Earlier this week, NASA released the latest images of the Pluto-Charon binary system, as the New Horizons spacecraft—newly recovered following a brief and heart-stopping loss of communications last weekend—approaches its quarry, more than 2.9 billion miles (4.6 billion km) from Earth, after a 9.5-year voyage from the inner Solar System to the cold, outermost reaches of the Sun’s realm. Its most recent images have revealed a curious, reddish-brown object, displaying a surface with startling albedo differences, a possible polar ice-cap, and an enigmatic “chain” of dark equatorial “spots.” In close attendance were its greyish binary companion, Charon, and a system of four tiny moons: Nix, Hydra, Kerberos, and Styx, all of whose names honor key figures and locations associated with the ancient Greek and Roman underworld. Next week, Pluto will become the last of the “traditional” nine planets to be inspected at close range by one of humanity’s robotic spacecraft, although our understanding of it has changed markedly even in the 114 months since New Horizons rose from Earth, back in January 2006. Now variously described as a “dwarf,” a “trans-Neptunian object,” a “plutoid,” a “Kuiper Belt Object,” and also by those who remain steadfast in their conviction that it remains a “true” planet, Pluto’s mysteries are such that even its exact nature continues to arouse fierce and ongoing debate. As the mission enters its final days before “Closest Approach” on 14 July, AmericaSpace’s New Horizons Tracker and a series of articles by Mike Killian, Leonidas Papadopoulos, Paul Scott Anderson, and myself will cover the discovery and exploration of Pluto and the unfolding developments as the spacecraft seeks to finally make this unknown world known.
Eighty-five years have now passed since the discovery of Pluto by U.S. astronomer Clyde Tombaugh, based at the Lowell Observatory in Flagstaff, Ariz., in February 1930, and its subsequent naming by English schoolgirl Venetia Burney, after the stern-faced god of the classical underworld. As described in a recent AmericaSpace Pluto history article, the decades which followed brought significant reassessments of Pluto’s nature, its estimated size, and its mass, which progressed in a downward direction. At first, it was thought to be about equivalent equatorial diameter as our Home Planet, but 1949 observations by the U.S. astronomer Gerard Kuiper, using the 200-inch (510-cm) telescope at Mount Palomar Observatory, operated by the California Institute of Technology in San Diego County, Calif., led him to the conclusion that Pluto was likely sized midway between Mercury and Mars, with a mass about one-tenth of Earth.
Subsequent discoveries in the 1970s and 1980s of the presence of highly reflective methane on the planet’s surface allowed for spectroscopic measurements of its brightness and known distance to reach conclusions about its mass, pegging Pluto at about 0.01 Earth-masses, far smaller than previously supposed, and as much as five times smaller than our own Moon. In more recent years, following the 1978 discovery of Charon, this has been refined yet, and in 2006 observations by the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) yielded an approximation of just 0.00218 Earth-masses.
By the closing years of the 20th century, the definition of “planet” was becoming increasingly difficult to maintain. The term had been deployed since time immemorial to describe strange “stars,” which appeared to “wander” across the night sky on regular, predictable journeys. Until the end of the 18th century, the “classical” planets of Mercury, Venus, Earth, Mars, Jupiter, and Saturn were well-known, after which the outermost “ice giants,” Uranus and Neptune, were detected by a combination of telescopic and mathematical means, followed by the search for a tantalizing “Planet X” in the early part of the last century. This search was conducted primarily in response to the widespread belief that the orbital motions of Uranus and Neptune were somehow perturbed from their predicted paths, possibly by an unseen planet, far from the Sun. Of course, it is now known through a better determination of Neptune’s mass and the observation of an entire orbit—84 years for Uranus, 165 years for Neptune—by both ice giants that many of these perturbations did not exist. Today, most astronomers no longer entertain the existence of a large planet with significant perturbative influence in the outer Solar System.
