with a diameter of 170 km. Shanghewan Crater, Jiutai County, Jilin Province, with a diameter of 30 kilometers. Crater in Shixing County, Guangzhou, with a diameter of 3 kilometers. A crater with a diameter of 6 kilometers was also found in Xinxing County, Guangdong Province.
Recently, some scholars wrote that Sichuan Basin is a huge crater.
Scientists also claimed that there were craters on the seabed, and boldly suggested that many ocean basins on the earth, even the Pacific Ocean and the Gulf of Mexico, had been hit by meteorites. However, this speculation is too inconsistent with the observed facts.
A meteorite thought to come from Mars.
In any case, the collision between celestial bodies and the earth has played an indispensable role in the evolution of the earth, which is a fact recognized and seriously considered by most scientists.
Scientists agree that near-earth small objects such as asteroids, comets and meteoroids pose the greatest threat to the earth. First, register all NEOs with diameters greater than 1 km, and strengthen observation and monitoring. The second is to screen out dangerous near-earth objects in time, and prevent and intercept them in time and effectively. Then it is necessary to build a "space early warning network". Establish astronomical telescopes with a diameter of not less than 2 meters around the world, and study and implement technologies to intercept, destroy and change the orbit of small celestial bodies. The most important thing is to give the early warning time of near-earth asteroids, so that humans may be fully prepared.
It is by no means an overnight success to carry out this work. It needs to attract the attention of the whole world, concentrate the wisdom of all mankind and participate in the action of defending the earth.
1in April 1993, astronomers held a special international conference in Aries, Italy, to discuss the possible impact of small celestial bodies on the earth. The meeting adopted and published the Aries Declaration, which attracted the attention of many countries and organizations.
Strategy 1: let the near-earth small objects deviate from their original orbits.
Firstly, determine the position of near-earth small objects and master their activity characteristics. Try to change the speed of celestial bodies with a small impulse, so that small celestial bodies deviate from their original orbits, so that they will not collide with the earth. Changing the orbit of a celestial body can be achieved by changing its mass. Specific measures are:
(1) laser beam. Through a huge laser device, huge energy is projected to the side of near-earth small objects. The laser beam makes the surface temperature of the projection surface rise sharply, causing it to crack and eventually separate, thus reducing the mass of the celestial body and changing its motion speed and trajectory. Of course, this technical requirement is still difficult to achieve at present.
(2) mass transfer device. Imagine installing a mass transfer device on a dangerous target, so that it can continuously mine on it for several years or decades and throw them into space, and finally achieve the purpose of reducing its mass and changing its trajectory.
Of course, this mechanical task is also very difficult at present.
Strategy 2: Use a nuclear bomb
To detonate a nuclear bomb hundreds of meters above the target, a huge interceptor missile with a nuclear warhead can be used to shoot above the target.
The principle of this method is the same as the "laser beam" mentioned above. The energy of the bomb caused the target side to heat up sharply and split.
Scientists have calculated various possibilities; Its weight reaches 65,438+00,000 tons (objects with a diameter of 65,438+0 km can be deflected) and its volume is as high as 65,438+00,000 tons (objects with a diameter of 65,438+00 km can be deflected). This technology is considered to be the most likely means of successful use.
Strategy 3: Direct Impact
Americans successfully hit a comet with a satellite with a copper warhead, which became a sensational news that year. This makes it possible to destroy or change near-earth asteroids by direct impact. Of course, a lot of research and experiments are needed to realize it.
Strategy 4: Leverage your strength.
This project can be called the big brother of the controversial Star Wars project "Smart Pebble". It suggests that asteroids should be accurately introduced into Earth's orbit and used to attack larger asteroids. This method can't be said to be whimsical, but it has not been carefully studied.
According to Zhao Haibin, a researcher at Purple Mountain Observatory of China Academy of Sciences, since the first NEO telescope in China was put into trial operation in June 2006, 72 1 asteroid has been discovered in more than three years. This is due to the Schmidt 1m NEO telescope dedicated to NEO search, which is mainly used to search for NEOs in the universe that may threaten the safety of the earth.
Zhao Haibin explained that asteroids are mostly composed of stones, metals and dust, ranging from pebbles to mountains, with irregular shapes and dim appearance. They should gather around the sun, in what astronomers call the "asteroid belt" between the orbits of Mars and Jupiter. However, due to its small mass, it is often disturbed by the gravity of the planets, far from the original orbit. In this case, the earth may become their target.
In order to search for NEOs that pose a potential threat to the earth in time, and join the international asteroid observation network, the Purple Mountain Observatory and Nanjing Astronomical Instrument Development Center jointly developed the 1m Schmidt NEO detection telescope. According to Zhao Haibin, up to now, they have reported nearly 300,000 observation data of more than 70,000 asteroids to the International Asteroid Center; Found 72 1 new asteroid with temporary number; Among them, 40 asteroids have accurately determined their orbits and obtained permanent numbers.
It is understood that among the more than 700 asteroids discovered by the Purple Mountain Observatory and temporarily numbered internationally, the near-earth asteroid "2007JW2" is the most striking. It is an Apollo-type near-earth asteroid with the shortest distance of 68 million kilometers from the sun and an estimated diameter of 1400 meters. It only takes 1.54 years to orbit the sun once.
Zhao Haibin said that in the international astronomical community, the ability to discover asteroids is one of the important signs of a country's astronomical observation ability. This work not only contributes to the research of astrophysics, astrochemistry and life sciences, but also helps to prevent the collision between asteroids and the earth.
According to experts, this telescope jointly developed by Purple Mountain Observatory and Nanjing Astronomical Instrument Development Center ranks first in China and fifth in the world. The aperture of the correcting mirror is 1 m, and the aperture of the main mirror is1.2m.. Near-Earth Object Detection Telescope is installed at Xuyi Observatory of Purple Mountain Observatory, located in Tieshan Temple National Forest Park Reserve at the junction of Anhui and Jiangsu provinces.