Solar System

A Bowdoin Van Riper. Scientific Thought: In Context. Editor: K Lee Lerner & Brenda Wilmoth Lerner. Volume 1. Detroit: Gale, 2009.

Introduction

The solar system consists of our sun and the bodies under its gravitational influence. These include eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their more than 150 known moons, thousands of asteroids, and tens of thousands of comets and protocomets. Up to a trillion more protocomets may surround the solar system in a formation called the Oort cloud, but its existence remains conjectural. Only the sun, the five closest planets, and Earth’s moon are visible to terrestrial observers with unaided eyes.

All the bodies orbit in the same direction, and in essentially the same plane, called the ecliptic. The planets are spaced in a roughly geometric progression, with each orbit twice as far from the sun as the one before it. Several of the rocky planets and larger moons are geologically active, a few have atmospheres, and some may have (or may once have had) liquid water on their surfaces. Earth is the only one known to meet all those criteria, and the only one known to harbor life. Simple life forms may exist—or have existed in the past—on Mars or Jupiter’s large moon Europa, but no evidence of them has yet been found.

Curiosity about extraterrestrial life, born with the Scientific Revolution in the seventeenth and eighteenth centuries, is a relatively new motive for studying the solar system. Newer still is the desire to understand other planets as twentieth-century scientists began to understand Earth: a complex, dynamic system rather than a simple ball of rock. Older motives were more practical. For thousands of years, human observers charted the motions of the sun, moon, and the five closest planets to track the passage of time, navigate unfamiliar territory, predict the future, and see into the minds of gods and men.

Historical Background and Scientific Foundations

The Greek Cosmos

Many ancient civilizations watched, tracked, and recorded the movements of the lights they saw in the sky. Beginning in the fifth century BC, the Greeks attempted to explain them. Ancient philosophers set themselves two separate problems, which they assigned to two branches of natural philosophy. Modeling the motions of the sun, moon, and planets was the province of astronomy. Describing the physical structure of the cosmos—what held up celestial bodies and caused them to move—was the province of cosmology. Workable solutions to both problems emerged in the century between the late 400s and the late 300s BC and became, for the next 2,000 years, the standard Western view of the cosmos. The two solutions rested on a single set of assumptions about the structure of the universe. Both assumed that Earth was fixed at the center of the universe, and both assumed that the universe was only slightly larger than the orbit of Saturn, the outermost planet.

Greek astronomers believed all heavenly bodies orbited in perfect, constant, uniform circular motion. A single circular orbit could not account, however, for the complex and sometimes erratic motions actually observed in celestial bodies. To bring their models into line with reality, therefore, Greek astronomers added additional circular motions. A planet might, for example, travel in a small orbit (an epicycle) around a point that itself traveled around Earth in a much larger orbit (a deferent). Both orbits were perfectly circular, and both the planet and the center of the epicycle moved at constant speeds, but the combination of the two gave the planet the kind of complex motion—slowing down, backing up, moving closer to or farther away from Earth—that astronomers actually observed. Other techniques for modeling complex motion evolved over time. Among them was the equant: an arbitrary, off-center point inside a planet’s orbit that divided it into four unequal sectors. The planet (or the center of its epicycle) would speed up or slow down at various points in its orbit, but cover each of the four sectors in the same amount of time, thus making its motion uniform with respect to the equant.

Claudius Ptolemy (AD c.90-c.168), an Egyptian-born astronomer of Greek heritage, perfected this approach in the mid-second century AD. His major work on astronomy—Mathematike Syntaxis (Mathematical arrangement)—completed around 150 and known today by an abbreviation of its Arabic title, Almagest, offered mathematical models of enormous accuracy and predictive power. Ptolemy’s models could be awkward, complex, and arbitrary, but they worked. They enabled those who used them to construct calendars, cast horoscopes, and navigate ships out of sight of land. As a result, they lasted. The Almagest remained the standard reference work for Western and Middle Eastern astronomers well into the 1400s.

Greek cosmologists, notably Aristotle (384-322 BC), envisioned the universe as a set of hollow, crystalline spheres nested one inside another. Earth stood, motionless, at the center. The outermost sphere marked the edge of the universe, and the stars—all equidistant from Earth—shone down from its inner surface. Each sphere rotated on its own axis. The moon, sun, and each of the planets was embedded at the equator of one of the spheres, like a tennis ball in the surface of a frozen pond, moving through the sky as if pushed by an invisible hand.

