When did the Comet Impact occur?

So far, the only celestial impact event that astronomers have accurately predicted and conducted detailed observations throughout is the large collision between comet S-L9 and Jupiter that occurred in July 1994.

For a week starting on July 16, 1994, astronomers from all over the world and astronomy enthusiasts observed the series of comet Shoemaker-Levy 9 (referred to as S-L9) towards Jupiter. collision. This once-in-a-lifetime astronomical event has aroused deep concern among scientists, because it has brought mankind's research on the impact of comets and asteroids on the earth, as well as the study of Jupiter, into a new stage.

Comet S-L9 is a periodic comet accidentally discovered by the famous American "comet hunters" Shoemakers and Levy. Eugene Shoemaker, born in 1928, was the head of the Astronomy Division of the United States Geological Survey when he retired. He has been searching for new comets and asteroids for more than 20 years and won the National Science Medal in 1991. His wife Caroline, who is 2 years younger than him, is a self-taught astronomer. In 1982, she joined her husband’s astronomical observation work without any pay. Their two children had grown up by this time. . Caroline is as meticulous as a silk and can often notice irregular and tiny spots of light in astronomical films, thus discovering new comets. She discovered 32 new comets in one day, which is still a personal world record.

At midnight on March 23, 1993, this like-minded couple searched for comets with David Levy, an amateur astronomer who assisted them in their work. Two days before that, the weather was bad, the clouds were thick, and they saw almost nothing. They used a 45-centimeter telescope at the Palomar Mountain Observatory in California, USA, to take a few random photos of the starry sky in the direction of Jupiter. Levy, who was born in 1948, owns an astronomical telescope with a diameter of 20 cm. He has discovered 8 comets alone and 13 comets in collaboration with others. That night, the few remaining photographic films they had were slightly "out", and it was expected that the shooting results would be difficult to guarantee. Only at the insistence of the optimistic Levi, the Shoemakers took a few pictures at will.

When she put the developed film under a stereomicroscope, Caroline noticed that the edges of the film were only slightly blurred due to light run-off. Carefully, she discovered that there was a star in an area near Jupiter on the film. String-shaped celestial bodies are obviously different from the usual point-shaped celestial bodies. "It appeared as a faint line with a dense tail, and looked like a squashed comet," she described. The three collaborators discussed the phenomenon for nearly an hour. It was finally determined that this was a newly discovered comet and not an asteroid.

Then, Levy sat in front of the computer and sent an email to Marsden, the head of the Astronomical Telegraph Bureau of the International Astronomical Union, through the computer network, requesting verification of their findings. Soon, Levy also called Scott, an astronomer at the University of Arizona, and asked him to conduct observations and verification of the sky near Jupiter. As a result, their findings were quickly confirmed by clearer CCD images taken by Scott. Fifteen minutes later, Scott confirmed on the phone with Levy that what he had observed was a comet. So Scott immediately wrote a letter to Marsden, confirming that the Shoemakers and Levy were the discoverers of the new comet, and that the comet fragments were distributed in a string.

Later it was learned that in the days before Shoemaker and his colleagues discovered the comet, three astronomical observation groups in Japan, Chile and California, USA, had also observed the comet. The phenomenon was not studied further, and a good opportunity to discover new comets was missed. In recognition of the contributions of the Shoemakers and Levy, the International Astronomical Organization named the comet after the three of them. This is the "S-L9" comet that has attracted worldwide attention this century.

After analyzing and studying the orbit of comet S-L9, astronomers believe that this comet does not orbit the sun like most other comets, but rather orbits Jupiter. It may have turned out to be a small satellite of Jupiter, or a member of the Trojan group of asteroids, which is rich in volatile materials. Some scholars also speculate that it may be an ordinary comet captured by Jupiter.

It is estimated that Comet S-L9 was only 43,000 kilometers away from Jupiter on July 8, 1991. Because it was so close to Jupiter, it was torn apart and disintegrated by the tidal force generated by Jupiter's strong gravity, and became a series of fragments. Based on 156 observations of comet S-L9 in the four months before and after its discovery, American astronomers Yeomans and Chodas calculated its orbit and predicted that these separated comet nuclei would approach again in mid-July 1994. There is at least a 99% chance of a collision with Jupiter. Later, based on further research and calculations, a very exciting and accurate prediction was finally obtained: the series of comet fragments will hit the atmosphere of Jupiter one after another within a few days from July 16 to 24, and Triggering a barrage of fireball explosions deep in the clouds. In the history of modern astronomy, calculating the orbit of a celestial body based on its observed displacement is a very mature business. On July 1, 1993, the "Hubble Space Telescope" operating in space took a clearer photo of the S-L9 comet. The photo clearly showed that each comet fragment was surrounded by a spherical dust cloud. It is estimated that any The diameter of a single fragment does not exceed 4 kilometers. From October 18 to 22, 1993, a special meeting was held in Colorado, USA. At the meeting, some astrophysicists made bold predictions.

