Reverse the Spaceship

Astronauts in a spaceship moving with a constant velocity $v$ observe a massive planet at a great distance also moving with velocity $v$ at an angle $120°$ with the direction of motion of the spaceship. The astronauts have to reverse the direction of motion of their spaceship. Determine how much maximum speed in reverse direction, the spaceship can acquire without expenditure of fuel with the help of gravitational field of the planet?
Answer comes in the form of $\alpha v$

Type your answer as $\alpha=?$