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 planethood. But New Horizons is the fastest space probe ever launched, and it is quickly closing that gap. Next July, its 5 billion-kilometer journey to Pluto will come to an end when it flies by the little guy and spends a frantic few hours quickly gobbling up as much data as possible. Then, the spacecraft will continue on to the icy Kuiper Belt, where more rocky targets wait in the darkness.
New Horizons entered a long period of mostly hibernation about one year after it launched, in 2006. In February 2007, it swung by Jupiter and grabbed a gravity assist from the most massive planet in the solar system. After collecting some data about the giant’s atmosphere, magnetic field, and enigmatic moons, the spacecraft whizzed away and silently slipped into slumber. But it awoke a few times during those seven years so scientists could check that its instruments were still healthy. Before now, Neptune was one of the last things New Horizons saw during a brief period of alertness. It was late August, and the spacecraft had crossed Neptune’s orbit 25 years to the day after Voyager 2 arrived at the ice giant.
This journey to the edge of our planetary system has been in the making for more than a quarter-century, and it promises a wealth of discoveries. Pluto is so far away that even the Hubble Space Telescope can’t make out its surface features. Is it a geologically active world, with geysers? And what about its many moons? There are at least five of them, maybe more, and no one knows what they look like. One of them, Charon, is so big in relation to Pluto that it actually forms a binary planet with the dwarf — the two bodies revolve around a point in the space between them.
There are so many questions waiting to be answered by New Horizons over these next months. But we will have to wait. For now, let the spacecraft shake itself from slumber and prepare to get to work. We have a mission to Pluto!