Visitors view the 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)
