Q&A: How a Skydiver Broke the Sound Barrier

Question: I just saw in the news that some skydiver broke the sound barrier, setting a world record for height and speed. Could you explain how anyone could fall faster than the speed of sound?  — CH, Hales Corners, WI

Answer: This was a truly amazing feat. I have to say though, your question somewhat stretches the intent of this blog. Nonetheless, a skydiver does qualify as “something you see in the sky, day or night.” So I should probably make an exception here. And it is an interesting question many people have asked since this story hit the news.

For those who haven’t heard … Felix Baumgartner, an Austrian daredevil, just set the world record for the highest skydive. He jumped from a helium balloon at an altitude of 39 km (24 miles). On his way down, it’s claimed he exceeded Mach 1, the speed of sound. Mach 1 = 1,110 kph (690 mph). According to onboard telemetry, his maximum speed was 1341 kph (833 mph), or Mach 1.28. Here’s a nice video portrayal of the event. It’s an animation, but scientifically accurate.

To explain how he could break the speed of sound, we’re going to need a little physics. The graphic at top shows how the air gets less dense with altitude. At 39 km up, the air is very thin. So thin, in fact, that he needed a pressurized suit to protect himself (and be able to breath).

The speed at which a skydiver falls depends on two forces: gravity, which pulls him down, and drag, which slows him. Gravity was essentially constant over the distance of that jump. But the amount of drag depends on air density, speed, and body orientation. And it’s the balance between gravity and drag that determines acceleration and maximum speed.

For a “normal” skydive in the lower atmosphere, those two forces get into balance at a speed of about 130 kph (80 mph). That’s what’s known as terminal velocity. Once you reach it, your speed of fall is constant. But at an altitude of 39 km, the drag is exceeded by gravity for quite some time because the air is so thin. Note how the size of each force is represented by the length of the arrows in the graphic.

I haven’t been able to find the exact altitude at which Baumgartner reached his top speed, but I’m guessing it had to be somewhere in the first few minutes of his total 10-minute descent. By the time he reached “normal” skydiving altitudes, he’d have decelerated to the usual 130 kph or so. Probably slower, because of his bulky (high drag) pressure suit.

It’s worth noting that meteors often break the speed of sound. And they’re falling objects too. Of course, they start from much higher up than 39 km, but the physics is the same. Many meteor observers have reported hearing faint sonic booms from high up in the atmosphere during meteor showers.

So I have no doubt it’s a valid claim. Fittingly, his jump occurred on the 65th anniversary of Chuck Yeager’s breaking the sound barrier in the first aircraft capable of that performance: the Bell X-1.

Next Week in Sky Lights ⇒ How Hurricane Sandy Became a Super Storm

Q&A: Why the Days are Getting Shorter
How Hurricane Sandy Became a Superstorm