Greek philosophers recognized that the rotation of a single sphere could not account for the motion of any major celestial body. Eudoxus of Cnidus (c.395-c.342), who first devised the nested-sphere cosmology, used a total of 27 spheres: three each for the sun and moon, four each for the planets, and the outer one for the stars. His student Calippus (c.370-c.300 BC) added seven additional spheres, for a total of 34. Aristotle, whose version remained the standard cosmological model in Europe until the early 1600s, used 55 spheres.

Greek cosmology lasted because it worked. It answered critical questions—What holds celestial bodies up? What makes them move?—in ways that, until the early 1600s, matched the best available observations of the night sky.

The Science: The Moving Earth

Nicolaus Copernicus (1473-1543) was an astronomer, priest, and frequent user of Ptolemy’s mathematical models. Attracted by their predictive power but frustrated by their awkward complexity, he set out to reform them using a radical solution: Treat the sun as the fixed center of the universe and Earth as one of six planets in orbit around it. This allowed Copernicus to draw simpler models. He still needed an epicycle for each planet, but was able to eliminate arbitrary constructions like the equant while retaining the Ptolemaic system’s accuracy. Aware of the radical nature of his ideas, Copernicus made sure his work, De revolutionibus orbium coelestium libri vi (Six books concerning the revolutions of the heavenly orbs, 1543) was only published after his death.

His publisher, also determined to avoid controversy, took the further step of adding a preface to the book claiming that heliocentrism was a mathematical convenience and not a description of reality. A generation passed before astronomers realized that the preface reflected the publisher’s view rather than Copernicus’s.

Johannes Kepler (1571-1630), a German astronomer and mathematician, was among those who recognized (and welcomed) Copernicus’s intent. Using the heliocentric model and the best-available planetary observations, Kepler plotted their paths and arrived at two startling conclusions: that their orbits are elliptical, not circular; and that their speeds are variable, not fixed. He embraced both conclusions, rejecting earlier astronomers’ insistence on uniform circular motion. His system achieved, as a result, the radical simplicity that Copernicus sought but never found. Each planet had a single orbit, and each planet’s motion could be summarized by three laws that now bear Kepler’s name.

Kepler’s work generated relatively little controversy because he wrote in Latin for an audience of fellow astronomers, published in a Protestant country (Germany), and did not go out of his way to promote heliocentrism or attack geocentrism. Kepler’s contemporary Galileo Galilei (1564-1642), on the other hand, wrote in everyday Italian for general audiences and spoke extensively and forcefully in public. Galileo lived in a Catholic country (Italy) at a time when the church was still recovering from the Reformation and deeply suspicious of other forms of “heresy”—even a scientific theory that appeared to conflict with the Bible by suggesting that Earth (not the sun) moved. He argued vigorously that the heliocentric model was true and the geocentric model was false, even when diplomatic course would have been silence. Not surprisingly, controversy swirled around him.

Galileo began his public advocacy of heliocentrism in 1610, using his telescopic observations to argue for the Copernican model and against the Ptolemaic and Aristotelian ones. Unofficial accusations of heresy began to circulate in Rome by late 1613, and by the end of 1615 Galileo felt obligated to travel there to clear his name. He presented his case to Cardinal Robert Bellarmine, a leading theologian and intellectual, who noted that heliocentrism contradicted orthodox views of both science and scripture, and that Galileo had no conclusive proof that it was true. Absent such proof, he directed, Galileo should teach the theory only in hypothetical terms.

Bellarmine also referred the question to a panel of distinguished theologians, who in early 1616 declared heliocentrism “foolish and absurd … and formally heretical since it explicitly contradicts [in] many places the sense of Holy Scripture.” The decision had, as the church intended, a chilling effect on discussions of heliocentrism. Copernicus’s Revolutionibus was withdrawn from circulation for a century, and researchers throughout Catholic areas of Europe fell silent on the subject. Even Galileo turned to other topics for the next 14 years.

When Galileo next took up the subject in 1630, the world had changed. The work of Kepler and other astronomers had, in Protestant countries at least, increased support for heliocentrism; Cardinal Bellarmine and Pope Paul were both dead; and Pope Urban VIII, Paul’s successor, was an old friend of Galileo’s. Urban invited Galileo to write a treatise comparing the geocentric and heliocentric models, but reminded him that he should present the latter only as a hypothesis. The resulting book, Dialogue on the Two Chief World Systems, Ptolemaic and Copernican (1632), was a scientific triumph but a political disaster. Galileo structured it as a debate between a committed Copernican named Salviati and a committed Aristotelian named Simplicio, with a neutral character named Sagredo acting as moderator. The eloquent and witty Salviati out-argues Simplicio on nearly every point, demolishing Aristotelian cosmology and promoting the Copernican alternative. The book ends with Salviati triumphant, Sagredo enlightened, and Simplicio delivering the Pope’s reminder that heliocentrism is “only a hypothesis.”

