In the afternoon.
Class 15A, Second Year of High School One.
Geography teacher Sun Zhi stood with his back to the class, writing points like perihelion, aphelion, and the obliquity of the ecliptic on the blackboard.
He was of medium height, a bit overweight, in his fifties, with a receding hairline, wearing glasses that perched on the tip of his nose as if they might fall off at any moment.
Suddenly, with a "snap!", Teacher Sūn felt something hit the top of his head. He looked down to see a piece of chalk, about half an inch long.
The students in the classroom burst into laughter.
Teacher Sūn stopped writing and turned to face the fifty students in the class.
"Who just hit me?" Teacher Sūn asked.
The students looked at each other, but no one spoke.
"What, you have the guts to do this, but not the courage to face the consequences?" Teacher Sūn said.
They continued to look at each other, remaining silent.
"You're covering for each other, none of you will speak up, right? Fine, this is easy to handle. After class, each of you will run ten laps on the sports field, 400 meters per lap. That's 4,000 meters. Once you're done, we'll consider this matter settled. What do you say?" Teacher Sūn said.
"Report, teacher. I know who hit you just now," said a female student sitting behind Zhāng Chūpíng, standing up.
"Who?" Teacher Sūn asked.
"It was him!" Chén Xiǎoxiǎo pointed at Zhāng Chūpíng. "I know he likes playing basketball the most. He was practicing his shots. He once said that if the chalk could hit the core of your head, it would count as a score."
The students in the classroom couldn't help but laugh again.
"Very good. Chén Xiǎoxiǎo is always honest. Please sit down," Teacher Sūn said.
Zhāng Chūpíng glared fiercely at Chén Xiǎoxiǎo upon hearing this.
Chén Xiǎoxiǎo stuck out her tongue at him.
"Teacher Sūn, I'm sorry," Zhāng Chūpíng said, standing up reluctantly. "I... it was just a slip-up."
"Zhāng Chūpíng, I know you're famous at school, a slam dunk master. How did your basketball shot end up hitting my head?" Teacher Sūn said.
"I... I won't dare next time," Zhāng Chūpíng said, scratching his hair.
"It's alright, I'm not that rigid. We can just have an open chat today, like good friends. Tell me, what have you been up to lately?" Teacher Sūn said.
"I haven't done anything," Zhāng Chūpíng replied.
"Really? I might have more information about you than you think. Don't assume I know nothing. I heard you often go to the sports field to play basketball after school. Playing basketball is good for your health, and I don't object to it. But, first, it can't affect your studies. Yet, you've failed your geography exams three times in a row. How can this be acceptable? Second, I heard your abdominal muscles are well-trained, and you have a habit of lifting up your number 23 jersey to show off your abs to the girls around you after every successful shot. Is this true?" Teacher Sūn asked.
"Uh—" Zhāng Chūpíng's face turned bright red. "So, Teacher Sūn has been paying quite a lot of attention to me."
"I suspect your 'Peacock Display' move is quite effective in attracting a lot of attention from the opposite sex, isn't it?" Teacher Sūn asked directly.
"Teacher Sūn, it's not that simple. Girls these days aren't that easily fooled," Zhāng Chūpíng said.
The students in the class burst into laughter.
"Someone also told me they saw you at the cinema with a certain girl. Is that true?" Teacher Sūn asked.
Chén Xiǎoxiǎo's eyes widened as she looked at him.
"No... no. I heard that movie, 'The Wandering Earth,' was very popular. I went alone. I happened to meet someone there; it wasn't a planned date. I can barely afford to eat these days, where would I get the extra money to treat someone to a movie?" Zhāng Chūpíng denied.
Chén Xiǎoxiǎo pinched his waist hard from behind, making him wince in pain.
"Is that so? Alright, I'll take your word for it," Teacher Sūn said. "They say laymen enjoy the spectacle, while experts appreciate the substance. You're all in your second year of high school and have learned a lot of geography. Since you mentioned 'The Wandering Earth,' let's talk about it. 'The Wandering Earth,' acclaimed as a milestone in Chinese science fiction cinema, is highly relevant for us high school students. Putting aside entertainment value, the most important reason is this: science fiction is increasingly becoming a hot topic in college entrance exams! Having science fiction literacy is becoming an increasingly important criterion for top universities in selecting talent in the future! Did you know that in 2018, the national college entrance exam for Volume 2 included a science fiction work as reading material for modern prose for the first time? The Ministry of Education, before the Spring Festival, announced 31 national-level competitions for high school students, and the Science Fiction Literature Competition replaced the New Concept Composition Competition, being rigorously included! Therefore, this work is very likely to be featured in college entrance exam questions."
"Teacher Sūn, you actually know a lot about this movie too," Zhāng Chūpíng asked.
