Another milestone passed! Today NASA's New Horizons spacecraft is 15.96 astronomical units (about 2.39 billion kilometers, or 1.48 billion miles) from the Sun - putting it halfway between Earth's location on launch day in January 2006, and Pluto's place during New Horizons' encounter with the planet in July 2015.

"From here on out, we're on approach to an encounter with the Pluto system," says New Horizons Principal Investigator Alan Stern, from the Southwest Research Institute. "The second half of the journey begins."

This is rare territory; New Horizons is just the fifth probe, after Pioneers 10 - 11 and Voyagers 1 - 2, to traverse interplanetary space so far from the Sun. And it's the first to travel so far to reach a new planet for exploration.

Humming along at more than 16 kilometers per second - more than 36,600 miles per hour - the spacecraft will next cross a planetary boundary in March 2011, when it passes the orbit of Uranus. Follow the journey.