Galileo may not have intended it as mockery, but mockery, along with flagrant public advocacy of a heretical idea, was what his enemies saw. Called to Rome to account for his actions, he was tried, convicted, and forced to publicly renounce his belief in heliocentrism—a harsh punishment for a proud man whose commitment to the idea was well known. He was sentenced to house arrest, and spent the last decade of his life at the villa of a wealthy friend. His last major scientific work, completed during that decade, was a treatise on physics that indirectly contributed to the final triumph of heliocentrism.

Newton’s Clockwork Universe

Astronomers were, by the time of Galileo’s death in 1642, increasingly concerned with the old central questions of cosmology: What keeps the planets in their orbits, and what moves them? Aristotle’s crystalline spheres, the standard answer for 2,000 years, no longer seemed plausible. They were incompatible with moving planets that had moons, incompatible with elliptical orbits, and incompatible with the growing realization that comets were solid objects whose paths crossed the orbits of the planets. In his last book, Galileo rejected Aristotle’s definition of “inertia” as the natural tendency of moving bodies to come to a stop, arguing instead that they would continue to move unless an outside force such as friction brought them to a stop. The lack of friction and similar forces in space meant that there was no need to ask what “pushed” the planets in their orbits. Once set in motion, they would keep moving forever. Galileo solved the other half of the problem by arguing that the planets’ inertial motion was “naturally” circular.

Isaac Newton (1642-1727) studied Galileo’s work while a student at Cambridge University, and adopted his revised definition of inertia. He also absorbed the work of Christian Huygens (1629-1695) and Robert Hooke (1635-1703), who hypothesized (but could not prove) that the elliptical orbits revealed by Kepler were the result of a centripetal force that pulled the planets toward the sun.

Newton took Hooke’s central idea—a centripetal force that diminishes in proportion to the square of the distance—and showed that a planet governed by such a force would obey Kepler’s second law. He then added his own critical insight: the centripetal force, gravity, acts between any pair of bodies with a strength proportional to their masses. His key insight was that gravity is universal: the force that pulls falling bodies toward Earth’s surface, the force that keeps the moon in orbit around Earth, and the force that keeps Earth in orbit around the sun are the same force, acting between different pairs of bodies. His 1687 book Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy), better known by the short form of its Latin title Principia, completed the heliocentric revolution by synthesizing Copernicus, Galileo, and Kepler.

Principia provided astronomers with mathematical tools of unprecedented power, which they used with enthusiasm. Edmond Halley (1656-1742), for example, compared records of comets that had appeared in 1531 and 1607 to his own observation of the comet of 1682 and concluded that they were in fact the same comet. Using Newtonian methods, he assessed the gravitational effects of the sun and planets on the comet and, in a 1705 book, predicted its return in 1758. He thus became the comet’s namesake, and the first person to recognize that some comets, at least, have closed orbits and return at regular intervals. Halley did not live to see his prediction confirmed, but the sighting of “his” comet in late 1758 guaranteed his fame and validated the power of Newton’s ideas.

Newton’s eighteenth-century admirers frequently compared his model of the solar system to an exquisitely made clock that, once set in motion, would run forever without adjustment. Those with traditional religious views were gratified by what they saw as proof of God’s creative power, but also disturbed by the apparent lack of any need or opportunity for God to act within the system. Some argued that God accounted for otherwise-inexplicable phenomena, only to become freshly disturbed when naturalistic explanations were found. Only deists—who believed that God, having created the universe, took no further role in it—were truly comfortable with the religious implications of Newton’s clockwork cosmos.

Telescopes and New Worlds

The publication of Newton’s Principia in 1687 settled with certainty the questions by the posed Greek philosophers. It provided a mathematical model, via Kepler’s laws, of how the planets moved and a physical explanation of why they moved the way they did. Galileo’s telescopic observations of 1609-1610 showed that there was more to the cosmos than the Greek philosophers had imagined. Later seventeenth-century observers with better telescopes showed that there was more than even Galileo had imagined, including rings and moons of Saturn and the Great Red Spot on Jupiter. Embodied in these changes was a profound shift in perspective. Aristotle had thought of Earth as the center of a small, enclosed universe. Newton and his successors considered it as one among several planets in a solar system that was itself a small part of a very large, perhaps even infinite, universe.