According to astronomers’ predictions, the impact point of the comet fragments is on the night side that marks the dividing line between day and night on Jupiter. That is, the initial impact cannot be directly observed on the earth, but people can see it. You can also see the flashes caused by impact explosions on Jupiter's moons, and you can also carefully observe the various scars caused by the collisions, as well as the explosive jets - mushroom clouds - that emerge from the edge of Jupiter. The first comet nucleus (about 1 km in diameter) hit Jupiter at 20:15 GMT on the 16th (4:15am on the 17th Beijing time). Subsequently, at intervals of about 7 hours, 4 hours, 6 hours and 20 minutes, and 4 hours respectively, the other four comet nuclei also collided with Jupiter.

Data collected from space and ground telescopes by the U.S. Space Telescope Science Institute and NASA's Goddard Space Flight Center show that multiple fireballs produced after the first impact stretched for nearly 1,000 kilometers. Theoretical calculations show that the comet nucleus containing rocks and ice hits Jupiter at a speed of up to 210,000 kilometers per hour, which can cause a strong shock wave in the upper atmosphere of Jupiter and may penetrate clouds composed of ammonia and dense hydrogen gas several kilometers below. layer, causing the temperature in the local area at the splash point to instantly rise to tens of thousands of degrees Celsius.

According to Japanese news media reports, the first fragmented comet nucleus (Block A) formed a huge mushroom cloud when it hit Jupiter. The high-temperature gas rushed to an altitude of 1,000 kilometers and remained on the surface of Jupiter. There were impact traces as big as the earth - black spots. The day after the first impact, NASA held a press conference, which Shoemaker and his wife, Caroline, and Levy attended and answered reporters' questions. Shoemaker said that the comet fragments crashed into the atmosphere of Jupiter, causing Jupiter to emit hot gas and rush into space. The spectacular sight of the clouds was very beautiful. Although the collision occurred at the southern end of the back of Jupiter and could not be directly observed from the earth, it Because Jupiter rotates very quickly, ground-based observatories can see changes in Jupiter's clouds in about ten minutes. The diameter of comet fragment A is between 0.8 kilometers and 1.6 kilometers, and the energy of impacting Jupiter is equivalent to the power of 200 billion tons of yellow explosives.

At 7:30 am GMT on the 18th (3:30 pm Beijing time), the seventh comet fragment hit Jupiter. The Nanjing Purple Mountain Observatory used a 60 cm reflection at 7:30 pm on the 19th. Telescopes have observed its "scar" on Jupiter. It is a "big crater" with a diameter of 20,000 kilometers, located at 44°19' south latitude of Jupiter, which is basically consistent with the location predicted in advance.

According to American astronomer Mullen, at 10:20 on the 19th (6:20 pm on the 19th, Beijing time), the fireball produced by the ninth fragment that hit Jupiter was also as bright as Block 7 produces quite a bit. He pointed out that the first time humans used the newly invented telescope to observe Jupiter was in 1610. Historical astronomical data shows that its appearance has not changed much since then.

A raised paraboloid made of dust clouds appeared in the sky above the landing point of the seventh comet fragment, and there was a circle inside the paraboloid. This appearance was vividly compared to an "eye" by astronomers ”, it immediately became the most prominent identifying symbol of Jupiter.

American female astronomer McFadden reported that the traces of Jupiter caused by the seventh fragment are so obvious that even amateurs using ordinary small-diameter telescopes can observe them in a clear night sky.

At 18:12 and 22:00 on the evening of the 20th (Beijing time), two fragments of the comet nucleus of Comet S-L9 hit Jupiter continuously. Between 23:11 and 12 pm that night, another comet nucleus hit Jupiter. This was the 13th impact since the early morning of the 17th. At that time, astronomers claimed that the southern hemisphere of Jupiter had been scarred by 10 consecutive violent "attacks" by S-L9. There were seven large wounds with a diameter of more than 10,000 kilometers, and one wound was estimated to have a diameter of tens of thousands of kilometers, which is much larger than the volume of the Earth. . An observational research report provided by Mr. Fu Chengqi, chief scientist of the S-L9 Comet Project of the Shanghai Observatory, said that due to the continuous bombardment of Jupiter by comets, the time for the atmospheric circulation near Jupiter's 45° south latitude to rotate once was shortened by about 45 minutes. Considering the huge size of Jupiter, it is roughly estimated that the storm speed caused by the impact in Jupiter's atmosphere reaches 100 to 150 meters per second.

