I want to create a trailing, ghosting like effect of a sprite that's moving fast. Something very similar to this image of Sonic (apologies of bad quality, it's the only example I could find of the effect I'm looking to achieve)

However, I don't want to do this at the sprite sheet level, to avoid having to essentially double (or possibly quadruple) the amount of sprites in my atlas. It's also very labor intensive.

So is there any other way to achieve this effect? Possibly by some shader voodoo magic? I am using Unity and 2D Toolkit, if that helps.