In all kinds of science fiction and movies, it is common to use spaceships or giant rockets to crash asteroids threatening the earth. But does it really matter? The National Aeronautics and Space Administration (NASA) will send a special spacecraft into space at the end of this month for practical verification by actually hitting an asteroid. The US "space" website said on the 6th that there are at least 25000 asteroids threatening the earth in space. Lindley Johnson, head of NASA's Planetary Defense Office, told reporters: "we don't want to put the earth at risk of being hit by planets at any time, so we must carry out relevant crash tests." it is said that NASA's "Planetary Defense" mission is named "double planet reorientation test (dart)", The cost of the whole project is as high as $330 million. According to the plan, the special spacecraft weighing about 550 kg will take off on SpaceX's Falcon 9 rocket on November 23 local time, and hit the target asteroid at a speed of 24000 kilometers per hour in space about 10.94 million kilometers from the earth before the end of September next year. Considering the latter's diameter of about 160 meters, it is expected that the impact will be irrelevant to it and can only change the flight route at most. But that's what scientists need. It is reported that the impact on the asteroid trajectory and the subsequent reaction can be observed by astronomical telescopes on earth. The spacecraft will also release microsatellites with cameras before the impact, and can also record the whole process of the impact. After this test, scientists will know whether the impact can change the asteroid trajectory and the preparation time required. (Xinhua News Agency)