\[\boxed{\mathbf{570}.}\]
\[m = \frac{m_{0}}{\sqrt{1 - \frac{v^{2}}{c^{2}}}}\]
\[c \approx 3 \cdot 10^{5}\ \frac{км}{с};\]
\[v = \frac{1}{2}\ с.\]
\[m_{1} = \frac{m_{0}}{\sqrt{1 - \left( \left( \frac{c}{2} \right)^{2}\ :c^{2} \right)}} =\]
\[= \frac{m_{0}}{\sqrt{1 - \frac{c^{2}}{4c^{2}}}} = \frac{m_{0}}{\sqrt{1 - \frac{1}{4}}} =\]
\[= \frac{2m_{0}}{\sqrt{3}} = \frac{2\sqrt{3}}{3}\ m_{0}.\]
\[Ответ:m_{0} \cdot \frac{2\sqrt{3}}{3}.\]