1612: Galileo incorrectly records Neptune as a fixed star during observations with his small telescope.
1846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight. Neptune's largest moon, Triton, is found the same year.
1983: Pioneer 10 crosses the orbit of Neptune and becomes the first human-made object to travel beyond the orbits of the planets of our solar system. The spacecraft remains on a trajectory heading towards the red star Aldebaran (in the constellation Taurus) and is expected to pass by it in about 2,000,000 years.
1984: Astronomers find evidence for the existence of a ring system around Neptune.
1989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 kilometers (2,983 miles) above the planet's north pole.
2002: Using improved observing techniques, astronomers discover four new moons orbiting Neptune: Laomedia, Neso, Sao and Halimede.
2003: Another moon, Psamathe, is discovered using ground-based telescopes.
2005: Scientists using the Keck Observatory take images of the outer rings and find that some of the ring arcs have deteriorated.
2011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.
2013: A scientist studying Neptune's ring arcs in archival Hubble Space Telescope images finds a previously unknown 14th moon of Neptune, provisionally designated S/2004 N 1.
2016: Scientists using the Hubble Space Telescope discover a new dark spot on Neptune, the first new atmospheric vortex seen in the 21st century.
“Exploration.” NASA, NASA, 26 June 2019, solarsystem.nasa.gov/planets/neptune/exploration/?
165 Earth years
15,299.4 miles | 24,622 kilometers
“In Depth.” NASA, NASA, 23 June 2020, solarsystem.nasa.gov/planets/neptune/in-depth/.