“New Horizons” Reaches Pluto


Artist's conception of "New Horizons at Pluto.

Artist’s conception of “New Horizons at Pluto.

The “New Horizons” spacecraft is a major NASA program to complete the initial exploration of all of the major bodies of the solar system. It was designed to help understand worlds at the edge of our Solar System by making the first reconnaissance of Pluto and the Kuiper Belt, the last major section in our solar system to be visited by spacecraft. It arrives at its closet approach to Pluto on July 14, 2015, “Bastille Day.” Is that a significant happenstance? Will it signal a major transformation in the same what the storming of the Bastille did for France?

Since no planetary spacecraft had been sent to Pluto or the Kuiper Belt, “New Horizons” was launched aboard an Atlas V rocket from Cape Canaveral Air Force Base, Florida, on January 19, 2006, and conducted a Jupiter flyby 13 months later to gain further acceleration.

The half-ton spacecraft contains scientific instruments to map the surface geology and composition of Pluto and its three moons, investigate Pluto’s atmosphere, measure the solar wind, and assess interplanetary dust and other particles. After it passes Pluto, controllers plan to fly the spacecraft by one or two Kuiper Belt objects. New Horizons carries several souvenirs from Earth, including some of the remains of Clyde Tombaugh (1906–1997), discoverer of Pluto.

Then, as part of an extended mission, “New Horizons” should visit one or more objects in the Kuiper Belt region beyond Neptune.

Artist’s impression of how the surface of Pluto might look. The image shows patches of pure methane on the surface. Credit: ESO/L. Calçada

Artist’s impression of how the surface of Pluto might look. The image shows patches of pure methane on the surface. Credit: ESO/L. Calçada

The Kuiper Belt, named astronomer Gerard Kuiper who theorized its existence, was not confirmed until the 1992 detection of a 150-mile wide body, called 1992QB1 located at the distance of the suspected belt. Several similar-sized objects were discovered thereafter, confirming that the belt of icy objects Kuiper has predicted did indeed exist. The planet Pluto, discovered in 1930 by Clyde Tombaugh, is only the largest member of the Kuiper Belt. Moreover, Pluto’s largest moon, Charon, is half the size of Pluto and the two form a binary planet, whose gravitational balance point is between the two bodies. Other named objects soon joined Pluto, including 1992 QB1, Orcus, Quaoar, Ixion, 90377 Sedna, and Varuna.

The New Horizons timeline.

The New Horizons timeline.

The discovery of these many objects, nearly as large as Pluto and occupying the inthe outer solar system, led the International Astronomical Union (IAU) in 2006 to redisignate Pluto from a planet—there would henceforth be eight of them in the Solar System—and call it by the new designation of “dwarf planets.” The first members of the “dwarf planet” category were Ceres, Pluto, and 2003 UB313. IAU members deliberated on this long and hard before reaching a definition of planets that included the following criteria: (a) it is in orbit around the Sun, (b) it has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) it has cleared the neighborhood around its orbit.

Its members also specifically commented that “dwarf planet” status of Pluto would hereafter be recognized as a critical prototype of this new class of trans-Neptunian objects. While this decision remains controversial, it represents an important recent step in understanding the origins and evolution of the solar system.

A protest of Pluto's demotion soon after the 2008 IAU decision.

A protest of Pluto’s demotion soon after the 2008 IAU decision.

By that time, of course, “New Horizons” was on its way to Pluto. Controversy remains on this “demotion,” and “New Horizons” lead scientist Alan Stern has been vocal in his criticism of the IAU’s decision.

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2 Responses to “New Horizons” Reaches Pluto

  1. Jordan says:

    Having seen the photos, it’s amazing how much more detail we now have of Pluto and its moon Charon. I do think it’s funny in that before New Horizons could reach Pluto, Pluto was “demoted” like that! Hopefully these photos will lead to more understanding of Pluto.

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  2. mike shupp says:

    Color me old-fashioned.

    The points that matter, it seems to me, are that Pluto is a reasonably large (i.e., spherical) and reasonably isolated (not in a “Belt”) object circling the sun, and detected from earth before 4 October 1957. By classical definition, as used since the time of Galileo and Newton, Pluto was a planet and all reasonably educated individuals would have regarded it as such.

    I think it follows that we can ignore the professional astronomers and define a set of “Classical Planets” which comprises Mercury, Venus, …, Uranus, Neptune, and Pluto. Now admittedly, Pluto is representative of larger bodies in the Kuiper Belt, which we didn’t know about before the dawn of the Space Age, and we probably ought to teach people about that as well. Memory tells me we manage to teach people that both Venus and Mars are somewhat like Earth as planets; I don’t recall anyone seriously suggesting that the similarity means that Venus or Mars — or Earth! — should be excluded from the list of planets.

    Yes, I know that many professional planetary scientists have thought deeply about this issue and decided in their wisest sagacious manner than “Pluto has not cleared its orbit!” — so unlike Earth, which has never been approached by an unruly, disorderly, out-of-place asteroid for countgless billions of years! Still, I have to wonder, of what benefit is this carefully phrased definition of a planet? Did the New Horizons spacecraft take a different trajectory in passing Pluto that we could never have imagined in far past 2006, when we innocently thought of Pluto as a planet? I’d marvel to see such computations!

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