No spacecraft has ever visited Pluto.
But that will change in July 2015, when NASA's New Horizons probe will fly within 6,200 miles of the dwarf planet after a nine-year journey. The probe is now just six months away from Pluto and will start preparing for the rendezvous when it wakes up from hibernation this Saturday at 3 pm EST. (Because the spacecraft is so far away, scientists won't receive a confirmation signal that the probe has been activated until 9:30 pm.)
As New Horizons nears Pluto in the coming months, the craft will collect new information about the dwarf planet's geology and atmosphere and take the most detailed photos we've ever seen of it — along with its five moons.
Even though Pluto seems very familiar to us, we know far less about it than any of the planets in our solar system. Two of its moons (Kerberos and Styx) have actually been discovered in the time since New Horizons left Earth in 2006.
In the 1960s and '70s, the 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, this summer, New Horizons will give us a close-up view of Pluto for the first time.
The New Horizons mission
New Horizons was launched in January of 2006, and has now traveled about 2.9 billion miles from Earth, powered by an engine that harvests power from radioactively-decaying plutonium pellets. In total, the mission will end up costing an estimated $700 million.
When the craft was launched, 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. (In fact, Alan Stern, the principal investigator for New Horizons, has argued that Pluto should still be considered one.)
On the way there, in September 2006, the probe flew by Jupiter, taking photos of the planet (including the first close-ups of the Little Red Spot) and its moons, and using Jupiter's gravity to slingshot out toward Pluto.
Since then, the probe has spent most of its time in hibernation mode, so as to conserve energy and extend the life of its hardware.
Beginning on January 15, 2015, as New Horizons gradually nears Pluto, the probe will begin using a number of instruments to gather data. Because of the vast distance between it and Earth, that data will take about 4.5 hours to arrive, and it actually won't be possible to send data at times, based on the probe's orientation.
What scientists hope to learn about Pluto
One of the most exciting things to come out of the mission will be the photos that New Horizons takes of Pluto. For now, the best images we have only show the dwarf planet as a blurry blob:
Starting in April, New Horizons will start taking photos better than the best photos we currently have (taken by the Hubble Space Telescope). As Emily Lakdawalla of the Planetary Society puts it, every photo the probe takes will be the best photo ever taken of Pluto. They may show polar ice caps, mountains, and perhaps even volcanic activity.
New Horizons will also take images of Pluto and its moons using visible infrared and ultraviolet wavelengths of light, which will tell scientists more about their composition.
Other instruments will detect particles escaping from the dwarf planet's atmosphere, while a radio antenna will send signals through it. By analyzing these signals after they pass through the atmosphere and reach radio dishes on Earth, we'll get a better idea of the gases present in the atmosphere. Because the craft will collect so much data — and because it takes so long to send it back — it will actually continue transmitting it until 2016.
Scientists will use this data for a few different things: They'll be able to calculate Pluto's mass more accurately, seek to better understand its geology, study clouds and wind movement on the dwarf planet's surface, and search for as-yes-undiscovered moons or rings.
If the craft is still operational after its Pluto flyby, scientists hope to use it to study more distant objects in the Kuiper belt — a region that contains more than 100,000 rocky objects, including Pluto. It's still uncertain, because it depends locating a suitable object within New Horizons' narrow flight trajectory, but the probe may help us learn more about one or more of these objects sometime between 2018 and 2022.