The accident that happened during take-off might have involved a big piece of ice that forms on the cone of the booster rocket (it holds liquid nitrogen, if I'm correct) before take-off. During take-off, that piece might have come off of the rocket and hit the space shuttle itself (according to CNN).

As for terrorism: the shuttle flew at an altitude of about 200,000 feet (or about 60 to 70 kilometres), which is much higher than any surface-to-air missile can go.