However, advances in the observing power of Earth-based instruments in the past few decades have allowed the detection of several objects which threw Pluto’s status as a “planet” into doubt. In October 1977, U.S. astronomer Charles Kowal discovered Chiron, a minor planet orbiting between Saturn and Uranus, which was subsequently reclassified as the first “centaur,” part of a population of tiny worlds existing between the Mars-Jupiter “asteroid belt” and the “Kuiper Belt” of the outer Solar System, beyond Neptune. Today described variously as an asteroid, a minor planet, and even a comet, Chiron’s equatorial radius has been determined at somewhere between 72.7 miles (117 km) by NASA’s Spitzer Space Telescope and 67.7 miles (109 km) by the European Space Agency’s (ESA) Herschel Space Observatory and raised new questions about the nature and possibly even the definition of what kind of object constitutes a “planet.” A second centaur, Pholus, was identified in January 1992, and it is estimated that over 44,000 such objects exist in the Solar System with diameters greater than 0.6 miles (1 km). However, the relative instability of their orbits and their dynamic lifetimes of just a few million years made it increasingly likely that they originated and were regularly replenished from an outer “reservoir” of material, raising the first suspicions of the existence of what is today termed the “Kuiper Belt.”
Suspicions that Pluto might not be a “true” planet were not new. In fact, they abounded in the weeks, months, and years after its initial discovery. In March 1930, measurements by the Dutch-U.S. astronomer Georges van Biesbroeck of the University of Chicago’s Yerkes Observatory suggested that Pluto resided beyond the orbit of Neptune and subsequent observations revealed a highly eccentric orbit, with a period of perhaps 3,000 years. This led some astronomers to speculate that Pluto was a long-period comet or, perhaps, a displaced asteroid. The U.S. astronomer Frederick Leonard of the University of California at Los Angeles (UCLA) later theorized that “in Pluto there has come to light the first in a series of ultra-Neptunian bodies, the remaining members of which still await discovery, but which are destined eventually to be detected.” Others, including the Irish astronomer Kenneth Edgeworth, noted as early as 1938 that Pluto was “too small to be classified as a major planet, in spite of its position” and in 1943 proposed the existence of “a very large number of comparatively small bodies” beyond Neptune in the Journal of the British Astronomical Association.
Several years later, in 1951, in the journal Astrophysics, Gerard Kuiper wrote of the possible presence of “one or more small planets,” which he likened to the dwarf world Ceres, residing between 38 Astronomical Units (3.5 billion miles or 5.7 billion km) and 50 AU (4.6 billion miles or 7.5 billion km) from the Sun, placing them far beyond the orbit of Neptune. However, Kuiper believed that this region would have long since been cleared by planetary gravitational perturbations and that few to none of these small planets would exist in the present era. Kuiper was apparently operating on the belief that Pluto was either Earth-sized or no smaller than Mercury, which would have allowed it long since eject such small objects out of its orbital neighborhood. Others disagreed, and in 1962 the Canadian astrophysicist Alaistair Cameron of Harvard University predicted the existence of “a tremendous mass of small material” in the outer Solar System, between 40 AU (3.7 billion miles or 5.9 billion km) and 50 AU (4.6 billion miles or 7.5 billion km) from the Sun. Shortly afterwards, fellow Harvard astronomer Fred Whipple independently postulated that a “comet-belt” might carry sufficient mass to account for the purported discrepancies in Uranus’ orbit, which had sparked the initial search for Planet X.