Galileo had used a refracting telescope, which uses lenses to bend and focus light. Newton, a master of optics as well as celestial mechanics, invented a reflecting telescope that used curved mirrors for the same purpose. Large mirrors were (relatively) easier and cheaper to make than large lenses. They could also be larger in absolute terms, and larger telescopes can “see” fainter and more distant objects than smaller ones. Telescopes grew larger in the eighteenth and nineteenth centuries, and as they did the solar system grew more and more crowded.

William Herschel (1738-1822), a German-born musician who moved to England and became an astronomer and telescope maker, was cataloging double stars in 1781 when he found the first new planet in recorded history. He initially mistook it for something else (as other astronomers had nearly 20 times in the preceding century), but quickly recognized and corrected his mistake. Using a powerful telescope of his own design, he realized that the “comet” he thought he had found had neither the gaseous “beard” nor the long “tail” characteristic of comets. Observing it over the course of many nights, he tracked its motion and concluded that it was a new planet, the seventh from the sun. Herschel proposed that it be named “George’s Star” in honor of the king of England, but scientists settled on the less nationalistic name Uranus.

Father Giuseppe Piazzi (1746-1826) of Sicily was also looking for stars when, on the night of New Year’s Day 1801 he found what he believed was a new planet between Mars and Jupiter. The existence of such a planet had been predicted by Kepler in 1609 and others since the large gap between Mars and Jupiter seemed to call for it, but the planet itself had never been sighted. Piazzi’s discovery was dubbed Ceres, after the Roman goddess of the harvest, but before it could be definitively studied German astronomer Wilhelm Olbers (1758-1840) discovered a slightly smaller body (soon named Pallas) orbiting nearby at nearly the same speed. This raised the possibility that what Piazzi had found was not a new planet, but one of a number of minor planets. Herschel confirmed that the new worlds were smaller than any of the known planets, and coined the beautiful but inaccurate word asteroid (“star-like”) to describe them. Two more fair-sized ones were quickly discovered—Juno in 1804 and Vesta in 1807—but they were the last for nearly forty years.

Plotting the movements of Uranus revealed perturbations in its orbit: small deviations from the path that Newton’s laws predicted. The most logical explanation, especially for astronomers raised on Newton’s celestial mechanics, was an eighth planet whose gravitational pull tugged Uranus as it passed. Urbain-Jean-Joseph Le Verrier (1811-1877) of France and John Couch Adams of England set out, independently and unaware of one another, to calculate where the eighth planet should be. They announced their results almost simultaneously (the perception that Adams could have “won” if the head of the Royal Observatory had taken him seriously caused a minor scandal in England). Astronomers turned their telescopes to the appointed place in the sky and, on September 25, 1846, picked out the dim blue disk that became known as Neptune.

Better telescopes that could gather more light and thus “see” dimmer and more distant objects helped astronomers locate Neptune and, shortly afterward, its large moon Triton. They also led, beginning in the late 1840s, to a flurry of asteroid sightings that lasted for the rest of the century. It took 50 years (1801-1851) to discover the first 20 asteroids, but thanks to rapidly improving telescopes the hundredth was charted in 1868 and over 300 were on the books by 1890. The rate of discovery shot up again in the 1890s, when Maximilian Wolf (1863-1932) attached a camera to the eyepiece of a telescope and made photographs of the sky for later study. Wolf himself logged the thousandth asteroid in 1923 and, fittingly, named it after Piazza.

The discovery of tiny, distant Pluto in 1930 was also made possible by the union of cameras and telescopes.

Clyde Tombaugh, searching for a hypothetical “Planet X” that lay beyond Neptune and disturbed its orbit the way Neptune disturbed Uranus’s, took telescopic photographs of the section of sky where he expected to find it. Comparing photographs taken on successive nights, he was looking for a “star” that shifted its position relative to those around it and so revealed itself as a body orbiting the sun. He succeeded and—since he was looking for a planet—his new discovery was hailed as the ninth planet.

On August 24, 2006, however, Pluto’s status was downgraded to that of a “dwarf planet” by the International Astronomical Union—the same designation as Ceres (a large asteroid) and 40 other bodies. The IAU changed Pluto’s status because its small body lacks the gravitational pull to dominate the space around it as its larger cousins do, and its orbit is less clearly defined than the other eight. “It’s going to be hard to find a new planet,” astronomer Mike Brown of the California Institute of Technology told National Geographic. “You’d have to find something the size of Mars.” “This will be an issue in the future,” added planetary scientist Andy Cheng. “Dozens of objects are going to be straddling this line.”

Swamps on Venus, Canals on Mars

Telescopes of greater power revealed the surface of Mars in increasing detail, but they could do nothing to penetrate the thick clouds that obscured the surface of Venus. Ironically, both circumstances led astronomers, writers, and (eventually) filmmakers and television producers to imagine that life existed on both worlds. The surface features on Mars seemed to suggest the possibility of life, and Venus’s impenetrable clouds invited extravagant speculations about what might lie beneath them.

