Planetary exploration is a historic endeavor and a major focus of NASA. New Horizons is designed to help us understand worlds at the edge of our solar system by making the first reconnaissance of Pluto and Charon - a "double planet" and the last of the traditional planets to be visited by spacecraft. Then, as part of an extended mission, New Horizons could visit one or more objects in the Kuiper Belt region beyond Neptune.
Our solar system contains three zones: the inner, rocky planets; the gas giant planets; and the Kuiper Belt. Pluto is one of the largest bodies of the icy, "third zone" of our solar system. In the early 2000s, the National Academy of Sciences placed the exploration of the third zone in general - and Pluto-Charon in particular - among its highest priority planetary mission rankings for the coming decade. New Horizons is NASA's mission to fulfill this objective.
In those zones, our solar system has three classes of planets: worlds of rock and metal (Earth, Venus, Mercury and Mars); the gas and ice giants (Jupiter, Saturn, Uranus and Neptune); and the ice dwarfs of the Kuiper Belt. There are far more ice dwarf planets than rocky and gas giant worlds combined - yet, no spacecraft has been sent to a planet in this class. The National Academy noted that our knowledge of planetary types is therefore seriously incomplete. As the first mission to investigate this new class of planetary bodies, New Horizons will fill this important gap and round out our knowledge of the planets in our solar system.
The ice dwarfs are planetary embryos, whose growth stopped at sizes (perhaps 500 to 2,500 or more kilometers across) much smaller than the full-grown planets in the inner solar system and the gas giants region. The ice dwarfs are ancient relics that formed over 4 billion years ago. Because they are literally the bodies out of which the larger outer planets accumulated, the ice dwarfs have a great deal to teach us about planetary formation. New Horizons seeks those answers.
Pluto's largest moon, Charon, is half the size of Pluto. The pair form a binary planet, whose gravitational balance point is between the two bodies. Although binary planets are thought to be common in the galaxy, as are binary stars, no spacecraft has yet explored one. New Horizons will be the first mission to a binary object of any type.
The Kuiper Belt is the major source of cometary impactors on Earth, possibly like the impactor that wiped out the dinosaurs. New Horizons will shed new light on the number of such Kuiper Belt impactors as a function of their size by cataloging the various-sized craters on Pluto, its moons, and, perhaps, on other Kuiper Belt Objects.
Pluto and the Kuiper Belt are known to be heavily endowed with organic (carbon-bearing) molecules and water ice — the raw materials out of which life evolves. New Horizons will explore the composition of this material on the surfaces of Pluto, its moons, and potentially on other KBOs.
Pluto's atmosphere is actually escaping to space, and nothing like this exists anywhere else in the solar system. It is thought that the Earth's original hydrogen-helium atmosphere may have largely been lost to space this way. By studying Pluto's atmospheric escape, we can learn a great deal about the evolution of Earth's atmosphere. New Horizons will determine Pluto's atmospheric structure and composition and directly measure its escape rate for the first time.
As the first voyage to a whole new class of planets in the farthest zone of the solar system, New Horizons is a historic mission of exploration. The United States has made history by being the first nation to reach every planet from Mercury to Neptune with a space probe. The New Horizons mission to Pluto and the Kuiper Belt - the first NASA launch to a "new" planet since Voyager more than 30 years ago - allows the U.S. to complete the reconnaissance of the traditional solar system.
New Horizons launched Jan. 19, 2006. It passed Jupiter for a gravity boost and scientific studies in February 2007, and will reach Pluto and its moons in July 2015. Then, if NASA approves an extended mission, the spacecraft would head deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region, at least a billion miles beyond Neptune's orbit. Sending a spacecraft on this long journey will help us answer basic questions about the surface properties, geology, interior makeup and atmospheres on these bodies.
A special panel of the National Academy of Sciences that was formed in 2001 to advise NASA on a planetary science strategy for the next 10 years (the so-called "Decadal Survey") ranked the exploration of Kuiper Belt Objects, including Pluto, as its highest scientific priority. The New Horizons mission is NASA's way to implement that recommendation.
Generally, New Horizons seeks to understand where Pluto and its moons "fit in" with the other objects in the solar system. We currently classify the planets into groups. Earth, Mars, Venus and Mercury are the "terrestrial" planets, which are mostly rock and metal objects. In contrast, the "gas giant" planets, which include Jupiter, Saturn, Uranus and Neptune, are dominated by thick, molecular hydrogen atmospheres, and Uranus and Neptune also contain abundant "ices" (actually water, methane, and ammonia).
Pluto belongs to a third category that could be called "ice dwarfs." They have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material (such as frozen water, carbon dioxide, molecular nitrogen, methane and carbon monoxide).
Pluto and Charon are among the very largest objects in the Kuiper Belt, a vast reservoir of icy objects located just outside of Neptune's orbit and extending out to at least 50 astronomical units from the Sun. The Kuiper Belt is thought to be the source of most short-period comets - those with orbits shorter than 200 years - so scientists really want to compare the composition and surface properties of Pluto and its moons to those of cometary nuclei.
Pluto and its moons are truly part of the current "frontier" in planetary science. No spacecraft has ever explored them, yet they promise to tell us much about the origins and outskirts of our solar system.