The possibility that such a belt might be the source of short-period comets, such as Halley’s Comet, with orbits of less than 200 years, was revived in the early 1980s by the Uruguayan astronomer Julio Ángel Fernandez. He realized that such a belt of cometary material beyond Neptune would prove a far more efficient source for short-period objects, because the required change in orbital energy would be far less than that for comets originating from the more distant Oort Cloud and, additionally, they would begin to evolve in orbits already close to the ecliptic plane. By 1988, other investigators were tracking the orbital mechanics of long-period comets from the Oort reservoir and short-period comets from the as-yet-hypothetical trans-Neptunian belt. “Although both sources could produce short-period comets, those from the Oort Cloud tended to have orbits with fairly large inclinations and thus appeared to be scattered over much of the sky,” explained Paul R. Weissman in The New Solar System. “On the other hand, comets that initially orbited in the ecliptic beyond Neptune tended to remain in low-inclination orbits as they approached the Sun—a characteristic very similar to that observed for the short-period comets.” University of Toronto astronomers Martin Duncan, Thomas Quinn and Scott Tremaine argued that this new cometary reservoir should be dubbed the “Kuiper Belt” and, in so doing, became the first to officially coin the name.
Efforts to telescopically identify objects beyond the orbit of Neptune began to bear fruit in the late 1980s, through observations performed at the Kitt Peak National Observatory, southwest of Tucson, Ariz., at the Cerro-Tololo Inter-American Observatory in northern Chile, and at the Institute of Astronomy at the University of Hawaii. Astronomer David Jewitt of the University of Hawaii and graduate student Jane Luu of the University of California at Berkeley subsequently worked at the University of Hawaii’s 7.5-feet-diameter (2.24-meter) telescope at Mauna Kea and on 30 August 1992 announced the discovery of the distinctly reddish-hued “(15760) 1992 QB1,” the first trans-Neptunian object to be discovered, after Pluto and Charon. It was found in the constellation of Pisces, at 23rd magnitude. Initially identified by Jewitt and Luu as “Discovery of the Candidate Kuiper Belt Object (KBO),” it opened the floodgates for more than 1,500 further bodies detected beyond the orbit of Neptune. “Computations … indicate that 1992 QB1 is currently between 37-59 AU from the Earth, but that the orbit … is completely indeterminate. Some solutions are compatible with membership in the supposed Kuiper Belt, but the object itself could also be a comet in a near-parabolic orbit,” it was explained by the International Astronomical Union (IAU). It was added that “Jewitt and Luu note that a comet-like albedo of 4 percent then implies a diameter of 125 miles (200 km) and that the red color suggests a surface composition rich in organics.” Following orbital computations, it was determined that (15760) 1992 QB1 lay 44 AU from the Sun, with an orbital period of about 292 years. Several months later, in March 1993, Jewitt and Luu identified a second KBO, identified as “(181708) 1993 FW.”
An entirely new Solar System appeared to be taking shape, and the number of observed celestial bodies was beginning to grow dramatically and by 1998 around 60 were known, with an average of ten others being telescopically identified each year. By the dawn of the new millennium—as NASA wrestled to secure funding for a succession of ill-fated flyby missions to Pluto—numerous KBOs had been discovered, including several which were very close to the dimensions of the ninth planet itself. “Where do Pluto and Charon fit into this picture?” Paul R. Weissman asked in 1999 in The New Solar System. “Many Solar System astronomers think of Pluto as the largest icy planetesimal to grow in the region beyond Neptune. While Pluto has a satellite and a thin atmosphere, traditionally considered proof of planetary status, we now know of asteroids with satellites and satellites with atmospheres. Moreover, Pluto is smaller than Titan, Ganymede, Callisto and Titan.”
Clearly, Pluto’s status as a fully-fledged planet was falling into question. As will be explored in tomorrow’s AmericaSpace Pluto history article, it was the detection of Eris in January 2005 which produced the first real mutterings of a need to reclassify the precise definition of what made a “planet.” A year later, in January 2006, New Horizons set sail on its long voyage to Pluto. Traveling in excess of 30,000 mph (50,000 km/h), and becoming the fastest human-made object ever to depart the Home Planet, it would require 9.5 years to traverse the orbits of Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, before reaching its quarry. Its remarkably smooth voyage would be strangely juxtaposed by a quite different voyage on Earth, as the very nature of Pluto came under intense and bitter scrutiny.
The second part of this article will appear tomorrow.
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