What Are Shooting Stars? 6 Questions About Meteors and Comets
Shooting stars aren't stars, and meteorites don't reach the ground on fire. Here are six ideas about comets, asteroids and meteorites that people often get backwards.
What a shooting star really is
A shooting star isn't a star. It's a grain of dust that vaporizes about a hundred kilometers up. Almost all of these grains are smaller than a grain of rice.
They enter the atmosphere at between 11 and 72 kilometers per second. At the top end, that's like traveling from Madrid to Toledo in a single second.
Why there are so many in August: the Perseids
In mid-August, Earth passes through the trail of dust left behind by Comet Swift-Tuttle. It's like driving through a cloud of mosquitoes: for a few nights, the windshield fills up.
The peak falls around August 12 or 13. Under ideal conditions there would be about a hundred meteors per hour. Under a real sky, you see far fewer. They're called the Perseids because they appear to spring from the constellation Perseus.
Tradition calls them the Tears of Saint Lawrence, because they arrive around his feast day, August 10. The comet is about 26 kilometers across and won't come near the Sun again until 2126.
Why they glow: compression, not friction
You'll often read online that they glow because of friction with the air. However, the main mechanism is compression.
The meteoroid moves so fast that the air in front of it has no time to get out of the way. It gets squashed against it, reaches thousands of degrees and gradually vaporizes the rock. Something similar happens in a bicycle pump: the bottom part, where the air is most compressed, gets hotter.
Meteorites don't fall in flames
Meteorites that reach the ground glow for only a few seconds, very high up. After that, the air slows them down and they go dark. They cover the last few kilometers in what is known as dark flight, like a thrown stone.
On the outside, they're left with a black crust about a millimeter thick. Underneath, the rock barely heats up, because it conducts heat poorly and there isn't enough time. It's like a finger passing quickly through a candle flame. That's why some witnesses describe freshly fallen meteorites as lukewarm or even cold.
The asteroid belt isn't like in the movies
It's a classic movie scene: spaceships dodging rocks in the asteroid belt, between Mars and Jupiter. The reality is different. There are millions of asteroids, but they're spread across an enormous volume.
There are usually hundreds of thousands of kilometers between one and the next, often more than the distance from Earth to the Moon. Pioneer 10 crossed it between 1972 and 1973 without maneuvering. Altogether they add up to around 3% of the Moon's mass. More than a third of that mass is in a single object: Ceres.
A comet's tail isn't a trail
A comet's tail points roughly away from the Sun. As the comet approaches, the tail trails behind it. As it moves away, the tail goes ahead of it. That's why a comet moving away from the Sun travels chasing its own tail.
It's like your hair when you walk with the wind at your back: it blows toward your face even though you're moving forward. For comets, that wind is twofold, which is why many have two tails. The gas tail, bluish and straight, is pushed by the solar wind. The dust tail, whitish and curved, is pushed by the pressure of sunlight.
Frequently asked questions
What is a shooting star?
It's a grain of dust, almost always smaller than a grain of rice, that vaporizes about a hundred kilometers up.
When do the Perseids peak?
Around August 12 or 13. Under ideal conditions there would be about a hundred meteors per hour, although under a real sky you see far fewer.
Is a freshly fallen meteorite hot?
Not necessarily. Inside, the rock barely heats up, and some witnesses describe them as lukewarm or even cold.
Why do some comets have two tails?
Because the solar wind pushes the gas tail, which is bluish and straight, and the pressure of sunlight pushes the dust tail, which is whitish and curved.
Informative article written with the help of AI tools from the verified script of the video. The video is in Spanish.