 Complete the following phrase:  Currently, 28 astronauts have been honored with the award, 17 of which were awarded posthumously for those who died in American spaceflight. Three died in the Apollo 1 fire, seven died in the Space Shuttle Challenger disaster, and seven in the Space Shuttle Columbia disaster. An asterisk indicates a posthumous award. Four of the twelve moonwalkers received the medal Armstrong, Conrad, Shepard, and Young, but only Neil Armstrong for his lunar mission. The New Nine class of U.S. astronauts the second group of astronauts selected by NASA has the most recipients of the medal, with seven. Second is NASA Astronaut Group 8 which received five awards, four for astronauts killed in the 