The US National Aeronautics and Space Administration (NASA) has previously announced that the asteroid 2011es4 flew over the earth on September 2, Beijing time. Its diameter is 22 meters to 49 meters, and the fastest speed can reach 8.16 kilometers per second. The nearest distance between the earth and the earth is about 120000 kilometers, less than one third of the distance between the earth and the moon. < / P > < p > “when the news of an asteroid approaching the earth is heard, the media and the public are often very excited, while scientists are generally calm. From the relevant information, it is likely that the asteroid 2011es4 just passed by the earth. In addition, it is very small, and even if it falls on the earth, it will be burned in the atmosphere, causing little impact. ” Dr. Wang Zheng, assistant researcher of the National Center for space science, Chinese Academy of Sciences, said in an interview with science and technology daily. Asteroid impact on the earth not only appears in science fiction movies, but also in the real world. Some of the giant craters on earth are the marks they left when they visited the earth. It is speculated that about 65 million years ago, an asteroid with a diameter of 10-30 kilometers plunged into the earth’s “embrace” at a speed of 20-40 kilometers per second. This extremely violent impact resulted in the earth’s dust blocking the sun for a long time, ending an era in biological history. Compared with the asteroids with a diameter of 10-30 km, the 2011es4 asteroid with a diameter of 22-49 meters is indeed a “small spot”. Some people may wonder, are all asteroids, why the difference in size is so big? Wang Zheng explained that the earliest quasars first formed smaller cores, and then gradually enlarged the surrounding materials by accretion. In this process, planetary stars constantly collide and aggregate to form individual planets of different sizes. Among them, the size of some planets is increasing, and most of the celestial bodies in their orbits are adsorbed on them, such as the eight major planets in the solar system. However, due to the temperature and other factors in the region, some celestial bodies fail to grow enough to gather and empty all the celestial bodies near their orbits, leaving a large number of small bodies, forming asteroid clusters, which are separated by accidental gravitational disturbance There are also some bodies that collide with each other to produce new fragments of different sizes and natural sizes. According to statistics, more than 1.4 million asteroids have been discovered, and this is probably only a small part of all asteroids. About 90% of the known asteroids have orbits in the asteroid belt. < / P > < p > “in fact, what you are most concerned about is those asteroids that may orbit very close to the earth, that is, near Earth Asteroids.” Wang Zheng introduced that in general, near earth asteroids are mainly divided into three categories: Amor type asteroids, Apollo type asteroids and Arden type asteroids. 2011es4 is an Apollo asteroid. < p > < p > amore asteroid, named after asteroid 1221 “Amor”. The perihelions of these asteroids are all outside the earth’s orbit, between 1.017 and 1.3 astronomical units (AU, about 149.6 million km). They are farther away from the apogee and, in general, do not threaten the earth. < / P > < p > the name of the Apollo asteroid is derived from the 1862 asteroid Apollo. When the asteroid Apollo was discovered in 1932, it was less than 0.07au away from earth. The perihelion of these asteroids is in the earth’s orbit, and the apogee is outside the earth’s orbit. The orbits often intersect with the earth’s orbit, and they are also the most dangerous group of asteroids. < p > < p > named after the first discovered asteroid “Arden”, the orbits of these asteroids are always within the earth’s orbit, that is, even if the distance between the apogees is less than 1AU. < / P > < p > what kind of asteroids pose a potential threat to the earth? Wang Zheng said that when the distance between it and the earth’s orbit is less than 0.5au and its diameter is not less than 150m, it will be considered as a potential threat asteroid. These asteroids may change their orbits due to various factors, thus colliding into the earth. < / P > < p > how to prevent asteroid invasion? Scientists have been looking for solutions. At present, the global observation network of near Earth Asteroids has been established to search for and monitor some potential asteroids, which can predict when they will approach the earth months or even years in advance. < / P > < p > at the same time, once the threat really comes, there are also plans to take the initiative. For example, the European Union has proposed a “low earth orbit shield” to defend against asteroids. The purpose of this project is to prevent asteroid impact and destruction by multiple means, such as gravitational impact and traction. < / P > < p > “in fact, many satellites and telescopes have been closely monitoring the movements of near Earth Asteroids. If there are asteroids with greater threat, they can also launch missiles for active defense. So there’s no need to worry about small probability events like asteroids hitting the earth. ” Wang Zheng said. Global Tech