The Collaborative International Dictionary of English v.0.48:

micrometeorite \micrometeorite\ n.
   A meteorite so small that it drifts down to Earth without
   becoming intensely heated in the earth's atmosphere.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

micrometeorite
    n 1: a meteorite or meteoroid so small that it drifts down to
         earth without becoming intensely heated in the atmosphere
         [syn: micrometeorite, micrometeoroid, micrometeor]