Thanks for your comments of course this is no more than an conceptual idea. yes lets see what is on mars before we compromise its surface. this is what terra forming does. if we find that the surface is totally locked out then why not try such an experiment. yes the collision will be very substantial if impact occurs at 20,000 K/hr and it weighs some tens of thousands of tonnes then the effect of course will be most interesting. yes material will be knocked off the surface and will cause a fantastic meteor shower here on earth. we just have to make sure that when the collision occurs that mars in on the other side of the sun to us! alternatively we could moderate the imapaction by splitting halley into two or three pieces each one then navigated into a an impact. this way the terra forming excercise could be engineered? Any further thoughts ladies and gentlemen?