Assuming Mars and the comet are perfectly round and have the same density, this comet will change the speed of Mars by about 0.004 feet per year, or about a quadrillionth of it's current speed. A body large enough and fast enough to destabilize a planets orbit (in the short term) would have a pretty good chance of breaking the planet into pieces.

Of planeraty size and mass.

anything bigger than bumble.

Or how fast.