I know that were been hit in the very past by these things..but you have to say...why is the earth relatively free from big hits.

All of the other planets and moons get hit through there periods...that includes the present.

I think there are measures that dictate if were going to get hit but not in our life times.

I'd even go to the extent to say that certain measures were put in place from ancient times to protect the earth from a large hits.

I don't think people realise how advanced these ancients were.