\[\boxed{\mathbf{572}.}\]
\[P_{h} = \left( \frac{R}{R + h} \right)^{2} \cdot p_{0};\ \ \ \]
\[R = 6400\ км;\ \ p_{0} = 50\ кг:\]
\[P = 50 \cdot \left( \frac{6400}{6400 + h} \right)^{2}\]
\[1)\ \]
\[2)\ Вес\ тела\ уменьшится\ вдвое:\]
\[\left( \frac{6400}{6400 + h} \right)^{2} = \frac{1}{2}\]
\[\frac{6400}{6400 + h} = \frac{1}{\sqrt{2}}\]
\[6400\sqrt{2} = 6400 + h\]
\[h = 6400\sqrt{2} - 6400 \approx 2651\ км.\]
\[Ответ:примерно\ 2651\ км.\]