After hibernating for much of the past seven years, the New Horizons spacecraft awoke today on Pluto’s doorstep. Near the margin of the observable solar system, the piano-size space probe is so far from home that even signals traveling at the speed of light take nearly 4.5 hours to get to Earth.

This evening, just before 6:30 p.m. Pacific time, one of those signals broadcast the beginning of a new phase in the spacecraft’s mission: Way out there, in dark, cold interplanetary space, New Horizons is awake and ready to begin studying Pluto.

Today, the probe is still 260 million kilometers from Pluto, the dwarf planet at the center of a simmering debate over the definition of