To convert the speed from miles per hour to miles per second, you can use the following conversion:
1 hour = 3600 seconds
Now, divide the speed in miles per hour by the number of seconds in an hour:
\[ \frac{21,000 \text{ miles/hour}}{3600 \text{ seconds/hour}} \approx 5.8333 \text{ miles/second} \]
Rounding this to the nearest mile per second gives you 6 miles per second.
So the correct answer is 6 miles per second.