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Planet Neptune goes around the Sun once every 164.8 Earth years, compared to once a year for Earth, which is one-quarter Neptune's diameter. It completes an orbit on Jul 12, arriving back in approximately the place where it was first observed officially on 23 Sep 1846. The blue ice giant is remote, but not too remote for visitors. In 1989 the Voyager 2 spacecraft passed some 2983 miles (4800 km) above its north pole. The New Horizons spacecraft is due to pass in 2014, 25 years after Voyager 2.

The spacecraft provided the first close-up views of Neptune and most of its 11 known moons. Scientists believe the planet, which does not have a solid surface, is made up chiefly of hydrogen, helium, water, and silicates. It is covered with thick clouds. Voyager 2 found a dark area made up of violently swirling masses of gas resembling a hurricane. Called the Great Dark Spot, it was similar to the Great Red Spot on Jupiter. But in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished.

For nearly a century Neptune was the planet farthest from the sun, losing that honor when tiny Pluto was discovered by Clyde Tombaugh in 1930. Now that the International Astronomical Union has downgraded Pluto's status, Neptune is once again the farthest known planet from the Sun.