NASA calls it a cosmic coincidence.

On Monday, NASA's New Horizons spacecraft crossed the orbit of Neptune on its way to Pluto. The celestial milestone occurred on the 25th anniversary of Voyager 2's historic flyby of Neptune.

It's the last major intersection for New Horizons, which is due at Pluto next summer after nearly a decade of travel.

Neptune wasn't exactly close to the spacecraft Monday. In fact, the planet was 4 billion kilometres away.

Scientist Alan Stern of the Southwest Research Institute in Boulder, Colorado, says this will be the first opportunity in a generation to explore a new planetary system up close. New Horizons will study not only mysterious Pluto, but also its moons, some of which might still be hidden.

Neptune and Triton on New Horizons spacecraft

NASA's Pluto-bound New Horizons spacecraft captured this view of the giant planet Neptune and its large moon Triton on July 10. Today, the spacecraft crossed the orbit of Neptune on Aug. 25, 2014 – its last planetary orbit crossing before beginning an encounter with Pluto in January 2015. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)