The Earth sits in a cosmic shooting gallery, constantly being bombarded by bits of space debris. Most of the time, the objects hitting the atmosphere are small, say the size of a grain of sand or even a small rock. They burn up in the atmosphere, leaving a streak as they go -- a shooting star on which you might make a wish.
However, some objects are a bit bigger. Rather than a silent streak as they rocket across the sky, they make a huge bang that can be heard for miles and, in extreme cases, even break windows.
One such object — a fireball — entered the Earth’s atmosphere this week. A fireball is a meteor with a brightness of the planet Venus or greater. Not only are they a sight to behold, but they can make noise, as did the recent one, which resulted in reports of a loud bang over central New York. So how does a fireball make that noise, while most meteors are silent?
To make a long story short, the sound released from the fireball is called a sonic boom. Sonic booms are caused by an object traveling faster than the speed of sound (about 767 mph at sea level). Because of their speed, they emit sound in an unusual way. Rather than producing sound waves in a spherical way like the ones that are created when a pebble is tossed into a pool of water, an object moving faster than the speed of sound emits waves in a cone-like pattern. It’s similar to the wake of a fast motorboat as water is pushed to either side.
When that cone of waves passes over an observer, they hear a sharp report, like a very loud and brief explosion. Since objects from space can hit the atmosphere at speeds of over 25,000 mph, which is far faster than the speed of sound, sonic booms are inevitable.
While meteors and fireballs both make sonic booms, meteors are generally much smaller and therefore quieter as they pass through the atmosphere. They often burn up at much higher altitudes where the air is much less dense, which also reduces the noise made by the meteor’s passage. Strictly from a sound point of view, a meteor is like a rifle bullet so far away that it cannot be heard.
In contrast, fireballs usually begin as bigger rocks, capable of producing a much louder sound and penetrating more deeply into the atmosphere where there is less distance between them and a person’s ear. Those features, combined with the fact that air is thicker at lower altitudes, is what allows fireballs to make a perceptible noise — sometimes a very loud one.
Are fireballs dangerous? Generally not. Even though they can make a startling noise, in their passage, they usually break up at high altitude, miles above the ground, with the altitude dependent on size, speed, and steepness of descent. Even better, most fireballs burn up in the atmosphere. Only a very few actually land.
Fireballs are not all that rare. Several thousand occur every day, spread across the entire globe. But, of course, 70% of Earth is covered by water and even on land the vast majority of the population lives on a very small portion of the globe. Toss in the fact that it’s rare for a fireball to be bright enough to see during the day, and it’s not surprising that they seem rare.
Even among experienced meteor observers — in other words, people who like to watch the night sky — a fireball like the one that passed over New York State will be observed about once per 20 hours of observation. (Because of the relatively small number of people reporting hearing the New York fireball, I assume it was of the dimmest type, the most frequent kind.)
But if seeing the dimmest of fireballs is uncommon, seeing a brighter one — perhaps as bright as a quarter moon — is much more rare, occurring about 0.4% as often. Such a bright fireball would take nearly 5,000 hours of clear night skies to see one — longer if you were unlucky.
Most meteors and fireballs have no impact (if you’ll pardon the pun) on people on the ground. They are fleeting and beautiful and remind us that empty space is not so empty. They are also a reminder that rarely a much bigger object can hit the Earth, from the 2013 Chelyabinsk meteor that broke windows and injured more than 1,000 people to the meteor impact 50,000 years ago that made Meteor Crater, to the huge impact 65 million years ago that killed the non-avian dinosaurs. But those big impacts are very rare. NASA is watching out for them and hopefully we will have some warning before they occur.
So, if you are lucky enough to see a fireball, enjoy it and remember it so you can tell your grandkids. They’ll love it.