# Calculation of Speed

OK, here’s one that should be simple, but I’m not so sure what the correct answer is. I heard an astronaut say the other day that the space shuttle goes around the Earth at 18,000 miles per hour. My question is, how is that calculated? Is it calculated as the speed if traveled on the surface of the Earth, or is it calculated as the actual speed in the orbit they are in? Consider the surface of any sphere. The route around the surface would be smaller than any orbit traversed. Any thoughts?

Its the tangental velocity. The rate of change from one point to another. In other words its the speed the shuttle would be moving if gravity suddenly ceased to force the craft into angular motion. NASA refers to all speeds in angular units and context. They dumb it down to “speed” for the masses. Its really a meaningless term, but it does give an impressive value for the media. Anyways, if you could imagine the shuttle moving in a circle tied to a string. At any instant the shuttle is moving at a speed perpendicular to the outstretched string. The speed they quote is that speed.