In 1977, the Voyager 2 spacecraft took advantage of an alternate route leaving Saturn that took it on a trajectory past Uranus and Neptune. It didn't reach Pluto. Nothing has. But now, decades later, we're finally on Pluto's doorstep, 75 days from close encounter. And it's going to be excellent.
The New Horizons mission was born from a 2003 National Academy of Sciences directive to study the outer Solar System and the Kuiper Belt, the group of rocky and icy bodies that orbits beyond the orbit of Pluto. New Horizons met that need. It's a flyby mission of the dwarf planet that will go on into the Kuiper Belt, visiting at least two distant bodies and possibly more depending on whether there are other good targets within the spacecraft's reach, the instruments keep working, and the mission gets an extension.
For the moment, New Horizons is just getting to the meat of it's prime mission. Its main goal is to characterize the relationship between Pluto and its largest moon Charon, looking specifically at both bodies' geology, morphology, composition, and atmospheres (assuming Charon has one).
But its the secondary objectives that will get into the fun, nitty gritty details. Like imaging goals. Photographs of both Pluto and Charon will be taken in stereo to measure surface topography, map their terminators, and resolve surface features in unprecedented detail; far more detail than anything Hubble has ever been able to show us. The science teams are also going to be looking for hydrocarbons and nitrites in Pluto's atmosphere, two compounds sometimes associated with the search for life.
Where Pluto's moons are concerned, New Horizons will study the largest moon, Charon, for evidence of an atmosphere, to determine its surface temperature, and to more closely determine its size, mass, and density. And of course, New Horizons will also look for rings and possible new moons around Pluto. We know of five — Charon, Hydra, Nix, Styx, and Kerberos — but we might find more when we're up close.
Of course, as we near Pluto, the question on everyone's mind is over the body's classification: is it a planet or a dwarf planet? Really, classification is just a name, and New Horizons' Principle Investigator Alan Stern doesn't have any qualms with the dwarf planet moniker. He coined the term. And either way, Pluto is still a fascinating body well worth studying. Classifying Pluto as a Kuiper Belt Object almost makes it more interesting. It's not the little oddball at the outer edge of our solar system anymore. Now it's representative of a whole class of bodies to discover, and that's sort of neat.
After nine years and three billion miles, we're finally on Pluto's doorstep. It's going to be a very exciting summer.