Beginning in the mid-nineteenth century, some astronomers identified greenish areas of the Martian surface with oceans or vegetation, and speculated that the seasonal changes in Mars’s polar ice caps meant that free-flowing water existed on the surface. Speculation about life reached a new level in the 1870s, however, when Giovanni Schiaparelli (1835-1910) used the Italian word canali (“channels”) to describe the dark streaks he saw on the planet’s surface. In the English-speaking world, canali was widely mistranslated as “canals,” which implied the existence of canal builders. Percival Lowell (1855-1916), a wealthy Bostonian with a passion for astronomy, seized on the idea with particular enthusiasm. Mars (1894), Mars and Its Canals (1906), and Mars as the Abode of Life (1908) reported his detailed observations of Mars, but also his belief that intelligent Martians had built the canals in a desperate attempt to save their dying civilization by tapping the polar ice caps for water. Variations on Lowell’s idea echoed in popular culture for years, through works as different as H.G. Wells’s (1866-1946) War of the Worlds (1898), Edgar Rice Burroughs’s (1875-1950) A Princess of Mars (1917), Ray Bradbury’s (1920-) The Martian Chronicles (1950), and Robert A. Heinlein’s (1907-1988) Stranger in a Strange Land (1961).

Speculations about possible life on Venus began later, and were more fanciful. Matriarchal societies predominated (since Venus was the goddess of beauty), as did lush, swampy jungles (since total cloud cover implied high humidity and frequent rain), but lizard-like humanoids were also popular. Edgar Rice Burroughs produced a Venus-based series of swashbuckling adventure stories beginning in 1931, and most of the major science-fiction writers of the next few decades—Heinlein, C.L. Moore (1911-1987), Arthur C. Clarke (1917-), and Bradbury—used a swampy Venus as a setting at least occasionally. Bradbury’s frequently anthologized story “All Summer in a Day” is the best-known example. Venus also figured prominently in low-budget science fiction films and television programs during the 1950s and early 1960s, since lush forests and scantily clad Venusian women were an inexpensive way to add visual interest to often-pedestrian plots.

Speculation about life on Venus and Mars grew, not coincidentally, as knowledge of remote areas of Earth increased. Nineteenth-century adventure fiction routinely placed lost cities and monsters “unknown to science” in unexplored parts of Africa, South America, or the Pacific, but by the early twentieth century there were few unexplored areas left. Other worlds—particularly Mars and Venus—took their place, at least until spacecraft began to demystify them in the 1960s.

The Edge of the Solar System

Aristotle’s idea that the universe ended just beyond the orbit of Saturn was still widely held in the early 1600s.

It began to unravel when Galileo’s telescope showed that the stars were not all equidistant from Earth and continued to unravel as astronomers gradually realized how far away they really were. The outer edge of the solar system also expanded, though more slowly. It was defined first by the orbit of Saturn, and then in turn by the orbits of Uranus, Neptune, and Pluto as each was discovered. Pluto’s tiny size, and the discovery that the anomalies in Neptune’s orbit were phantoms created by measurement errors, suggested that there were no more planets. Whether anything else lay beyond Pluto remained an open question.

Irish astronomer Kenneth Edgeworth (1880-1972) speculated in the 1940s that a belt of small icy bodies might orbit the sun just beyond Neptune. Gerard Kuiper (1905-1973; rhymes with “viper”) also suspected that large numbers of such bodies existed, but argued in a 1951 paper that they were randomly scattered and lay well beyond the orbit of Pluto. Chilean-born astronomer Julio Fernandez (1954-) worked out Edgeworth’s idea of a trans-Neptunian belt in more detail in a 1980 paper, but it remained a hypothesis until astronomers began to find trans-Neptunian objects in the early 1990s. More than 70,000 such objects with diameters greater than 62 miles (100 km) have been found since 1992, nearly all between 30 and 50 astronomical units from the sun (1 AU = 93 million miles [149,668,992 km], the distance from Earth to the sun). The vast majority of the objects are masses of ice and dust similar to the nuclei of comets, and the majority of short-period comets that pass through the solar system are believed to come from what is now called, completely illogically, the Kuiper Belt.

