On November 17, 1998, Voyager 1 became the most distant man-made object from the Sun when it reached a point further away than NASA probe Pioneer 10, which launched in 1972.
Although it is not, as is commonly believed, the only man-made object visible from space, China's Great Wall still amazes with its sheer size, and remains impressive as a testament to architectural will, and to the need to keep out the Mongol hordes.