<P> All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth . Through these unmanned missions, humans have been able to get close - up photographs of all the planets and, in the case of landers, perform tests of the soils and atmospheres of some . </P> <P> The first artificial object sent into space was the Soviet satellite Sputnik 1, launched in 1957, which successfully orbited Earth until 4 January the following year . The American probe Explorer 6, launched in 1959, was the first satellite to image Earth from space . </P> <P> The first successful probe to fly by another Solar System body was Luna 1, which sped past the Moon in 1959 . Originally meant to impact with the Moon, it instead missed its target and became the first artificial object to orbit the Sun . Mariner 2 was the first probe to fly by another planet, Venus, in 1962 . The first successful flyby of Mars was made by Mariner 4 in 1965 . Mariner 10 first passed Mercury in 1974 . </P> <P> The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973 . Pioneer 11 was the first to visit Saturn, in 1979 . The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980--1981 . Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989 . The Voyager probes are now far beyond Neptune's orbit, and are on course to find and study the termination shock, heliosheath, and heliopause . According to NASA, both Voyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun . </P>

When did exploration to the inner solar system begin