WASHINGTON, USA, The American ex-astronaut, David Walker, who performed four space shuttle missions, one of which launched "Magellano", in 1989, mapping Venus' surface, died. Walker was 56 and had cancer. The astronaut was employed by NASA in 1978, to direct shuttle's missions. After the first mission, in 1984, in which he piloted the space shuttle, he became the commander of the missions operated...