Pluto's moon Charon was unknown until the late '70s.
Our understanding of the universe around us has increased exponentially over the course of the past century. A thousand years ago, Earth was the center of the Universe, and the stars were just pinpricks in the firmament. A hundred years ago, our galaxy was the entire extent of the known cosmos; now we know that galaxies number in the hundreds of billions, and each contains hundreds of billions of individual stars. And there are those among us who can remember when only eight planets were known -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. This changed in 1930, when farm boy-turned-astronomer Clyde Tombaugh discovered the ninth planet, which he named Pluto after the Roman god of the Underworld. The first two letters of the name also served to posthumously honor his patron, astronomer Percival Lowell, whose calculations predicted the existence of a ninth planet, based on observed perturbations of Neptune. Interestingly, when quantum mechanical effects are taken into account, Neptune's orbit no longer shows such perturbations -- but Tombaugh found Pluto anyway.
For decades, little was known about Pluto other than the bare fact of its existence. Although it dips inside the orbit of Neptune for 20 years out of every 248, Pluto is generally the most distant of the nine known planets. Pluto's extreme distance -- nearly forty times the distance from the Earth to the Sun -- made unlocking its secrets a Herculean task. Gradually, however, data began to emerge: the Plutonian year was calculated to be 248 times longer than Earth's, and its day turned out to be 6.4 times longer than ours. In addition, astronomers eventually realized that this remote world had an axial tilt of 122 degrees -- in other words, it rotates tipped over on its side (as does Uranus). However, Pluto's size remained uncertain until 1985, when it was determined to be about 2,274 km in diameter (1384 mi): smaller than seven moons in the solar system, including Earth's! In 1985, it was discovered that Pluto possesses a tenuous nitrogen/methane atmosphere.
Our understanding of Pluto's true size came about primarily because of a prior discovery, the most exciting one since Tombaugh's original find: the fact that Pluto has a moon, and a huge one at that. In mid-1978, astronomer Jim Christy noticed an odd-looking bump on images of Pluto recorded at the U.S. Naval Observatory in Arizona. Initially it was thought that the images were flawed, but Christy found the bump on numerous images recorded over the previous decade. After noting that the feature appeared in different spots on Pluto's surface in different images, he did some calculations and found that it made a full circle in 6.4 days -- exactly the same length as the Plutonian day. Clearly, Christy had discovered a tide-locked satellite of Pluto. Christy's first thought was to name the new moon in honor of his wife, Char (short for Charlene), but the established international naming rules for astronomical objects prevented this. However, fate intervened: while looking through a book on Greco-Roman mythology, Christy was amazed to discover that the name of the ferryman who piloted dead souls across the River Styx and into Pluto's realm was named Charon. The name was ideal, melding the official nomenclature requirements with his desire to honor his wife.
Charon and Pluto soon came under intense scrutiny, and discoveries about these two icy bodies at the edge of the solar system quickly accumulated. Charon's orbit was determined based on the information discovered by Christy, revealing that it circles at a distance of 18,800 km (11,280 mi) from Pluto. Knowledge of the orbit also allowed astronomers to make a rough computation of Pluto's mass for the first time. It turns out that Pluto masses just two-tenths of one percent as much as the Earth, indicating a somewhat lower density, on par with a mixture of ice and rock; Charon's mass is about 15% of Pluto's.
It soon became clear that Pluto and its moon comprise the nearest thing to a double planet that exists in our solar system, far outclassing its nearest competitor, the Earth-Moon system. A double planet is a two-body system in which the moon closely approaches the size of the mother planet, and the Pluto-Charon system excels in this regard. In 1985, observations during a stellar occlusion (that is, an event during which Pluto and Charon passed in front of a star) allowed astronomers to determine the actual sizes of both Pluto and Charon. Pluto, as mentioned before, was determined to be 2,274 km (1384 mi) in diameter. Charon proved to be 1172 km (703 mi) across, making it about half as large as Pluto. Because of its great size relative to Pluto, planet and moon are locked into a dynamic orbit whereby Charon always hovers above the same area of Pluto's surface, and both bodies keep the same faces to each other. Because of this, Charon would never be visible from some parts of Pluto's surface.
As a double world, Pluto and Charon orbit a common center of mass. Moons and their planets always share a common enter of mass, but it usually lies deep inside the parent planet. This is not the case for Pluto and Charon: their center of mass lies between the two worlds, and it is this center of mass that orbits the sun.
Charon's presence provides interesting clues to mysteries that have been plaguing astronomers since Pluto's discovery. For example, why is Pluto lying down on the job? With the exception of Uranus, its axial tilt is the second largest of all the planets -- a huge 122 degrees (Venus's 178 degrees wins the prize). Every planet except Mercury exhibits some axial tilt; Earth's own tilt of 23 degrees, 27 minutes is what causes the seasons. But 122 degrees is extreme. Assuming that the planets condensed out of the primordial dust cloud with little or no axial tilt, what could have happened to cause this?
The same thing that happened to all the other planets, of course: something big hit Pluto, in this case hard enough to knock it over and possibly to cause the observed variation in its orbit (another big mystery). The early solar system was filled with large bodies that had not yet stabilized in their orbits. One such body appears to have struck or otherwise interacted with Uranus, knocking it over and reversing the direction of its rotation. A similar interaction may have been so traumatic for Venus that it literally flipped upside down. It's thought that the Moon formed when a Mars-sized body hit the Earth, sending material from both bodies splashing into Earth orbit. In Pluto's case, the culprit was probably a similar icy body that eventually became Charon.
Recent finds have shown that Pluto and Charon are merely the largest known of a whole family of similar, icy objects hidden in the farthest reaches of the edge of the solar system. Some have likened these objects to giant comets or "dirty snowballs," consisting as they do of mixtures of ice and rock. However, they far exceed the size of comets, and seem more analogous to asteroids, that swarm of minor bodies orbiting (mostly) between Mars and Jupiter. The largest known trans-Neptunian objects are approximately 300 km (180 mi) across, and hundreds are now known. The presence of thousands or millions more are suspected. Their prevalence gives credence to the theory that Pluto and Charon were initially independent bodies that were bound gravitationally by a close interaction. If there are huge numbers of little Plutos out there, it seems likely that (like the asteroids) they bump into each other occasionally.
Charon is now known to consist of slightly different materials than Pluto -- another plus for the collision theory of its origin -- and it's suspected to have a very tenuous atmosphere. It may even share an atmosphere with Pluto, since they orbit so closely together. For the first time, the Hubble Space Telescope has recently been able to produce sharp images that show Pluto and Charon as distinct bodies. Even as you read this, astronomers and other scientists are teasing out new morsels of information about Charon and its mother world. Because of its inherent ability to fascinate, this unique double planet will continue to be studied for a long time to come.
Many projects to study Pluto are currently underway or in their planning stages. Most exciting is NASA's planned mission to Pluto, the Pluto-Kuiper Express, which may get off the ground as early as 2003, returning images by 2013. If all goes as planned, this single spacecraft will provide more data than all our seventy years of ground-based observation combined, and Pluto-Charon will finally be more than a faint speck on the distant borders of our solar system.
