Make A Wish

Popular Science looks at the science of shooting stars

In the first video we see footage of a fireball generated by a large meteor recently sighted careening over the skies of western Canada. Impressively bright! Since we get only a brief glimpse of the action we’ve also included another amazing video, below, of a meteor streaking over Guadalajara. It’s a common misconception that the heat generated from meteors impacting the atmosphere is due to friction. In fact it’s due to a thermodynamic process known as adiabatic compression. Let’s see how this works.

According to the First Law of Thermodynamics, the change in internal energy (U) of a system is equal to the amount of work (W) done on (or by) the gas plus or minus how much heat (Q) is added to or removed from the system. Mathematically:

ΔU = Q + W

And yes, the First Law is definitely a statement of the fundamental physical principal of conservation of energy.

Anyway, what’s happening here is that, as a meteor encounters the thicker regions of the atmosphere at speeds in the neighborhood of 30,000 miles per hour, the fluid (air) in front of the meteor is compressed rapidly. (Pressure generated this way is referred to in fluid dynamics as ram pressure.) The meteor is doing a huge amount of work on the air. If this work is done rapidly, there is not sufficient time for heat to escape from the compressed mass and the process proceeds “adiabatically.” An adiabatic process is one in which the system is insulated and heat is not exchanged with the surroundings.

Therefore, according to the First Law of Thermo, all of the work done by the meteor in compressing the air goes into driving up its total internal energy and its temperature (which is a measure of the average internal energy of the individual air molecules). The air in front of the meteor reaches temperatures of around 3000 degrees Fahrenheit, which in turn heats up the meteor.

The fireball you see is actually mostly due to the glowing air surrounding the meteor rather than the object itself. This makes sense when you realize that the objects that create the shooting “stars” in a typical meteor shower are generally about the size of a grain of sand, and burn up about 60 miles above the surface of the Earth. These kind of meteors are usually debris left over from a comet, while the more rare and dramatic flame balls shown in the videos are a result of larger rocky chunks (up to several feet across), which probably originate in the solar system’s asteroid belt.

So remember, if you ever see a streak of flame ripping across the sky, don’t freak out: it’s probably just a large meteor adiabatically heating the surrounding air as it blasts through the atmosphere. (Most of these “big uns” never even reach the Earth’s surface to achieve the status of meteorite.) It’s not the end of the world — at least not unless the rock is larger than a few hundred meters in diameter. Then we might be in big trouble; just ask the dinosaurs.

Adam Weiner is the author of Don’t Try This at Home! The Physics of Hollywood Movies.