The Beijing Observatory and the 22nd Institute of the Ministry of Electronic Industry jointly observed a strong radio burst caused by the impact of comet nucleus No. 15 on Jupiter's magnetosphere in Xinxiang, Henan. In the 25 MHz band, its intensity was 10,000 times greater than the background intensity. . From 18:24 to 29 on the 19th, the radio wave burst caused by the impact of Comet No. 12 nucleus on Jupiter was observed in the 29 MHz band, with the intensity increased by about 50 times. From 21:00 to 22:00 on the 18th, the Xinglong Observatory of the Beijing Observatory used a 60-centimeter telescope to observe a dark spot of about 4,000 kilometers at the impact point of comet nucleus 17 on the surface of Jupiter. At the same time, it used a 2.16-meter telescope to observe the spectrum of the impact point. There are obvious changes in the band from 6640 Angstrom to 6660 Angstrom.

The last fragment of comet S-L9 hit Jupiter shortly after 8:00 GMT on July 22 (4:00 pm Beijing time on the 22nd). Two astronomers from the University of Chicago in the United States transmitted information through a computer network and confirmed that they used an infrared telescope set up in the Antarctic region to observe that the fireball produced by the last fragment of S-L9 (the 21st fragment) hitting Jupiter was not as bright as the previous ones. Some other fragments were so bright after impact. Sugek said at a press conference on the eve of the last comet fragment hitting Jupiter: Comet S-L9 was initially about 10 kilometers in diameter and had a mass of about 500 billion tons. There were at least 21 fragments of the comet. The fragments fall towards Jupiter at a speed of about 210,000 kilometers per hour, releasing a total energy equivalent to the energy produced by the explosion of 40 trillion tons of TNT, and the instantaneous high temperature may be close to 30,000 degrees Celsius.

Scientists used the opportunity of this comet-Jupiter collision to analyze the chemical composition of the atmospheres of comet S-L9 and Jupiter. They have found sodium, sulfur, nitrogen, hydrogen sulfide, hydrogen cyanide and trace amounts of moisture. While observing comet collisions, astronomers found ammonia gas above the impact site, but did not find the expected large amounts of water vapor. This raises a question: Is S-L9 a comet? If it is not a comet, why does it have a comet tail? And why does it disintegrate into 21 pieces?

McFadden of the United States It is believed that finding water is the key to understanding Jupiter's atmosphere. People had expected to see bright white clouds as each fragment hit Jupiter, but what was observed did not match the expectation that water would be found. Some scholars therefore suspect that S-L9 is not a real comet. Comets are generally composed of gas, ice and dust, and often come from the edge of the solar system. Their distinctive feature is that they "grow" a tail when they approach the sun. Asteroids are large rocks that orbit in the inner solar system. They are generally believed to have no water. McFadden said it's also possible that existing instruments aren't precise enough to detect water.

The fireball created by the impact resulted in the formation of a mixture of charged particles that emitted powerful radio signals as they rotated with Jupiter's magnetic field. The flash caused by the collision also made it possible for scientists to conduct chemical analysis, but it will take some time to determine which compounds came from the comet, which came from Jupiter's atmosphere, and which were produced by the fireball. By observing how the dark spots on Jupiter eventually disappear, it may be possible to gain more information about Jupiter's climate and wind direction. The comet collision also produced a huge shock wave on Jupiter. This shock wave covered nearly a quarter of Jupiter's area and moved at a speed of about 734 meters per second.

American astronomer Ranney said that the observation results showed that the comet fragments did not penetrate deeply into Jupiter's atmosphere and liquid hydrogen layer. He said that when the comet fragments hit Jupiter at a high speed of 60 kilometers per second, they were blocked by Jupiter's thick atmosphere.

After the S-L9 comet hit Jupiter, humans had to pay more attention to these comets and asteroids that would pose a threat to the earth. According to astronomers, there are currently about 2,000 comets and asteroids that can pose a threat to the earth. Comets and asteroids, because their orbits may coincide with the Earth's orbit. But it is estimated that only about 100 can be discovered in time. In August 1994, NASA began to catalog comets and asteroids that are larger than 1 kilometer in diameter and may collide with the earth, so that scientists can promptly detect and track these uninvited guests from the sky and protect mankind's beautiful home. 4. The constellation explosion that created 10,000 earths

A rare supergiant star named "RhoCas" 10,000 light-years away from us experienced the explosion on January 7, 2003 A large-scale outbreak occurred. Astronomers say that the debris generated by this explosion is enough to create 10,000 "Earths".