The lowest altitude by an Earth observation satellite in orbit is . 

If a satellite has to orbit the earth in a circular path every $6 \mathrm{h}$, at what distance from the surface of the earth should be satellite placed (radius of earth, $R_{\mathrm{e}}=6400 \mathrm{km}$) (assume, $\frac{GM}{4{\pi }^2}=8.0\times {10}^{12} \mathrm{N} {\mathrm{m}}^{-2} {\mathrm{kg}}^{-1}$, where $G$ and $M$ are gravitational constant and mass of earth and ${10}^{1/3}=2.1$),