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Update, July 15, 2015 at 00:53 UTC: NEW HORIZONS IS ALIVE! The "I'm OK" signal has been received at Earth, letting us know the probe survived closest encounter. That's fantastic news! That's all we'll get for a while; the probe has been very busy taking data while it's still close to Pluto and its moons, and turned away just long enough to let us know it's working. Since it's all one big piece, it can only do one thing at a time, and this time is best spent doing what it does best: looking at Pluto. This was a quick burst of data, and it will send images and more data later tonight.
My very sincere and tremendous congratulations to everyone on the New Horizons missions! This has been an AMAZING day.
At 11:49:57 a.m. UTC on Tuesday, a robotic scout for humanity brushed past the gateway to the outer solar system. At that moment, the New Horizons spacecraft passed 12,500 kilometers from the surface of Pluto.
I write this mere minutes after the event, and we won’t know how the spacecraft did until later tonight (around 9 pm. ET). We must wait for two reasons: One is that it takes 4.5 hours, the time it takes for light, the fastest thing in the Universe, to travel the 5 billion kilometers back to Earth. The other is that right now New Horizons is busy taking vast amounts of measurements of Pluto and its moon Charon, and has its hands full. It will phone home once all the close approach business is settled.
But by all accounts now, the probe is healthy and functioning well. Monday night, it sent back the above picture of Pluto, the highest resolution color photo ever taken of the icy world. NASA released it on Instagram, and it was taken when New Horizons was still 16 hours and 766,000 km from Pluto. The huge heart-shaped feature is prominent, and craters abound. The albedo (brightness) features are fascinating, and no doubt due to the atmosphere of Pluto periodically freezing out in winter and thawing in summer, migrating across the surface over time.
The color is real! The reddish hue is due to tholins, organic (carbon-based) molecules crated when methane, abundant on Pluto, is hit by ultraviolet light from the Sun. This breaks apart the simple molecule and allows it to reform into more complex molecules.
This picture is the highest resolution image we will see from New Horizons until the close approach data are sent back, painfully slowly due to both the great distance and the low data rate of the probe (which, to be fair, is also due to great distance).
Never before have we been where we are now: Standing on the threshold of the outer solar system, awaiting word that the mission was a success. Other probes have gone this far, and farther, but none have gotten this close to an object in the remote and forbidding Kuiper Belt, the region of the solar system beyond Neptune containing billions of these small icy worlds. Pluto may be the king of them all, and today, if all goes well, we will know it for the first time.