# Miles-per-second around the earth vs. miles-per-hour? [closed]

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?

## closed as primarily opinion-based by kim holder, Hohmannfan♦, Fred, TildalWave, PearsonArtPhoto♦Mar 2 '16 at 22:46

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• Km/s, km/h or m/s gives even more sense... – Hohmannfan Mar 2 '16 at 14:03

'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?).

• 1 newton is defined as the force needed to accelarate 1kg at the rate of 1m/s^2. That's why m/s is very commonly used in engineering / science. The km/s is just a derivative, probably to give laymen a better feel of the speed. I don't think it has anything to do with preventing having to say thousand. – ventsyv Mar 2 '16 at 20:50

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.

• I nearly downvoted this simply for both answering a question and participating in closing it (the saving grace is that it actually feels like a good answer to the question). Will you please not do that again? Pick one or the other, but not both; a question is either a good enough fit to be welcome on the site, and if so could be answered, or it isn't and then should be closed, not answered. I can't see any valid argument for both answering and closing (not even really voting to close, but actually closing, given your diamond) the same question within the span of 12 seconds. – a CVn Mar 3 '16 at 8:37