The scene of comet Shoemaker-Levi 9 hitting Jupiter was very shocking.

Humans live on a metal ball wrapped in rocks, which flies in space at the speed of 107 km per hour. Knowing that the ball is full of people, there is no room left, and human anxiety has not been alleviated. Just like a car running at full speed through a crowded city street, the earth's orbit around the sun is also dangerous, because there are millions of projectiles in its path-from pebbles to boulders the size of Ukraine. In the past 4.5 billion years (about this indescribable time span, we will talk about it later), the earth has been moving at a fast and steady speed under the challenge of these galaxies, so it is not surprising that there are some collisions on the way, even occasional head-on collisions. In fact, such a cosmic serial collision is the core part of the earth story. We can see that since the birth of the earth, the conflict around the earth has been the driving force of the earth's change, thus making it a planet suitable for human habitation and providing us with the elements of life-even life itself. When looking up at the sky, few people will feel that the earth is spinning like a shell, because we are fixed on this planet under the invisible pull of gravity.

Familiar constellations twinkle in the starry night sky, and this little blue dot in space is moving quietly. Any noise, collision and impermanence around seems to have nothing to do with it. However, there are always planets that harass it from time to time.

1994 July 16 is the 25th anniversary of Apollo 1 1 first manned landing on the moon. On the same day, comet Shoemaker-Levi 9 collided with Jupiter, which was completely expected. This comet was discovered in 1993 by American astronomers Eugene Sumek and Caroline Sumek and astronomy enthusiast David Levi. About 60 years ago, this ordinary comet from outer space was absorbed into Jupiter's orbit and began to circle slowly along this orbit, while waiting for an upcoming disaster. However, the nature of this collision is unexpected. First of all, comets are not very big. When comet Shoemaker-Levi 9 approached Jupiter, Jupiter tore the comet into about two pieces with its amazing gravity. These huge fragments are arranged in a row, the small ones are about 300 meters wide and the big ones are about 2000 meters wide. In the next few days, astronomers from hundreds of observatories around the world are observing the impact of these fragments on Jupiter's southern hemisphere. Astronomers witnessed the tragedy through the Hubble telescope: comet fragments kept falling into Jupiter's atmosphere, and each impact produced a huge fireball. Dust clouds mixed with debris rose from Jupiter's surface, leaving large dark scratches. This large-scale celestial collision excited scientists. As an astronomer later said, "we are thinking about observing the sky with a microscope in the future ... but the explosion happened in front of our eyes ... it's unbelievable." The most powerful impact is a piece of debris called "comet nucleus G", which impacts Jupiter with the energy equivalent to 6 million tons of TNT-75 times the energy of current nuclear weapons. Although the size of "comet nucleus G" is only as big as a hill, it can throw a fireball as big as the earth.

The scene of comet Shoemaker-Levi 9 hitting Jupiter is very shocking, and it is a clear and true observation. Until that moment, the corner of space where we live seemed to be a quiet resting place, but now, the earth is very fragile because the surface is not covered. If a comet hits the earth instead of Jupiter, civilization will end. On the contrary, we can see from the safety zone 42 million kilometers away that the universe is actually a violent place.

The article is selected from the BBC popular science trilogy.