An unstable particle at rest breaks into two fragments of unequal mass. The mass of the lighter fragment is $2.50 \times 10^{-28} \mathrm{~kg}$, and that of the heavier fragment is $1.67 \times 10^{-27} \mathrm{~kg}$. If the lighter fragment has a speed of $0.893 c$ after the breakup, what is the speed of the heavier fragment?