The first signals were received at the mission's control center at Johns Hopkins University's Applied Physics Laboratory in Maryland via a giant radio antenna in Australia just before 9:30 p.m. ET, nearly four and a half hours after it was sent by the piano-sized probe. It takes that long for signals to travel between there and here at the speed of light.
Later readings confirmed that New Horizons was fully awake.
New Horizons has been spending about two-thirds of the time since its launch in 2006 in hibernation, to save on electronic wear and tear as well as operational costs. Every few months, the spacecraft's systems have been roused to wakefulness for a checkup, or for photo ops such as its Jupiter flyby in 2007.
The probe also has been sending weekly blips known as "green beacons" — to let the mission team know it's not dead, but only sleeping.
From now on, New Horizons will remain awake continuously through its Bastille Day flyby of Pluto and its moons next July 14. After a few weeks of preparation, the probe's instruments will start making long-range observations on Jan. 15.
The spacecraft is currently about 162 million miles away from Pluto, but as that distance shrinks, the observations will get better and better. By next May, New Horizons' images of Pluto should be sharper than the best pictures taken by the Hubble Space Telescope. And in July, the probe may catch sight of the clouds and ice volcanoes that scientists suspect may exist on the dwarf planet.