I assume that NASA measures distances (besides light year) in space using the metric system because that is exact and cientific.
Nautical miles are not part of the metric system, and the metric system is no more "exact and scientific" than any other system of measurement. All are based on arbitrary units and relationships. The metric system does relate different types of measurement in convenient ways (mass, volume, distance), but some of that is accidental. It also uses base 10, which is easy for calculating things in one's head, but which is
less useful for many scientific applications. Ask a mathmetician sometime to explain the advantages of base 12 or 16 which "break up into neater pieces."
Lots of Americans use the metric system every day, especially those in scientific, engineering, or military fields. But most of them still tend to "think" in standard American measurements. An artillery officer or naval gunnery officer might report the distance to an enemy target to his gun-crew down to the meter, then turn to the guy next to him and say, "They're still a mile out of range." And folks rarely whip out laser measuring tools to check things to the milimeter in real life. In fact, given that absolute accuracy is rarely needed and given our fondness for nice, round, numbers, we rarely express
any quantity exactly. Saying the station is "five miles long" doesn't mean "five miles exactly down to the inch", so your metric conversion would be an approximation in any event - even if the show didn't already give you the information you were looking to derive. (And I'll bet even the ISN graphic is rounded off.) Go measure any city's "Miracle Mile" or look up the actual duration of "The Hundred Year's War" or check the start and ending years for "The War of 1812" if you want to get an idea of mankind's devotion to absolute accuracy.
Regards,
Joe