Ask a Physicist: Slingshot Blueshift

Bruce, from the Netherlands, wants to know:When manmade probes are sent out into our galaxy, they are sent in such manner that they take advantage of the 'slingshot' gravitational effect of large orbiting masses (planets) in order to accelerate. We know that that same gravitational force impacts upon light rays, bending them as they pass those large orbiting masses. Therefore, why is the velocity of light not also accelerated by the same 'slingshot' effect?Bruce,Thanks for writing in! This quest...