The ship will use the same amount of energy to decelerate at its
destination. Note that this calculation assumes a perfect efficiency in
converting the energy of matter annihilation into the kinetic energy of
the space ship. The mass consumed is
m = E/c² = (3.6 × 10²²)/(9 × 10¹⁶) = 4.0 × 10⁵ kg
almost half the spaceship mass.