If the acceleration due to gravity experienced by a point mass at a height h above the surface of earth is same as that of the acceleration due to gravity at a depth $\alpha \mathrm{h}\left(\mathrm{h} \ll \mathrm{R}_{\mathrm{e}}\right)$ from the earth surface. The value of $\alpha$ will be $\_\_\_\_$ . (use $\mathrm{R}_{\mathrm{e}}=6400 \mathrm{~km}$ )