In the 1960s and '70s, NASA's Mariner missions showed us Mars, Venus, and Mercury, and in the 1970s and '80s, the Voyager missions showed us Jupiter, Saturn, Neptune, and Uranus. In much the same way, New Horizons will give us a close-up view of Pluto for the first time on July 14.
New Horizons launched from Cape Canaveral in January 2006. At that time, Pluto hadn't yet officially been demoted from planet to dwarf planet, and the mission was initially billed as a visit to the solar system's only unexplored planet. That's not the only thing that has changed here on Earth since the launch. Two of Plutos five known moons (Kerberos and Styx) have also been discovered since then.
New Horizons will pass within 6,200 miles of Pluto on July 14th. Until this mission, Voyager 1 was the spacecraft that flew closest to Pluto, and New Horizons will be 158,000 times closer than that. It will return detailed images of Pluto and its moons, far better than the blurry pictures our telescopes have managed.
New Horizons began sending back photos and other scientific data shortly after its flyby. But even traveling at the speed of light, it takes about 4.5 hours for those signals to reach Earth. On top of that, the huge 3-billion-mile distance means the signal is extremely faint and must be transmitted very slowly: An image that's 1024 pixels wide takes about 42 minutes to come through.
The spacecraft also collected lots of data on Pluto's temperature, atmosphere, and interactions with the solar wind (the charged plasma released by the sun), as well as the five moons. It's sent back several photos and many scientific measurements, but it'll take 16 months for it to relay everything.
All the while it will be speeding toward an encounter with another Kuiper belt object, probably early in 2019. In fact, thanks to its plutonium power source (an element named for the planet, of course), New Horizons should have enough juice to operate though the 2030s, by which point it will be about twice as far from sun as it is now. Eventually it will escape the solar system entirely, joining NASA’s four other spacecraft that are also headed to or already in interstellar space: Pioneers 10 and 11, and Voyagers 1 and 2.
What We've Learned So Far
We've only received about 1 to 2 percent of New Horizons' data — but it's already revealed all sorts of surprises about Pluto, and created a number of mysteries.
The biggest surprise is the fact that parts of Pluto's surface seem relatively young, with little or no craters. It also has ice mountains that are 11,000 feet tall — as high as the Rockies. Together, these observations tell us that some sort of underlying geologic process is going on, generating fresh terrain and features over time, perhaps even volcanic plumes.
But geologic activity requires some internal source of energy — and previously, scientists assumed Pluto wouldn't have any. Models suggest it's too small to still have large amount of radioactive materials left over from its creation (these materials decay over time, releasing heat).
And it's not orbiting a large planet, which can lead to tectonic activity in places such as Jupiter's moon Europa. That works through a phenomenon called tidal heating, in which the moon is squeezed by the gravity of the planet that it orbits, generating energy. Ultimately, Pluto's activity remains a big mystery at the moment.
The latest data download, meanwhile, revealed that the large, heart-shaped feature on Pluto's surface — nicknamed Tombaugh Regio, after Pluto's discoverer Clyde Tombaugh — has an extremely high concentration of carbon monoxide.
What makes all these findings especially intriguing is that Pluto is our first taste of a much broader class of objects — the thousands of chunks of rock and ice that orbit the sun in a region called the Kuiper belt, beyond Neptune.
That means that if Pluto has, say, an ocean hiding underneath its ice — one possible explanation for its mysterious energy source — lots of other worlds at the outer edge of the solar system could, too.
Additionally, scientists believe that Pluto was created at the same time as the rest of our solar system, from the same materials. It likely formed much closer in to the sun — going through the same early stages of growth as Earth and the other rocky planets — before being flung outward billions of years ago.
This means that all the data collected on Pluto's geology, atmosphere, and moons will help scientists refine their ideas about this early era in our planet's history. "We know that the Earth went through the stage of growth that Pluto stopped at," Alan Stern, New Horizons' principal investigator, told me in April. "This will help us connect the dots."
» Scientific American: "New Horizons Emerges Unscathed from Pluto Flyby"