In a little more than four weeks’ time, on 14 July, NASA’s New Horizon spacecraft will sweep past the dwarf planet Pluto, its large binary companion Charon, and a system of at least four tiny moons—Nix, Hydra, Kerberos, and Styx—as it reaches the climax of a 9.5-year voyage to unveil a group of the Solar System’s farthest known celestial bodies. By so doing, New Horizons will bring full-circle our first-time exploration of each of the traditionally accepted nine planets in the Sun’s realm. Although Pluto was formally demoted in 2006 to the status of a dwarf planet, a trans-Neptunian object, and the largest known body in the Kuiper Belt, fierce debate still rages as to whether it ought to be reinstated as a “planet” or retain its somewhat less lofty descriptor. Over the coming days and weeks, AmericaSpace’s New Horizons Tracker and a series of articles by Mike Killian, Leonidas Papadopoulos, and myself will cover the discovery and exploration of Pluto to date, the trials and troubles faced by those who desired to send a spacecraft there, and the unfolding developments as New Horizons seeks to make this unknown world known.
As described in last week’s Pluto articles, the discovery of this diminutive world—whose estimated equatorial diameter is only about 1,470 miles (2,370 km), making it substantially smaller than our Moon—came about following unexplained perturbations in the motions of Uranus and Neptune, which prompted speculation that a more distant planet, popularly dubbed “Planet X,” might be exerting a gravitational influence upon it. Astronomers Percival Lowell and William Pickering predicted its location and suggested its possible size and even physical composition, but their efforts to find it in the early years of the 20th century proved fruitless. Then, in February 1930, a young astronomer named Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Ariz., successfully located a star-like object and his discovery of the mythical Planet X caused a sensation. Several weeks later, inspired by the suggestion of a young girl from Oxford, England, the planet was named “Pluto,” in honor of the classical Greek and Roman god of the underworld.
However, it soon became apparent that Pluto was not what it seemed. As early as 1932, the German-born U.S. astronomer Armin Leuschner suggested that its dimness and high orbital eccentricity—its path carried it out of the ecliptic plane, ranging as close as 30 Astronomical Units (AU), about 2.7 billion miles (4.4 billion km), to as far as 49 AU, roughly 4.6 billion miles (7.4 billion km), from the Sun—made it a more likely candidate for an asteroid. In fact, for certain periods of its orbit, including a 20-year period from January 1979 until February 1999, Pluto was situated farther from the Sun than Neptune. The new world was six times dimmer than Lowell had predicted in his observations and proved so tiny and so dark that it revealed no visible disk to even the largest and most sophisticated Earth-based telescopes.
In the decades after its discovery, estimates of its size moved steadily downward. At first, it was suggested that Pluto may be about the same equatorial diameter as Earth, but 1949 observations by 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 conclude that it was sized midway between Mercury and Mars, with a mass about one-tenth of our own world. However, even with such powerful instruments, it was virtually impossible until the late 1970s to discern much detail about Pluto, other than an approximate rotation rate of about 6.4 Earth-days and a handful of details about the nature and mechanics of its highly elliptical orbit.
That situation began to change in 1976, when frozen methane was spectroscopically detected on the planet’s surface by Dale Cruikshank, Carl Pilcher, and David Morrison of the University of Hawaii at Honolulu. Their work marked the first occasion that anything other than water-ice or frozen carbon dioxide had ever been discovered via spectroscopy, anywhere in the Solar System, and the presence of frozen methane was strongly suggestive that Pluto’s surface was highly reflective. Following this line of argument, the measured brightness—together with the planet’s distance from the Sun—implied that Pluto must be about 0.01 Earth-masses, far smaller than previously supposed and far smaller than our Moon. Similar conclusions had earlier been reached by Dennis Rawlins, who measured albedo variations between Pluto and Neptune’s large moon, Triton, and suggested that both celestial bodies were likely to be of similar mass.
Two years later, on 22 June 1978, U.S. Naval Observatory astronomer James Christy was in the process of examining an enlarged photographic plate of Pluto, taken in April 1965, in tandem with more recent images, acquired by the 61-inch (1.5-meter) Kaj Strand Astrometric Reflector at the U.S. Naval Observatory Flagstaff Station (NOFS). His goal was to better determine the planet’s orbital elements. However, he identified a noticeable “bulge,” apparently emanating from one side of its disk. For more than a decade, the anomaly was assumed to represent a default in the image, perhaps induced by improper alignment, but upon closer examination Christy noticed that only Pluto itself appeared elongated in the image and he concluded that the bulge betrayed the presence of a large moon, orbiting at an approximate distance of about 12,100 miles (19,600 km) from its host.
