1. Does the Heat of a Falling Fireball Come from Friction?
The Hayabusa 2 capsule is a small container about 40 cm in diameter, 20 cm high, and weighing about 16 kg [7]. When it entered the atmosphere, it was observed as a fireball resembling a shooting star from Earth [7]. Many people believe it glows because it rubs against the air. However, sources explaining re-entry heat state that the main cause is the strong shock wave formed in front of the vehicle, which suddenly compresses the air [2]. This explanation suggests friction is not the primary factor.
Because friction at lower speeds and light emitted by hot gases must be considered separately, we cannot say "friction is zero." Still, the starting point for understanding is that most of the heat comes from compression [2].
2. How Hot Does Air Get When It Is Compressed?
Try pushing down on a bicycle pump with the outlet closed. The air inside shrinks and pushes back strongly. Compressed air also rises in temperature. During re-entry, this happens instantly in front of the vehicle. As air passes through the shock wave, its density increases by about 50 to 100 times depending on speed, and the compressed gas is said to reach over 6,000 K (about 5,700°C) [2].
It is important to distinguish locations when reading these numbers. The temperature of over 6,000 K refers to the gas in front of the vehicle. In contrast, the surface temperature of the Hayabusa 2 capsule’s front face is estimated at around 3,000°C [1]. These are different measurements for different places, so neither number is wrong.
3. Why Blunt Capsule Shapes Reduce Heat
We often think fast-moving objects should have pointed tips. However, capsules returning from space are blunt and rounded. The reason is that a strong shock wave forms away from the vehicle in front of a blunt shape [4]. A thick layer of hot air gets between the shock wave and the vehicle, allowing heat to dissipate into the flow ahead, which reduces the heat transferred to the craft [4].
This idea is introduced as having been thought up by Harvey Julian Allen at NASA’s Ames Research Center from studies of ballistic missile re-entry. Some sources note it was a joint effort with other researchers, so it cannot be attributed to one person alone [4]. In 1958, Maxime Faget proposed re-entering the atmosphere with a simple blunt shape to help astronauts avoid dangerous heating and acceleration. This was a major shift from airplane designs at the time [5]. According to NASA records, Faget argued that this design would allow astronauts to avoid hazardous conditions [5]. This concept was applied to the Mercury spacecraft in America’s first crewed space program.
4. How Eroding Heat Shield Materials Keep the Inside Cool
Even with a blunt shape, the front face can reach thousands of degrees. To handle this, heat shields were made from materials that erode. When heated, the surface burns, melts, or turns into gas, carrying heat away in the process [2]. The Mercury, Gemini, and Apollo capsules used this type of ablative heat shield [2].
For the Hayabusa 2 capsule, the heat shield is made of carbon fiber-reinforced plastic (CFRP) in two pieces: front and back. When high heat melts the surface into charcoal and breaks it down into gas, some of the heat energy is used up, making it harder for heat to reach the inside [1]. Because the front gets hotter, charring progressed more on the front side than the back [1].
During flight measurements, the capsule’s interior was kept within a range not far above room temperature. Officials stated that the inside of the heat shield remained very clean [1]. This contrast between the hot exterior and nearly room-temperature interior shows the effectiveness of eroding heat shields.
5. How Hot Do Crewed Capsules Get When Returning to Earth?
The outer surface of the Apollo command module reached about 5,000°F (about 2,760°C) during re-entry [3]. The outer surface of Orion’s heat shield also reaches about 5,000°F, which is roughly half the temperature of the sun’s surface [3].
The capsule returns at about 25,000 mph (about 40,000 km/h) and slows down to about 300 mph (about 480 km/h) in just a few minutes. Despite this, the back side of the heat shield stays around 200°F (about 93°C) [3].
The material Avcoat is an ablator that burns and erodes in a controlled way during re-entry, carrying heat away from the vehicle. It covers the exterior with blocks 1 to 3 inches thick (about 2.5 to 7.6 cm). NASA reports that this heat shield was developed over about 15 years with 1,000 tests [3].
6. Why the Space Shuttle Used Ceramic Tiles for Reuse
Eroding materials are consumed after one use. The Space Shuttle, designed for multiple flights, used silica (glass-like) ceramic tiles instead of ablative shields [6]. Black tiles withstand up to about 2,300°F (about 1,260°C), and white tiles up to about 1,200°F (about 650°C) [6].
About 20,000-plus tiles were attached. Since the exact number varies by source, treat this as an approximate figure [6]. In 1979, it was found that the tiles lacked sufficient strength to withstand the pulling forces of a single flight. Consequently, in October of that year, the method changed to densifying and strengthening the tiles, with strength tests performed on all attached tiles [6]. About 13% of the tiles failed to meet standards and were replaced during these tests [6].
7. How to Feel Air Compression and Check the Original Sources
You can test compression with an adult’s help by closing the outlet of a bicycle pump and pushing it down slowly. You will feel the air inside shrinking and pushing back, and the pump may feel slightly warm. However, friction from the moving piston is also involved, so this alone cannot prove that heat comes only from compression [1]. Do not push hard or rush; take care to stay safe.
To learn more, you can read Japanese articles analyzing the Hayabusa 2 capsule [1] and comparing them with other Japanese articles introducing the same capsule [7]. You will notice that each article focuses on different points. Opening JAXA’s Japanese pages alongside these may also lead to new discoveries.