New Horizons, a spacecraft launched to Pluto in 2006, has passed the orbit of Neptune, on its way to the dwarf planet.
Neptune was nearly 2.5 billion miles away from the spacecraft when the orbit was crossed by the vehicle.
New Horizons is scheduled to reach Pluto and its system of moons on 14 July 2015. It will become the first spacecraft to ever visit the icy dwarf planet. When the mission was launched in January 2006, Pluto was still classified as a planet - the last one not visited by a robotic vehicle. In August of that year, the International Astronomical Union (IAU) demoted the tiny world to dwarf planet status.
Flyby of Pluto will last several weeks, providing astronomers with their first-ever close-up views of the remote body.
The orbit of Neptune has only been crossed by one other spacecraft - Voyager 2, which made a close encounter with the gas giant in 1989, 12 years after launch. New Horizons easily broke the speed record, covering the distance in just eight years. The new vehicle crossed Neptune's orbit on 25 Aug., the 25th anniversary of Voyager 2's encounter.
"It's a cosmic coincidence that connects one of NASA's iconic past outer solar system explorers, with our next outer solar system explorer. Exactly 25 years ago at Neptune, Voyager 2 delivered our 'first' look at an unexplored planet. Now it will be New Horizons' turn to reveal the unexplored Pluto and its moons in stunning detail next summer on its way into the vast outer reaches of the solar system," Jim Green, director of NASA's Planetary Science Division, said.
When Voyager 2 explored the system of Neptune, astronomers discovered a ring system too faint to be seen from Earth, as well as a giant storm, dubbed the Great Dark Spot. The vehicle also explored Triton, one of the moons in the system. This natural satellite of the gas giant might be similar, in many ways, to the upcoming dwarf planet.
"There is a lot of speculation over whether Pluto will look like Triton, and how well they'll match up. That's the great thing about first-time encounters like this - we don't know exactly what we'll see, but we know from decades of experience in first-time exploration of new planets that we will be very surprised," Ralph McNutt of the Johns Hopkins University, said.
New Horizons will also explore objects in the Kuiper Belt, a disk-shaped collection of icy rocks that orbit the Sun at the outskirts of the Solar System.