"Meteors" and "spacecraft", the brave ones flying through the air

Meteor showers have always been a romantic and mysterious existence in the universe. Many people look forward to witnessing the beauty of meteors streaking across the sky. Generally, there are still many meteor showers that can be observed at the end of the year, such as the Orionid meteor shower in November and the Geminid meteor shower in December.

When watching these meteor showers, have you ever thought about a question: Why do meteors get hot and glow?

Beautiful and charming meteor shower (Photo source: veer Gallery)

Many people may think that the light of a meteor is caused by the friction between the meteorite and the air in the atmosphere. In fact, this is largely wrong. In fact, most of the heat generated by the meteorite after entering the atmosphere is generated by the compression of the air, and the heat generated by the friction of the air is only a small part. The heat generated by the meteorite compressing the air eventually causes the meteorite to burn and glow, forming a short-lived meteor. Most meteorites eventually burn out under high temperatures. Let's explain this process below.

After entering the atmosphere, the meteorite compresses the air in front of it, causing the temperature of the air in front of the meteorite to rise rapidly to several thousand degrees, and then ignites the meteorite, causing it to glow and become the meteor we can see. In other words, it is the compressed air that creates the beautiful "last song" of the meteor, not the friction of the air.

When a meteorite enters the atmosphere, the first part to burn is the part where the air in front of the meteorite is most compressed (Image source: https://sputniknews.com/science/201804291064008986-five-asteroids-heading-past-earth-today/)

Why does compressed air generate a lot of heat? This is actually related to the law of conservation of energy. When a meteorite enters the atmosphere, its speed is very fast (usually tens of kilometers per second). At this time, the air in front of the meteorite cannot be quickly "squeezed" around the meteorite, so it will be compressed quickly. According to the law of conservation of energy, the work done on the compression of the gas will be converted into the internal energy of the gas, which will then manifest as an increase in the temperature of the gas.

This phenomenon is very common in daily life. For example, when we use a pump to inflate a tire, the air temperature in the pump will rise because we are compressing the gas to do work, so the pump will heat up. Similarly, because the air in front of the meteorite is compressed very strongly, the temperature will also rise very high, usually around 2000 degrees Celsius. At such a high temperature, most meteorites will be burned, and life on Earth is therefore effectively protected.

Of course, there are still a few relatively refractory meteorites that fall to Earth before burning out. These are the meteorites we see in various space museums. Such meteorites are generally very rare and are very valuable scientific research materials.

When various man-made spacecraft (including artificial satellites, manned spacecraft, and even space stations) return to the atmosphere, they emit light and heat. The principle is similar to that of meteorites. Their efficiency in compressing air is even higher than that of meteorites. Here we use the return capsule of a manned spacecraft as an example. The shape of the return capsule of a manned spacecraft is generally as follows:

The return capsule of the Apollo lunar spacecraft (Image source: http://www.pinsdaddy.com/apollo-command-module-model_3Zsivj3AOYNXNR0BgInRlmH9JmWQBRPkGA8FmDvWHbQ/)

Or something like this:

The return capsule of the Shenzhou 10 manned spacecraft (Photo source: People's Photo Network, photo by Fang Yang)

Although they may differ in details, they all look similar - small on top and large on the bottom, with a very blunt surface at the bottom. Such a blunt surface ensures that the heat generated by the compressed air is mainly concentrated on the blunt side of the spacecraft, and will not be lost to the back of the spacecraft, thereby effectively protecting the astronauts or other scientific equipment behind. Therefore, the return capsule of any manned spacecraft is now designed to be blunt. When the return capsule returns to the atmosphere, its heating conditions are roughly as follows:

Schematic diagram of compressed air in the return capsule (Image source: https://www.quora.com/Why-does-a-spacecraft-heat-up-during-reentry)

It can be seen that its heat is mainly concentrated in the front part, so the astronauts at the back are effectively protected. Of course, after the return capsule enters the atmosphere, it will produce friction with the atmosphere, which will definitely generate some heat, but this part of heat is negligible compared to the heat generated by compressed air. In addition, the outer shell of the return capsule of the manned spacecraft is made of fire-proof and high-temperature resistant materials, so it will not burn like a meteorite, but the high temperature will still scorch the outer shell of the return capsule.

As shown in the picture below, there is the return capsule of my country's Shenzhou manned spacecraft. You can compare it with the return capsule before entering space in the picture above, and you will find that this is a "scorched" return capsule.

The Shenzhou return capsule after returning to Earth (Image source: https://commons.wikimedia.org/wiki/Shenzhou)

For some other spacecraft that are not flame-resistant, although the shape and return capsule are different, the principle is almost the same. After entering the atmosphere, after a period of time, due to the high temperature generated by the compressed air, the entire spacecraft will burn. For example, when the former Soviet Union's "Mir" space station and China's "Tiangong-1" space station crashed, most of the parts were burned to ashes in the atmosphere, and only a small part was scattered in the Pacific Ocean.

Some students may ask, why does the spacecraft not heat up when leaving the earth? The principle is actually very simple, because various spacecraft (including manned spacecraft, space stations, satellites, etc.) are carried by rockets when leaving the earth. The shape of the rocket is streamlined, which can easily "squeeze away" the air in the direction of travel, and the heat generated by friction is relatively small, so the rocket does not experience a very high temperature test when leaving the earth.

Long March series rockets. It can be seen that the rockets all have pointed heads. One of the purposes is to better expel the air in the direction of movement and reduce resistance (Image source: http://calt.spacechina.com/n482/n498/index.html)

Of course, we can also use rockets to slow down the return capsule on its way back to Earth, but that means doubling the fuel, which not only increases the cost of the launch, but also seriously reduces the rocket's payload, which is a thankless move. Besides, nature has already provided us with such a good free "brake" - the atmosphere, so why not use it?