Satellite or partner?
Discovered by Jim Christy in 1978, Pluto’s satellite Charon has the distinction of having the largest known satellite-to-primary-planet ratio in the solar system. Charon is nearly half the size of Pluto, which results in some ambiguity regarding whether it should be classified as Pluto’s satellite or the smaller member of a binary planet.
Whatever its classification, Charon is in synchronous orbit with Pluto, which means the moon’s orbital period is the same as its rotational period. Thus, the same side of Charon faces Pluto in the same way as one side of the Moon always faces Earth.
Pluto’s mass, about 0.0021 that of Earth, and low surface pressure make it likely the planet can’t hold much of an atmosphere; however, an ethereal, gaseous mixture of nitrogen, methane, and carbon monoxide may be present above its surface while it is closest to the Sun (perihelion). These gases would probably be frozen throughout much of Pluto’s orbit. New Horizons will attempt to measure the escape rate of Pluto’s atmosphere directly, which may tell us more about how Earth’s atmosphere evolved.
Scientists disagree about Pluto’s classification as a planet. Some believe it should be labeled an asteroid, a comet, or a large Kuiper Belt object. Its orbit is inclined 17° above the ecliptic plane and is highly eccentric, or more elliptical than the other planets’ orbits. The 2005 discovery of Sedna and other large Kuiper Belt objects poured renewed energy into this ongoing debate.
Mysterious region no more
The Kuiper Belt is a disk-shaped orbital region beyond Neptune at about 30 to 50 AU from the Sun. It’s home to a bevy of small objects, possibly planetary debris left over from the solar system’s formation.
New Horizons is due to arrive at the Kuiper Belt and encounter its objects by 2016. Cataloging these objects is of the utmost importance as astronomers think they’re the main source of comets that occasionally impact Earth, sometimes with catastrophic results. Additionally, the mission will study organic molecules (those containing carbon) in an effort to learn more about the evolution of life on Earth.
NASA’s New Horizons mission to the Pluto-Charon system is set to launch in mid-January 2006.
The mission
The 1,050-pound (476 kilograms), piano-size New Horizons spacecraft will use Jupiter’s gravity to slingshot toward the Pluto-Charon system, where it will arrive in mid-2015. For 5 months, New Horizons will study the system’s global geology and geomorphology, map both worlds’ surface features, compositions, and temperatures, and try to identify their atmospheric structures and compositions. Also on the agenda is an examination of two smaller moons (S/2005 P1 and S/2005 P2) discovered in 2005 by a Hubble Space Telescope team.
The spacecraft’s science payload includes infrared and ultraviolet imaging spectrometers, two particle spectrometers, a multicolor camera, a long-range telescopic camera, a space-dust detector, and a radio experiment.
An unusual planet
Pluto is neither a terrestrial planet like Earth nor a gaseous giant planet like Jupiter. It lies at an average distance of 39.5 AU (1 AU is the average Earth-Sun distance) — a distance so great not even the Hubble Space Telescope can resolve surface features. Pluto is a small frozen world known as an “ice dwarf.” While Pluto’s composition remains unknown, its density suggests a combination of 70-percent rock and 30-percent water ice. Bright areas on Pluto’s surface indicate the presence of frozen nitrogen, methane, ethane, and carbon monoxide. Dark surface areas are also present, but their makeup is unknown. Surface temperature on the “ice dwarf” ranges between –391° and –346° F (–235 to –210 C).
December 21, 2005
NASA’s New Horizons spacecraft will give the solar system’s ninth planet the up-close-and-personal treatment in January 2006. This first-ever mission to Pluto will explore the distant planet, its moon Charon, and the Kuiper Belt. The New Horizons spacecraft will then hurtle out into deep space on a one-way journey into the cosmos beyond the Kuiper Belt.
Why is the New Horizons mission so important to planetary scientists? “Exploring Pluto and the Kuiper Belt is like conducting an archeological dig into the history of the outer solar system, a place where we can peek into the ancient era of planetary formation,” says Alan Stern, New Horizons principal investigator, of the Southwest Research Institute in Boulder, Colorado.