It remained entirely possible, of course, that the distortion might be due to Pluto having an irregular shape, but by an amazing instance of serendipity and good fortune, in the mid-1980s Charon’s orbital plane was seen edge-on from Earth. In effect, the moon would appear to pass in front of Pluto, “transiting” its parent, then vanishing behind it, in an “occultation,” on no less than five occasions from 1985-1990. The first of these, on 17 February 1985, served to verify the existence of the moon, which Christy had earlier named “Charon,” after the mythological boatman who ferried the souls of the dead across the River Styx to Pluto’s realm, Hades. Despite being originally pronounced in Latin in a similar fashion to “Karen,” the name also proved helpful to Christy’s marriage, for his wife, Charlene, was nicknamed “Char.” The name remained unofficial for several years, before eventually receiving formal approval in January 1986.
It was a name that was perfectly in keeping with Pluto itself. In ancient times, a coin—known as “Charon’s obol”—was traditionally placed onto the tongue of a dead person, in order to pay the boatman for his service in conveying the soul from the realm of the living into the realm of the departed. The transits and occultations of Pluto and Charon during the early years after the moon’s discovery allowed photometrists and spectroscopists to monitor the combined emissions from both celestial bodies, in an effort to identify specific chemical constituents and more carefully determine their respective sizes. These yielded an equatorial diameter for Pluto about two-thirds of our Moon, around 1,430 miles (2,300 km), with Charon almost exactly half the size of its host. Combined densities of both worlds revealed a composition of about 60-70 percent rocky materials and 30-40 percent ice, predominantly in the form of frozen water-ice.
In the years after Charon’s discovery, estimates were made by Robert Harrington of its orbit and the mass of Pluto itself; in effect, the moon had provided a significant breakthrough in our understanding of the ninth planet. Its relationship with Charon was less of a host-satellite one, and more of a “binary-planet.” Charon orbits about 11,700 miles (18,800 km) from Pluto’s surface, circling every 6.4 Earth-days, exactly matching the rotation rate of its partner. In the same manner as our Moon, Charon keeps the same face directed toward Pluto in perpetuity. However, unlike our planet, which spins 29.5 times for each lunar cycle, one side of Pluto always faces Charon and vice versa. An observer on Pluto’s Charon-facing side would see the moon hanging in the sky, unchanging its position over the years, whilst an observer on the space-facing side might not even know that a moon existed.
Surprisingly, Charon’s north-south motion—indicative of the fact that the pole of its orbit resides within the plane of the ecliptic—provided clear evidence that Pluto’s own rotational axis was tipped very closely to the ecliptic, in a similar fashion to Uranus. Nor was the good fortune of being able to witness five mutual eclipses in 1985-1990 overlooked, for these events occur only twice in Pluto’s 248-year orbit of the Sun. “Had Charon been discovered in, say, 1993,” explained Dale Cruikshank in his chapter on Pluto, Triton and Charon for the fourth edition of The New Solar System, “we would had to wait until the 22nd century to witness the next series of overlappings.”
Moreover, the rotational locking of the two worlds was suggestive of Charon being a large body, relative to its host, and celestial dynamicists have suggested that the size ratio between the two is closer than any other host-satellite pairing in the Solar System. Present estimates show Charon to be a little more than half the size of its host, and about 22 percent of the mass, with an equatorial diameter of about 750 miles (1,200 km), compared to Pluto’s 1,470 miles (2,370 km). Spectroscopy has also revealed Pluto to be predominantly coated with ices of nitrogen, methane, and carbon monoxide. This can be compared to less volatile water ices in the case of Charon, with patches of ammonia hydrates and water crystals detected in 2007, suggesting the possible presence of active cryovolcanism or cryogeysers, perhaps not dissimilar to those observed on Neptune’s moon, Triton. Current thinking suggests that Charon’s overwhelmingly icy composition may indicate that it was formed as a consequence of a giant impact into Pluto’s icy mantle.
In six decades of observation, our knowledge of Pluto had expanded from a distant, star-like point of light into the kernel of a real world, with an attendant moon, on the very edge of the known Solar System. Yet with no flyby mission on the horizon, it was the arrival of the Hubble Space Telescope (HST) in April 1990 which would contribute substantially—and which continues its contributions, to this very day—to our understanding of Pluto’s realm. With Hubble, we would see Pluto, literally, in a very different light.
The second part of this article will appear tomorrow.
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