Using the equation: 

(Kepler's constant for earth):  
K = GM/4π^2  
K = 1.01 x 10^13  


Finding time (r = 6.4 x 10^6 m (radius of earth)):

C = r^3/T^2  
T = sqrt(r^3/C)  
T = 5095 s  
T = 85 minutes  

So 85 minutes is approximately the minimum time for a satellite to orbit the earth.  Why can't it just go faster to take less time?  Would that mean it would fall out of orbit?