The radius in kilometer to which the present radius of earth ($R = 6400 \mathrm {~km}$) to be compressed so that the escape velocity is increased 10 times is $\_\_\_\_$.
The radius in kilometer to which the present radius of earth ($R = 6400 \mathrm {~km}$) to be compressed so that the escape velocity is increased 10 times is $\_\_\_\_$.