Well i do believe something crashed into the earth...however i dont believe it had any life on it..at least not anything above a microorganism...
lets look at it...
and hold a few things true...first...there is intelligent life somewhere in our galaxy...and further within our universe
So:
1) we are so far way from other solar systems that an unmanned spacecraft is most likely the only thing that could make it across the distance
2) it is most likely an unmanned spacecraft which crashed at roswell..if anything at all
on the other hand, if there exists a race of intelligent beings somewhere else...they may have developed methods of travel which we are only beginning to look at...The Einstein-rosenburg tunnel for instance...if so...then yes we could have been visited countless times as it would take zero travel time to reach us
just my two cents...