"Then, I'd like to ask you, there are many scientific concepts in this movie. Can you explain a few?"
"Helium flash. The sun emits light and heat through hydrogen fusion in its core. When the hydrogen elements inside are nearly depleted, the high temperature and pressure inside will trigger helium fusion. At this point, the sun will undergo a helium flash. Simply put, the sun is aging and exploding. This means the sun accelerates its aging process. After the helium flash, the sun will transform into a vast but dim red giant star. It will then expand and become so enormous it could swallow the Earth. In fact, the sun wouldn't even need to swallow the Earth; the Earth would have vaporized during the helium flash. This is why engines were installed on Earth in 'The Wandering Earth.' Our sun is nearly 5 billion years old, and astronomers estimate it has another 5 to 7 billion years of lifespan. The first helium fusion will occur in 1.2 billion years. By then, humanity will either be extinct or will have long since fled," Zhāng Chūpíng said.
"You're correct. Anything else?"
"Gravity assist. Why was Earth captured by Jupiter's gravity? This is where the gravity assist effect comes in. A gravity assist (also known as gravitational slingshot) uses a planet's gravitational field to accelerate a spacecraft, flinging it towards its next target, essentially using the planet as a 'gravitational booster.' It's somewhat similar to centrifugal force. Taking a spacecraft as an example, when traveling to inner planets, heading towards the sun provides acceleration; for spacecraft traveling to outer planets, even with rocket boosters, flying away from the sun means resisting the sun's gravity, so their speed gradually decreases. Therefore, when a spacecraft flies towards an inner planet (like Venus; Mercury is not suitable as it lacks an atmosphere), this principle is used for deceleration. When flying towards an outer planet, acceleration is needed. The first time a spacecraft approaches a planet from a long distance, below orbital velocity, it uses rocket boosters to ultimately generate kinetic energy exceeding orbital velocity. The resulting motion is as if the spacecraft is bounced off the planet. This series of maneuvers is called planetary gravity assist. In layman's terms, imagine the orbiting planet is a gang boss. You have to match my speed to join the gang. Once you're here, you'll obediently be my subordinate (satellite). If your speed is too fast and you surpass me, get out of here, go as far as you can... Many spacecraft in human history have achieved this, such as Mariner 10, which passed Venus on February 5, 1974. After a gravity assist deceleration, it reached Mercury. Otherwise, due to the sun's gravity, its speed would have been too high, and it would have 'overshot.' The Cassini probe also flew past Venus twice, then traversed Earth and Jupiter before finally reaching Saturn. 'The Wandering Earth' substituted spacecraft with Earth, but the principle remains the same. The only difference is that to overcome the Sun's gravity with the kinetic energy generated by Earth's engines and boosters, it was necessary to enlist the help of the massive Jupiter to slingshot Earth away," Zhāng Chūpíng said.
"That's very well explained. Now, tell me, what is the Roche limit?" Teacher Sūn asked.
"The Roche limit, though it sounds profound and mysterious, is actually quite simple. The Roche limit refers to the minimum distance at which a celestial body, held together by its own gravity, can approach a second celestial body without being torn apart by tidal forces. For example: when the distance between Earth and Jupiter is at the Roche limit, Earth's own gravity and Jupiter's tidal force are equal. If their distance is less than the Roche limit, Earth would tend to break apart and become part of Jupiter's rings, much like how Saturn's rings formed. It's named after Édouard Roche, the first person to calculate this limit. Therefore, when Earth passes Jupiter, it cannot get too close. If it gets too close and exceeds the Roche limit, Earth will break apart. However, if it's too far, the gravitational pull might not be enough to achieve the third cosmic velocity needed to escape the solar system. It would only loop back and continue orbiting the sun. Thus, taking the risk of approaching Jupiter was Earth's only way to escape the solar system. (If you run fast enough, Jupiter can help me escape the Sun's grasp.) It's clear that the Roche limit is the critical point of doom, serving as the common 'countdown to explosion' trope in movies..."
Subsequently, Zhāng Chūpíng went on to explain concepts like engine flameout + earthquake, meteor showers, the five steps of Earth's escape, and what happens to the moon. He spoke fluently and articulately.
"Zhāng Chūpíng, you're amazing. That was very well explained. But I just don't understand, why do you always fail your exams?" Teacher Sūn asked after patiently listening to him.
"Because movies are interesting, and the knowledge in the geography textbook is too dry and boring. My head hurts just thinking about memorizing it," Zhāng Chūpíng said.
"So, you're indirectly saying my geography class is boring? Alright, you're really something! All other students are excused from the 4,000-meter run. But, Zhāng Chūpíng, you must run the full distance for me, not a single lap less!" Teacher Sūn said.
"Yes, I guarantee I'll complete the task," Zhāng Chūpíng replied.