As with the MESSENGER probe I discussed last time, New Horizons will be taking awhile to get out to the far reaches of the solar system, arriving in 2015. But while this seems leasurely, it's still the fastest probe to date. New Horizons reached the distance of the moons orbit in a scant 9 hours. For comparison, it took several days for the Apollo astronauts to traverse the same distance.
So what do we know about Pluto currently? The answer is, not much. Even the best telescopes available, ie. the Hubble, can barely show Pluto as anything more than a fuzzy speck.
It was originally discovered in 1930 by Clyde Tombaugh and named by Venetia Phair. It was immediately given the title of planet because Percival Lowell had predicted that perturbations in Neptune's orbit indicated another planet further out. Perturbations in Uranus' orbit had led to the discovery of Neptune. However, Pluto only happened to be near the expect placement of the mystery planet but did not have enough mass to be the actual cause. This perturbation has lead many conspiracy theorists predicting that the missing planet is still out there.
However, this designation has recently become a hot topic of dispute. One of the major reasons for this is the recent discovery or similarly sized objects at even greater distances such as Quaoar (2002), Sedna (2004), and "Xena", formally named (2005). Quaoar and Sedna are slightly smaller than Pluto, while Xena is slightly larger. (NOTE: Xena is only the informal name and yet to be approved by the International Astronomy Union. Its current official designation is 2003 UB313) This has sparked a debate about what should be called a planet.
The general consensus today is that Pluto, if discovered today, would not be deemed a planet and instead would be considered a Kupier Belt Object (KBO). However, the last I heard, Pluto was going to retain its title as a planet for historical reasons. (Astronomers love historical significance which explains why the magnitude system runs backwards as well as being annoyingly logarithmic.)
The Kupier Belt has long been hypothesized as a repository for icy bodies that orbit at great distances from the sun. Although, until recently, these bodies had never been observed. Yet their existance stood on solid scientific ground. The reason for this is because short period comets all had their greatest distance from the sun in around the same region. However, the discovery of objects like Quaoar, Senda, and the tenatively titled Xena, has shown that there truly is a Kupier Belt.
Ironically, several young earth creationist websites still try to use an argument that the solar system must be young since comets would all evaporate if the solar system were older than a few thousand years. To justify this they've had to ignore the Kupier Belt and have previously dismissed it claiming none have been observed.
Eventually, the New Horizons mission will fly by Pluto and head to explore this Kupier Belt. However, it will first drop studying Pluto, it's moon Charon (which is almost the same size as Pluto making it more of a double planet if anything), and its two newly discovered moons.
While it may seem odd that such a small planet could have things orbiting it, it's not uncommon. In fact, even some asteroids, such as Ida and its sattelite Dactyl.
But overall, know one knows what we're going to find at Pluto and beyond.
So that's all of the NASA probes currently headed around our solar system. In my next astronomy lesson, I'll quickly talk about the European Space Agency's Venus Express that was recently launched. From there, I think it might also be fun to touch on some of the earlier probes that have long since disappeared. From there, I expect I'll want to discuss how astronomical images are created and how science is actually gleaned from them. But we'll see where things go.