Retired NASA Astronaut Jon McBride speaks to the crowd in front of the NASA Mars rover concept vehicle during Mars Day at the National Air and Space Museum in Washington D.C., the United States, on July 21, 2017. The 5,500-pound, 11-foot-tall Mars rover concept vehicle is on disday at the National Air and Space Museum as part of its annual Mars Day, which celebrates the Red Planet with a variety of educational and fun family activities. (Xinhua/Ting Shen)
