That changed seven months later, when astronomers decided that Pluto was not a planet. For the time being, New Horizons is at least the first mission to a dwarf planet -- the new class of objects into which scientists dumped Pluto.

But that doesn't mean it will be the first spacecraft to visit a dwarf planet. Under the new definition (it's still unclear), Ceres may be upgraded from asteroid to dwarf planet, and if NASA's Dawn mission launches as planned next summer, it will arrive at Ceres in February 2015, five months before New Horizons gets to Pluto.

In the meantime, New Horizons' mission remains the same: to unlock one of the solar system's last, great secrets. The spacecraft will cross the orbits of all the planets from Earth to Neptune and fly by Pluto and Charon in July 2015.

Charon had been generally regarded as Pluto's moon, but the new definition of planet may change its status as well. Apparently, not even the astronomers are entirely sure.

Regardless, the seven science instruments on the piano-sized New Horizons probe will shed light on the bodies' surface properties, geology, interior makeup and atmospheres.

Mission Milestones
+ February 2007 Jupiter gravity assist
+ March 2007 - June 2015 Interplanetary cruise
+ July 2015 Pluto-Charon encounter
+ 2016-2020 Kuiper Belt objects encounter

Related Links
Where is New Horizons Now
Lost Among A Million Outer Planets