Even the Kuiper Belt, however, may not be the end. Jan Oort (1900-1992) observed in 1950 that the highly elliptical orbits of long-period comets seem to extend outward from the sun in all directions to distances of about 50,000 AU. Oort suggested that the outermost portion of the solar system is a huge spherical cloud of protocomets: small bodies composed of ice and dust left over from the nebula out of which the solar system was formed. Calculations suggest that the Oort cloud (as it is now known) might contain as many as 1 trillion of these bodies, and that their combined mass might equal that of Jupiter. Its existence remains a working hypothesis, however, and the tools needed to look for direct evidence of it have yet to be invented.

The existence of the Kuiper Belt and the possible existence of the Oort cloud show that there is no sharp boundary where the solar system ends and interstellar space begins. They support the widely held theory that the solar system formed from a cloud of gas and dust. They also reveal—like much else in modern science—that a gray area, not a bright line, separates us and the world we know from the rest of nature.

Exploring the Moon, 1959-1975

The exploration of other worlds by spacecraft was shaped by the Cold War space race between the superpowers. Beginning with the Soviet Union’s launch of Sputnik 1 in October 1957, both governments saw space exploration as a venue for demonstrating their technological superiority. The Union of Soviet Socialist Republics (U.S.S.R.) went from success to success in the early years of the space race, launching the first artificial satellite, the first animal, and the first human into orbit. Luna 2 and Luna 3, which took the first closeup pictures of the moon in 1959, were part of that string. President John F. Kennedy’s May 1961 call for a manned lunar landing by the end of the decade was a calculated risk: It redefined victory in terms of a specific goal that, the president’s advisors privately assured him, the United States could reach before the U.S.S.R.

Most of the unmanned missions to the moon in the 1960s were linked, in some way, to set the stage for later manned landings. The Soviets did it with the later Luna-series probes, such as Luna 9 (the first to make a soft landing) and Luna 10 (the first to orbit the moon). The Americans did it with the Ranger, Surveyor, and Lunar Orbiter series of missions. The technologically sophisticated Luna missions of the early 1970s were intended to restore the Soviet space program’s luster after the failure of its manned lunar landing. Lunas 16, 20, and 24 returned to Earth carrying a total of three-quarters of a pound (0.34 kg) of lunar soil. Luna 19 and 21 deployed six-wheeled robotic rovers that could take photographs and analyze samples. NASA, Soviet officials pointedly noted, had the capacity to do neither.

Project Apollo itself was shaped in more subtle ways by Cold War pressures. The first two landing missions, Apollo 11and Apollo 12, stayed on the moon for comparatively brief periods and did only limited scientific work. Their purpose was to show that a landing and safe return was possible; science had to be fitted in around the edges of those goals. The remaining four landing missions—Apollo 14 through Apollo 17—stayed longer and did more, but scientific objectives remained secondary to operational ones. Mission planners consistently rejected astronauts’ requests to stay outside longer and travel farther than the conservative schedules allowed and turned down scientists’ requests for landings in remote sites of high scientific interest. Dr. Harrison Schmitt, the only scientist-astronaut to walk on the moon, got his seat on Apollo 17 only after intense lobbying by scientists and only after the last three landing missions (Apollo 18-20) had been canceled.

Neither the Apollo landings nor the unmanned missions were science driven. Nonetheless, they returned an extraordinary amount of scientific data: hundreds of thousands of photographs, some showing features as small as a millimeter across, and nearly 850 pounds (386 kg) of rock and soil samples. The data confirmed theories that the moon had been formed at the same time and from the same materials as Earth, and revealed that it had been geologically dead—and thus unaltered, except for meteorite impacts—for 3.2 billion years. The endless recycling of Earth’s crust by erosion and plate tectonics means that the very oldest terrestrial rocks are only about 3 billion years old. The moon became an invaluable source of information about the early history of the solar system, not only because of its ancient rocks but because of the 4-billion-year history of meteorite impacts recorded—again, unaltered by erosion or plate tectonics—on its airless surface. Scientists are still learning from the results of the lunar missions of the 1960s and early 1970s, but once those missions were over, the superpowers’ interest in the moon evaporated. It had been 20 years since the last Soviet mission, and 23 since the last American one, when the spacecraft Clementine entered orbit in 1995.

Exploring the Planets, 1962-1989

Missions to other planets were less influenced by Cold War politics. Sending the first spacecraft to Venus, Jupiter, or even Mars did not have the same propaganda value as sending the first human into orbit or making the first manned flight to the moon and back. NASA proposals for sending humans to Mars by the end of the century were scrapped in the early 1970s, and manned missions to other worlds were not even proposed. Unmanned missions to the planets were not, therefore, constrained by any need to serve as scouts for the manned variety. Robot spacecraft were free to go anywhere in the solar system, and scientists took full advantage of the opportunity.

