Complex sentence: An asteroid, meteor, or comet would remain intact through the atmosphere by virtue of its shear mass. However, an object smaller than would have to have a strong iron composition to breach the lower atmosphere.
Split into simpler sentences: An asteroid, meteor, or comet this size does not have to worry about making it through the atmosphere intact; its shear mass would get it through. On the other hand, an object that is smaller than would have to have a strong iron composition to breach the lower atmosphere.