Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus. In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space. Since then, humans have landed on the Moon during the Apollo program; the Apollo 13 mission marked the furthest any human has been away from Earth at . All eight planets have been visited by space probes; the outer planets were first visited by the Voyager spacecraft, one of which (Voyager 1) is the furthest object made by humankind and the first in interstellar space. In addition, probes have also returned samples from comets and asteroids, as well as flown through the Sun's corona and made fly-bys of Kuiper belt objects. Six of the planets (all but Uranus and Neptune) have or had a dedicated orbiter. See also List of gravitationally rounded objects of the Solar System List of Solar System objects by size Lists of geological features of the Solar System List of Solar System extremes Outline of the Solar System Notes References External links If the Moon were only 1 Pixel: A Tediously Accurate Map of the Solar System (web based scroll map scaled to the Moon being 1 pixel) NASA's Eyes on the Solar System NASA's Solar System Exploration NASA's Solar System Simulator Planetary science Space science Planetary systems with eight confirmed planets