Over the course of 15 years, from the launch of Mariner 2 in 1962 to the launch of Voyager 2 in 1977, the United States and the U.S.S.R. dispatched probes to every planet except Pluto. Like the missions to the moon that overlapped them, they returned a wealth of data. Their collective impact on science was even greater than that of the lunar missions, however, since the worlds they visited were more distant and less visible. Much of what is known about Venus, Jupiter, and Saturn, and virtually all of what is known about Uranus and Neptune, was uncovered by these spacecraft.

Early flybys of Venus and Mars revealed mountains taller than Mount Everest and valleys deeper than the Grand Canyon. The Pioneer and Voyager missions to the outer solar system confirmed the existence of ring systems around Jupiter, Uranus, and Neptune, and revealed Saturn’s rings to be more complex than telescopic observers had ever dreamed. They also discovered more than a dozen additional moons orbiting the outer planets, and returned the first detailed pictures of the larger moons that had been discovered by telescope.

Pictures and instrument readings from the spacecraft also allowed scientists to make critical inferences about the internal processes of planets and moons. Voyager 1 and Voyager 2 discovered sulfur-spewing volcanoes on Jupiter’s moon Io, tectonic forces remaking the rock-and-ice crust of Ganymede, and the possibility of liquid water beneath the icy surface of Europa. Pioneer 11, which became the first spacecraft to fly by Saturn in 1979, discovered that the planet radiates more heat than it receives from the sun, suggesting that it has an internal heat source. Ten years later, Voyager 2 made a similar discovery at Neptune.

Even as the spacecraft sent back evidence of new wonders, however, they also wiped away old ones. The Soviet Union’s Venera-class probes of the 1970s had revealed the surface of Venus as a living hell of four-digit temperatures, crushing atmospheric pressures, violent electrical storms, and sulfuric acid rain. The most successful of them lasted just over an hour before their systems began to fail and transmissions ceased. Mars—the subject of so many speculations about water, canals, and intelligent life—was an even greater disappointment. Mariner 9, which began orbiting in 1971, and Viking 1 and 2, which landed in 1976, revealed a barren, rocky landscape nearly as desolate as the moon. The idea of a solar system teeming with exotic life quietly faded from popular culture as the first wave of planetary probes reported back.

The pace of planetary exploration slackened considerably in the late 1970s. Missions already in progress, like Voyager 1and 2, kept going. A limited number of new ones, such as the Pioneer Venus mission to map the surface of Venus using radar, were approved. New missions were the exception, however, and in the 1980s and early 1990s the flow of new data diminished from a torrent in the 1970s to a modest stream or even a trickle. The problem was money. Both the United States and the Soviet Union faced economic problems in the 1970s and embarked on major arms buildups in the 1980s. Space exploration budgets shrank, and unmanned, science-oriented programs had to compete for funds with manned programs such as reusable spacecraft and Earth-orbiting space stations. Manned programs had military applications, and were seen (as they had been in the 1960s) as potent symbols of a nation’s technological sophistication. Unmanned programs, no matter how cost-effective, were seen as luxuries and frequently lost out in the competition for funds.

Exploration on a Budget, 1990-2006

In the early 1990s, NASA announced a radical shift in its approach to planetary exploration missions. Summed up in director Dan Goldin’s mantra “better-faster-cheaper,” it called for flying more missions with smaller and simpler spacecraft than those used in the 1960s and 1970s. Better-faster-cheaper was expected to produce a greater variety of missions to a wider range of destinations, and (by reducing costs) to increase the chances that funding could be found for any given mission. Flying large numbers of low-cost missions would also make equipment failures less traumatic: If this year’s Mars orbiter failed, another one would already be scheduled to launch in 18 months.

The new approach coincided with a series of highly public failures, not all of which were connected to it. It also, however, produced spectacular successes. Clementine, which arrived in orbit over the moon’s poles in 1995, found evidence of extensive deposits of water ice. The Mars Pathfinder, which followed the Viking landers by 20 years and cost one-twelfth as much, revealed that at some time in the past Mars had been warm and wet, with a thicker atmosphere and liquid water on its surface. The NASA probe Mars Odyssey, which arrived in orbit in 2001, detected large amounts of hydrogen—an almost certain sign of water ice—in the soil around the Martian South Pole. The European Space Agency orbiter Mars Express confirmed the presence of both water and carbon dioxide ice the following year. Early in 2004, the American rovers Spirit and Opportunity sealed the issue by uncovering rocks that had clearly been formed underwater sometime in the planet’s distant past.

