Watching Astronaut Scott Kelly and his crew members emerge from the Soyuz capsule tonight, I'm curious why television experts say their free-fall in orbit is at a whopping 5 miles per second instead of the more surface-appropriate 18,000 miles per hour?
'five' is easier to say and wastes less space than 'eighteen thousand'. With numbers this far out of normal human experience, I don't think people have an easier time understanding 18,000 mph versus 5 miles/s.
Also, in the spaceflight community, using miles/s or km/s is common. Again, brevity plays a role. km/s is also easier to use in calculations (saves having to divide by 3600 before you can get anywhere).
In engineering, it's common to use SI prefixes to shorten numbers. Unfortunately 1 Mm (=1000 km) never got any traction. Prefixes for miles even less so (kilomile?).
On Earth we use "Miles per hour" because it gives us a good idea of how long it will take to get to our destination as well as we are used to car/train speeds in miles per hour.
We do know pretty much how long a second is (a very short time) and can conceptualise how far 5 miles is (a longish distance) so telling us it moves 5 miles in just one second gives a great idea of the ridiculously high speed of the spacecraft.
And even though 18,000 is a very large number, 18,000 miles is almost meaninglessly large as very few of us have experienced that distance, so it doesn't really help to visualise the speed.
If, however, one asked how long it took to orbit the earth, then the answerer might start to talk in miles per hour since the orbit is thousands of miles and we want to know how many hours it will take.
Miles per second, or more commonly, km/second, is used because it makes the math easier. Miles per Hour is used on Earth because it makes the math easier. Common tasks in earth travel are in miles per hour because it takes hours to get from place to place, or at least many minutes. Miles per Hour will allow you to roughly estimate how long it takes to get to places on Earth.
As for space, it makes sense because the units of force are typically measured in seconds, and many of the tasks involved in spaceflight thrusting. The units of thrust are either pound or newtons, both of which convert thrust using seconds. Acceleration typically m/s^2, and so hours makes the numbers very large, besides, no thrust keeps constant for time period of that long.
Bottom line, it makes more sense.