1. How a Stone Entering at 14 km/s Was Found the Next Morning
On the night of February 28, 2021, a fireball appeared in the sky over the UK. The stone was found and collected by a family on their private driveway within 12 hours of the fall [5]. The stone is a type called a carbonaceous chondrite. It entered the atmosphere at about 14 km/s, which is more than 100 times the speed of a Shinkansen. Despite this speed, researchers state that the interior was cold and unchanged [5]. The term "cold" here refers to the researchers' description that the material inside was not altered by heat; it is not a measurement of how it felt to the finder's hand. Because it was collected before getting wet from rain, it could be studied in a state close to how it arrived from space. Why is a stone that passed through hot air not burned all the way through? First, we look at why most stones do not reach the ground.
2. Why Most Space Rocks Burn Up Before Reaching the Ground
Rocks coming from space vary in size from dust to fragments of asteroids. If they burn up in the atmosphere and glow, they are called "meteors"; if they reach the ground, they are called "meteorites" [1]. NASA estimates that about 44 tons of meteoritic material falls to Earth every day, but most of it evaporates in the atmosphere [1]. Most of this 44 tons evaporates in the atmosphere and is not the amount of meteorites that can be picked up [1]. The main cause of the light is said to be not "frictional heat" but rather the air compressed and heated ahead of the stone, and vapor from the melting, evaporating stone. Models show that air heated by shock waves can reach tens of thousands of degrees [2]. However, the stone loses mass and slows down, often breaking apart. To reach the ground, researchers describe the conditions as being large enough, mechanically strong, and having a relatively small entry speed [2]. Small particles like dust can slow down high up before evaporation begins, and it is said they settle slowly without becoming meteors [2]. So, where is it decided whether a stone survives?
3. How a Meteorite Falls Quietly After Its Light Disappears
When the stone enters the denser air in the lower atmosphere, it brakes sharply, and the light stops. There is an explanation that this height is around 30 km, but it differs with each fall. For the Narashino meteorite, it was about 22 km, and NASA materials also describe a height of about 7 miles (about 11 km) [9][3]. For this article, consider it to be "roughly 10–30 km". The flight after the light stops is called "dark flight". During this time, it falls almost solely under gravity and air resistance; deceleration continues, the trajectory approaches a near-vertical fall, and it is carried by winds at high altitudes [2]. The impact speed of meteorites weighing 0.1–100 kg is typically 10–100 m/s [2]. The height where the light ends has great significance. Yasuo Shiba, who studies fireballs in Japan, provides a rough guideline that if the height differs by 10 km, the mass falling to the ground differs by about 1000 times, and the size by about 10 times. For stony meteorites, if the height is 20 km, the mass is about 10 kg; if 30 km, it is about 10 g [6]. Conversely, even if a bright fireball appears, it does not necessarily mean a meteorite fell.
4. Why Only the Meteorite's Surface Melts While the Inside Stays Cold
Freshly fallen meteorites have a black, thin skin on the surface. This is formed when only the surface melts in the latter half of atmospheric entry and then solidifies; it is called a "fusion crust". The thickness varies by meteorite, but it is a very thin layer [3]. Several reasons are listed for why heat does not conduct easily to the inside. Space is very cold, rocks are poor conductors of heat, and the glowing time is short, lasting only a few seconds to 10 seconds. Furthermore, the heated material on the surface is immediately blown away behind the stone (ablation). Therefore, there is little time for heat to transfer to the inside [4]. The dark flight, which lasts about 1 minute after the light stops, also passes through air that is not hot [4]. It is reported that freshly fallen meteorites are at most warm to the touch, and some people drop them because they are cold. However, it cannot be said that they are "always cold". The surface can sometimes be temporarily warm.
5. The Narashino Meteorite: Its Path Across the Night Sky and Where It Fell
Around 2:32 AM on July 2, 2020, a large fireball appeared above the Kanto region. In an apartment building in Narashino City, Chiba Prefecture, a resident found a 63 g stone (about 45×30×25 mm) in the shared corridor in the morning, and a 70 g stone was found in the courtyard. Gamma-ray measurements by the National Museum of Nature and Science confirmed it was a recent meteorite fall [8]. This fireball was captured by automatic cameras in a citizen observation network and cameras from a high school astronomy club. Researchers calculated the trajectory and fall location from the footage [6]. Analysis by Iwata Minami High School in Shizuoka Prefecture showed that the light began at a height of about 84.4 km and ended at about 20.4 km, lasting 7.16 seconds, with an average speed of 13.6 km/s [7]. After the light disappeared, the stone took several minutes to fall. The westerly winds at about 10 km above Japan push the stone east, but while winds can reach 100 m/s in winter, they were 30–40 m/s on this summer day [6]. The fact that the shock wave reached the ground is evidence that meteorite material reached a low altitude at high speed. By morning, a stone from the fireball had been found in the corridor. The height of the light, the direction of the wind, and the numbers in the footage: many clues overlap, and the stone is found.
6. How to Investigate a Fireball and Its Meteorite Yourself
If you see news about a fireball, look for whether the "height where the light disappeared" is mentioned. There is a guideline that the lower the height, the more likely a large stone remains. Even for a bright fireball, a meteorite is not always found. If there is a museum exhibiting meteorites, compare the black, thin skin on the surface (fusion crust) with the color of the broken face. The National Museum of Nature and Science publishes announcements and explanations about meteorites. NASA's Meteors & Meteorites page also has information organizing meteors and meteorites.