A new generation of spacecraft also returned to the outer planets. Galileo reached Jupiter in 1995 and spent eight productive years in orbit before controllers dove it (intentionally) into the atmosphere. The multinational Cassini-Huygens mission reached Saturn in 2004 and separated into an orbiter (Cassini) scheduled to make 74 trips past Saturn and Titan by 2008, and a lander (Huygens) that parachuted to a soft landing on Titan’s surface. New Horizons, launched in January 2006, will spend five months examining Pluto in 2015 before proceeding into the Kuiper Belt. New Horizons barely survived budget cuts, however, and a long-planned mission to explore Europa has been postponed for lack of funds. NASA’s 2007 science budget was cut to free up money for manned programs, and the long-term impact of plans to send astronauts to Mars remains to be seen.

Modern Cultural Connections

When money becomes available for further planetary exploration missions, promising destinations abound. The water ice found on the moon, if we knew how much of it there was and where to find it, could supply the oxygen (for breathing) and hydrogen (for fuel) to sustain a permanent base. Mars’ ancient rocks and Titan’s methane atmosphere might, if we knew more about them, tell us much about the early history of Earth (of which only fragmented records exist here). Understanding Venus’s plate tectonics, Io’s volcanoes, and Europa’s oceans will give us a better sense of how rare (or common) planets like ours might be elsewhere in the universe. The question of life elsewhere in the solar system also remains unsolved, and will until Mars, Europa, and perhaps even Titan are explored in detail.

Dozens of questions about each of those destinations (and others) remain to be answered, and the answers—whatever they turn out to be—are likely to substantially alter our understanding of the solar system. If that happens, it will be only the latest step in a process that started more than 450 years ago, when Copernicus began to dismantle Aristotle’s Earth-centered universe.

Primary Source Connection

The following article was written by Mark Sappenfield, a staff writer for the Christian Science Monitor. Founded in 1908, the Christian Science Monitor is an international newspaper based in Boston, Massachusetts. The article describes the discovery of Xena, an icy world farther from the sun and larger than Pluto, and the resulting debate over what constitutes a planet by definition. In 2006, the International Astronomical Union (IAU) adopted a definition of the term “planet” that excluded Pluto, and Pluto’s exact classification remains debated. The IAU has suggested that Pluto serves as the basis for a new classification called plutons—smaller planetary bodies with orbits farther from the sun than Neptune.

WHAT DEFINES A PLANET? NEW FINDS PUT THE ANSWER IN DOUBT

The discovery of an icy world beyond Pluto, and a moon circling it, points to an unforeseen diversity of objects.

WASHINGTON—the discovery of a tiny moon circling the most distant object seen in the solar system is further proof that the view of a tidy solar system with nine planets—enshrined in science-fair dioramas and school textbooks—is headed toward almost certain revision.

In July, astronomers announced the discovery of what they considered the 10th planet, an icy world that swings 9 billion miles away from the sun and is almost certainly larger than Pluto. This weekend, they declared this object, informally known as Xena, also has a most planetlike feature: a moon.

Whether Xena is in fact a planet will be the decision of the International Astronomical Union (IAU), which could instead begin a far more fundamental reexamination of what a planet is.

Whatever its final classification, though, Xena is but one in a series of new discoveries in the solar system that point to an unforeseen diversity of intriguing objects beyond the nine planets.

In the asteroid belt between Mars and Jupiter, for instance, scientists recently found the first “triple asteroid”—two asteroids orbiting a third.

In addition, they discovered that the largest known asteroid, Ceres, is probably a failed planet. It is not just a glob of rock. It is almost perfectly round, suggesting that it has a rocky core separated from an outer crust—like Earth. Only Jupiter’s disruptive gravity prevented Ceres from accumulating more mass and becoming a planet.

Then again, Xena could redefine what a planet is. “It’s going to reignite the planet debate,” says Marcos van Dam, who helped discover Xena’s moon, Gabrielle.

The planet debate dates back to 1930 and the discovery of Pluto. Even then, Pluto was seen as an oddity—a tiny ball of ice wheeling among gas giants on an unusually elliptical orbit tilted far above and below the plane of the other eight planets. Yet in 1930, Pluto was unique, so it was deemed a planet.

Now, astronomers have found other worlds like Pluto in the Kuiper Belt—a band of frigid and far-flung objects beyond Neptune. Last year, they found Sedna, a curiously reddish body with its own moon. Now, with Xena, they have found a Kuiper Belt object larger than Pluto, and they could find scores more such “planets”—leading IAU to reconsider the term.

None of them, however, will probably be named after TV show characters, like Xena the Warrior Princess. Xena is technically named 2003 UB313 and will remain so until the IAU decides whether it is a planet. And IAU chooses the name itself.