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The speed of sound is one of the key speed barriers in physics. It is also important because it one of the few natural speed barriers mankind has been able to break. However when we here the term Mach 1 it is easy to assume it is the speed of sound through Earth’s atmosphere. However this term is more loaded than you might think. The truth is that the Mach number is a ratio rather than an actual direct measurement of speed. Why is it that way? It is because the formula for Mach speed numbers is the speed of the object breaking the sound barrier divided by the speed of sound. This ratio of speeds is the Mach number.
Like most ratios there are approximations and categories depending on the speed of the object. This gives us the categories of subsonic, transonic, supersonic, and hypersonic. This categorization system is often used to classify air craft or space craft with different Mach numbers. The minimum requirement is that most of the craft classified have the ability to approach or exceed the speed of sound. The reason is that the speed at which an object travels is important to ability to break the sound barrier. On Earth the speed of sound is 334 m/s or around 750 miles per hour.
Subsonic craft are the exception as they cover just about every air craft that travels below the speed of sound. This includes most commuter airline jets and small air craft. Since these craft never meet or exceed the speed of sound the fall into the subsonic category and have a Mach number less than 1
Transonic air craft are planes that have certain part that move at the speed of sound. Not the entire plane actually moves at the speed of sound. These air craft are considered to be moving at the cusp of the speed of sound so they have a Mach number Mach 1. However most craft that move at such speeds can simply move at supersonic speeds since air compression is high at this speed.
The next category is supersonic air craft. These are craft that moved beyond the compression of air that is the “sound barrier.” These craft generally have a Mach number of greater than 1. Among the craft that have this designation are fighter jets.
The last category is hypersonic this is anything that travels at a speed greater than Mach 3. Very few air craft can move at such speeds. The most common are craft leaving the Earth’s atmosphere.
We’ve recorded an episode of Astronomy Cast all about the space shuttle. Listen here, Episode 127: The US Space Shuttle.