A spacecraft travelling with speed V approaches a planet of mass M along a straight line whose perpendicular distance from the centre of the planet is p. When the spacecraft is at a distance c from the planet, it fires its engines so as to multiply its current speed by a factor k (0<k<1), its direction of motion being unaffected. [You may neglect the time taken for this operation.] Find the condition that the spacecraft should go into orbit around the planet.