Sentence:  Since there is a continual loss of micrometeoritic material in space because of the radiation effects , there must be a continual replenishment : otherwise , <WSD>micrometeorites</WSD> would have disappeared from interplanetary space . 
Meaning of the word marked with <WSD> tags:  a meteorite or meteoroid so small that it drifts down to earth without becoming intensely heated in the atmosphere