Many theories predict it was comets that first deposited water, and even the organic material necessary for life, onto the planet Earth. Much research has recently been focused on comets, as these moving 'space rocks' still hold mysteries. NASA has sent a number of missions to comets. AAAS Member Michael A'Hearn has been at the lead of a number of these expeditions. He was the principal investigator for the Deep Impact Mission. He is also a distinguished university professor emeritus at the University of Maryland, where he taught astronomy and continues to research comets.
Using images from NASA, A'Hearn shares the most recent discoveries and realizations about comets, and shows what we still don't know about these moving orbs of ice and rock. (Photos 2-14 courtesy of NASA, Photo 1 courtesy of Michael A'Hearn)