A NASA spacecraft speeding across the solar system has officially covered half the distance of its trip to Pluto and its moons.
On Thursday, NASA's New Horizons probe zoomed past the 1.48 billion-mile (2.39 billion-km) mark, completing half the travel distance between Earth in 2006, when it launched, and where Pluto will be when the spacecraft arrives in July 2015.
"From here on out, we're on approach to an encounter with the Pluto system," said New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. "The second half of the journey begins."
New Horizons has been billed as NASA's fastest mission to visit another world. It is zooming across the solar system at about 36,000 mph (nearly 58,000 kph). Next month, it will cross the orbit of Uranus.
The spacecraft is headed to study the dwarf planet Pluto and its three moons Nix, Hydra and Charon. New Horizons will not stop and orbit Pluto.
Instead, it will record detailed observations during a flyby and then head out into the Kuiper Belt on the edge of the solar system to study the icy objects lurking in that cosmic realm.
Thursday's distance marker is the latest in a string of mission milestones for New Horizons. In December 2009, the probe hit its first waypoint directly between the sun and Pluto's location. On April 20, the spacecraft will be at the midpoint between the sun and where Pluto will be in 2015.
On Oct. 17, New Horizons will have completed the first half of its decade-long trip, by flight time.
Pluto is an oddball world that was discovered 80 years ago this month by astronomer Clyde Tombaugh. In 2006, it was downgraded from a planet to dwarf planet.
New images of Pluto released earlier this month and taken by the Hubble Space Telescope revealed that the icy world's seasons change and affect its color.
- Top 10 Extreme Planet Facts
- Pluto, Discovered 80 Years Ago, Still a Big Mystery
- Images: New Horizons Bound for Pluto